Expansion chambers and the RG500

By Randy Norian

This is a VERY long article I wrote on pipes, I have been adding to it on and off, for years.
Time to fnish it up, and set it out in the light of day!

This is just my take on expansion chambers, and my search for
the best pipe on my Gamma. A lot of this information I have
read, some has been gathered from talking to tuners who take
this stuff more seriously than I do, some of it is just my best
guess as to how things work. Do not take this article as the
last word on 2 stroke exhausts! Although I mention a lot of
things to make you aware of them, that's often the extent of my
knowledge. I do not intend to submit this paper to the SAE and
so am not going to footnote every reference... however, I am
drawing largely on 3 published sources. Gordon Jennings wrote
an excellent article for Cycle Magazine years ago, entitled "Do
you really want to know about expansion chambers?" There is no
date on my old photocopy, however the scanned piece is available
for download along with other information later in this
article. "Two stroke performance tuning" by A. Graham Bell
(ISBN 0 85429 329 9) is required reading for aspiring 2 stroke
tuners, although some of the specific information may be getting
a bit dated. John Robinson's "Motorcycle Tuning: two-stroke"
(ISBN 0-7506-1806-x) is another essential source of 2 stroke
tuning info.

My background: I am not a professional tuner, but have been
trying to get my 2 strokes to go faster since I bought my first
RD350 in 1978.Aas far as 2 strokes go, I've owned an H2 750,
RD350 (several) , RD400 (several) , CR500, and
RG500, and have succeeded in making them all either run faster
or blow up violently. Although I've been experimenting with
altering expansion chambers for many years, I've only made a few sets
from scratch (and with help), so my practical experience
with this stuff has its limits. Take everything with a grain of
salt, and if your experience disagrees with what you find here,
well then, you should trust what you've seen with your own eyes.
But here's what we'll cover!


How expansion chambers work (basic theory)
More things to consider (slightly advanced)
pipe temp
back pressure
cone angles
multistage cones
stalling
crankcase volume
transfer port angles
body waves
internal stingers


Yet more things non-pipe things that affect pipe/engine
performance
compression ratio
Ignition timing
blowdown
disc timing
Xo-Io crossover

Designing 2 stroke pipes (programs, websites, formulae)
widening the powerband
exhaust valves
adjustable pipes
ignition timing
exhaust throttling
water injection


--------------RG500 case study-----------------
RG500 stock exhaust system (dimensions, analysis)
improving stock pipes
Other pipes I've tried
Wolf vs stock- identical??
Swarbrick
Power Pro
Nikon
Graphical Pipe Analysis
Designing the best RG500 pipe Ever Known To Man
Darcy Rosentreter (AKA The Amazing Darcy)
(and why it won't fit on your bike)
dyno results and pipe experiments


Okay, here we go!
How expansion chambers work (basic theory)

I'm assuming that anyone who has made it this far knows the
basics of 2 stroke engine operation. If you already know how
chambers work, as well, skip this part and move onto the
advanced stuff.

The exhaust system is critical to the operation of a 2 stroke
engine. A 2 stroke relies on pressure changes in the exhaust
system to help draw air through the cylinder- first to draw
exhaust out and fresh charge into the cylinder, and then a
moment later to cram any excess fuel mixture back in through the
exhaust port, just before the piston seals the port shut. A
well designed expansion chamber is worth at least a 25% power
gain over a simple tubular exhaust on a 2 stroke, and I'd guess
it's probably more like 50+ %. No matter what you do to the
rest of the engine, the exhaust will make or break the package,
and can be used to tune the engine's characteristics to a large
extent.

There are a few basic underlying principles involved in all this
exhaust pipe stuff. Here they are:

Waves and Reflections
Sound waves are just pressure pulses that travel through the
air-- just like a shock wave traveling through a slinky. They
travel faster or slower depending on the density and temperature
of the medium (the stuff they're moving through). In fact, any
pressure wave travels (propagates) through the air at the speed
of sound. They're pretty much the same thing. Why don't low
pressure weather systems rush around at 600 mph, then? I have
no idea.

Waves traveling down a tube will reflect back the way they came
when they get to the far end. If the end of the tube is closed,
there will be a positive reflection. Like a ball bouncing off a
wall, the wave returning will look just like the one that made
it. If the end of the tube is open, there will be a negative
reflection, and you'll get a pressure wave that has the opposite
value of the one that made it. As it happens, anytime the pipe
gets smaller it will reflect a partial + wave back , and any
time it gets larger it will reflect a partial - wave back.
Weird, but that's how it works, and that's one of the keys to
understanding expansion chambers.

An example (skip this if you already get it):
Suppose we have a hollow tube, and at one end we introduce a +5
psi, (positive) pressure wave. (pop!!) We'll say this wave is
short, maybe 0.1 inch long... At room temperature, the wave zips
down the tube at about 1100 feet per second (fps). If the far
end of the tube is closed with a flat cap, the wave will hit and
return (approximately) a +5 psi reflection back up the pipe,
also 0.1 inch long. (pop!!) Now we try that again , but at the
end we remove the flat cap and install a conical end cap 4
inches long. This new end cap tapers to a point, like a needle.
Again we introduce a 0.1" long pressure pulse into the pipe.
(pop!!) it travels down the pipe and hits the start of the
cone... as the pipe squeezes together, a long, but weaker + wave
is reflected back up the pipe. The reflection begins as soon as
the (pop!!) enters the conical section, so the original pressure
wave is still moving down the pipe while a partial reflection is
already beginning to travel back up. By the time our (pop!) has
reached the end of the needle, there is a 4" long wave already
returning up the pipe (that's half the length of the returning
wave). Back at the front of the pipe, where all this started,
we will be greeted by a weak pooooooooooooop wave 8 inches long.
It has all the energy of the original wave, but now it's spread
out over an 8" length, so it is lower in amplitude.
The important thing is this: we can trade the amplitude
(strength) of the returning wave for duration. Strong, short
reflection, or weaker, but longer reflection. This is The Big
Compromise in designing an expansion chamber.

Back to Pipes
The typical expansion chamber consists of a few basic parts.
The headpipe, which has a shallow angle of around 1.7 degrees
(3.4 degrees included) the diffuser (expanding section) which
has an angle of around 7 degrees (14 degrees included) a belly
or center section of constant diameter, the baffle (converging
section) which is around 12 degrees (24 degrees included), and a
stinger and muffler assembly, which varies widely but can be
anywhere from 8 inches to 2 feet or more in length, and about an
inch in diameter. These are gross generalizations, pipes vary
widely according to intended usage but these are ballpark specs
for an RG500 exhaust.

A trip thru the exhaust/intake cycle
The exhaust port opens suddenly around 85 degrees after TDC. At
that point, a high pressure wave shoots into the headpipe. The
pressure wave is traveling at around 1700 fps due to the high
temperature and pressure in the exhaust pipe. The pressure moves
down the pipe till it gets to the diffuser, the first major
expanding section of the exhaust system. Part of the wave's
energy is reflected back, in negative form, up towards the
exhaust port, while the remainder of the original wave continues
down the pipe.

The negative reflected wave reaches the exhaust port just in
time to draw lots of fresh mixture up through the transfer ports
and into the cylinder. In fact, it can pull so much fresh
mixture that it actually fills up the cylinder and pulls the new
charge right out the exhaust port and into the exhaust pipe.
(This is obviously wasteful)

Now we get back to the original wave, reduced in strength but
still moving down the exhaust pipe. It reaches the baffle cone
(converging) and begins to reflect a + wave back up the pipe.
The baffle cone is usually about 2X as steep as the diffuser and
so creates a pretty strong, but short duration, pulse. This
is often known as the "stuffing" pulse, and here's why: just as
the piston is starting to close the exhaust port, trapping our
excess fresh charge in the headpipe, the stuffing pulse arrives
and literally crams a fair amount of it back into the cylinder.
With luck, the stuffing pulse holds out until the exhaust port
is closed, and now we have much more mixture in the cylinder to
be compressed than the motor could have pumped on its own. The
exhaust system on a 2 stroke acts like a supercharger from the
exhaust side, and it's this little trick that allows 2 strokes
to make the tremendous power that they do.

More things to consider (advanced)
Once we know how the pipe works, we need to get a little more
specific as to just when all this sucking and blowing occurs.
When everything is right, it all goes just like the scenario
described above. Unfortunately, the vacuum and pressure pulses
arrive back at the exhaust port at fixed intervals after the
exhaust port opens-- this means that the timing will be right at
some RPMS, and will be very wrong at other RPMs, leading to the
notorious "all or nothing" power delivery that many expect from
a 2 stroke powerplant.

Determining the operating range of an expansion chamber
The most significant factor in this case is the placement of
the baffle cone. The timely arrival of the stuffing pulse at
the exhaust port is crucial to making good power. For this
reason, most basic equations for analyzing expansion chambers
simply deal with the 'tuned length' of the pipe. Jennings gave
this formula:


Lt = Eo x Vs / N
Lt in inches (tuned Length)
Eo is exhaust open duration in degrees
Vs = speed of sound in fps (Jennings uses 1700 fps as an average)
N = engine speed in RPM
for Metric fans, Robinson uses Lt =3D Eo x 42525/ RPM , where Lt =tuned length in mm

Lt is the distance from the edge of the piston to a point
halfway down the baffle cone, if we pretend the baffle cone
continues to make a point.

If we consider a stock RG500, this
distance is about 84cm, (33 inches) and Eo is 188 degrees.
Using Vs of 1700 fps, this formula predicts a peak power RPM of
9684 RPM. This is a pretty good estimate, as my bike peaked at
9500 rpm in stock form.
Just to juggle things a little bit, let's raise the exhaust port
2mm on this motor. This increases Eo to about 196 degrees.
Recalculating with the same pipe gives a new peaking RPM of
10100 RPM.

Obviously, there are a lot of things that affect the workings of
the expansion chamber.

Wave timing
The diffuser section generates a vacuum pulse that helps to draw
mixture up through the transfer ports. Just when this wave
should arrive depends on what we want it to do. At 10,000 RPM,
there are just 3.1 milliseconds (mS) from the time the exhaust
port opens, and the exhaust pulse heads down the pipe, until the
exhaust port closes prior to ignition. The useful vacuum pulse
has a duration of about 7 mS. The stuffing pulse, about 5 mS.
These need to be timed precisely to arrive when needed. Keep in
mind that the wave timing is for all practical purposes fixed,
and the vacuum and pressure waves do their thing, with the same
timing, regardless of engine speed.


Fresh mixture is pumped up from the crankcase through the
transfers as the piston descends. After BDC, however, the
rising piston wants to suck the mix back *into* the transfers.
Only the inertia of the flowing gas tends to keep it moving into
the cylinder.. unless it's helped out by the timely arrival of
a vacuum pulse at the exhaust port. The vacuum pulse can be as
strong as -7 psi, and really pulls fresh mixture up into the
cylinder from the crankcase. Using a less aggressive diffuser
will make a weaker but longer duration vacuum pulse, which will
be in synch over a wider RPM range. At lower RPMs, the vacuum
pulse arrives increasingly before BDC, and flow through the
transfers after BDC is reduced. At low enough RPMs, there may
be no vacuum available after BDC at all. As the pipe comes into
phase, the vacuum pulse arrives just in time to keep the charge
flowing before and after BDC, and helps to overfill the
cylinder. As RPMs increase too far, the wave will not begin to
arrive until after BDC, and at high enough RPMs the transfers
will close before the vacuum pulse is done, so some of the pulse
is wasted.

Ideally, the fresh charge fills the cylinder and then spills out
into the headpipe as the cylinder is 'overfilled'.
The stuffing pulse should be timed to arrive shortly before the
exhaust port closes. At lower rpms, the pulse arrives too soon,
before the cylinder is done filling. If the rpms are low enough
it can not only force exhaust gases back into the cylinder, it
can prevent fresh mixture from moving up through the transfers.
As RPMs rise and the pipe comes into synch with the motor, the
stuffing pulse will arrive just in time to push the fresh
'overfilled' mixture back into the cylinder before the piston
seals the port shut, and power rises dramatically. As rpms
increase futher, the piston closes the exhaust port before the
stuffing pulse can get there, and the supercharging effect is
lost. At this point, the motor usually falls flat on its face.

Body waves
No, this is not a fashion consideration. Inside the pipe, the
original pulse hits the baffle and heads back up towards the
exhaust port. but what happens when it gets back to the
diffuser section? To a wave travelling up the pipe, the diffuser
represents a decrease in pipe size... and so part of the wave
reflects back down the pipe, as a + wave. That wave then hits
the baffle, and reflects back up the pipe, and so on and so
forth. The result is a series of decreasingly strong waves
resonating inside the center of the pipe. This is called the
body wave. The body wave is fed by a fresh exhaust pulse every
engine cycle. At certain rpms, the body wave is in synch with
the exhaust pulses coming from the enngine, and it reinforces
them. This can lead to an even higher, super peak in the torque.
At other rpms, most noticeably just before the powerband
begins, the body wave can be out of step with the motor, and can
cause a terrible drop in torque output. This is often the cause
of the 'pre-powerband hole' that bikes without exhaust valves
get to enjoy. Adjusting the center section of the pipe to
affect body wave timing can be used to tune out dips and spikes
in the powerband.

Exhaust temperature
The temperature of the gases in the exhaust pipe affect Vs (the
speed of sound). Higher temps =3D higher Vs, and in turn, higher
peaking rpm.

The temperature in the pipe is affected by several things,
including ignition timing and pipe outlet restriction. Outlet
restriction is affected by the stinger length and diameter.
Pipe temperatures can also be altered by wrapping the expansion
chamber with insulating material, or by using an insulative
coating applied to the metal itself.

Backpressure
A certain amount of backpressure is desirable in the 2 stroke
exhaust. Backpressure slows the speed of the exhaust gases
flowing down the pipe, making it a little easier for the
stuffing pulse to stop the flow, turn it around, and stuff it
back into the cylinder. In fact, increasing backpressure
usually seems to increase peak power. Higher backpressure also
raises the density of the gas in the pipe, and also
temperatures, (both of which raise Vs) and thus peaking RPM. As
always, though, there are disadvantages to offset the gains.
More backpressure makes it easier for the stuffing pulse to do
its job, and generally boosts peak power, but it also increases
exhaust temps, and causes piston temperatures to skyrocket. Too
much backpressure will melt pistons, for sure. Too much
backpressure also puts drag on the motor, making it gain revs
more slowly. Heat is a big killer for 2 stroke pistons, and it
doesn't take a big problem to quickly allow too much heat to
build up in the piston. An engine that will be used for high
speed, top end runs might use a pipe with less restriction to
let the motor live under those conditions. A bike that will be
used on a race course, with periods of on and off throttle, can
often push things further because there won't be as much time
for heat to build up.

Cone angles
It would be nice if we had great, strong exhaust pulses to pull
and push mixture and and out of the cylinder whenever needed.
Only so much energy can be extracted from the exhaust pulse as
it moves down the pipe, though, so we have to choose how much we
want to use, and when. As mentioned before, steeper cones will
create stronger pulses, but of shorter duration. The powerband
will be stronger, but the pipe will only be 'in phase' , or what
I call resonant, over a narrower RPM band. Not only that, the
'off-pipe' or anti-resonance, will be even worse. The pipe
which has potential to pull the most mixture into the cylinder
can also foul things up the worst at lower rpms. It's all a
tradeoff, as usual.

There are other limits to how steep you can make the cone
angles, especially on the diffuser side. Too sharp of a
diverging angle can 'stall' the exhaust pulse, as it is not able
to follow the rapidly diverging walls of the pipe. I'm not
exactly sure what happens, but I picture it like nonlaminar
flow across an airplane wing.

Multistage cones
There are a few ways to help in this situation.
One way is to ease the pipe into a steep angle
by using a series of increasingly steeper cones- in fact, 2 and
usually 3 stage diffuser sections are the norm these days.
Another way is to increase backpressure via the stinger outlet
restriction, generally by using a smaller ID stinger. This is
sort of a band aid, however, and the drawbacks have already been
mentioned.

Crankcase volume/ pipe diameter
Crankcase volume is another critical parameter to juggle, along
with everything else in the pipe design game. Back in the old
days, 2 stroke tuners relied on the descending piston to blow
most of the fresh charge up into the cylinder. Without a strong
vacuum pulse from the pipe to assist, tuners had to rely on
kinetic energy in the transfer stream to keep things flowing
after BDC. To achieve this, and to get all that charge in the
case up into the cylinder where we figured it ought to be,
motors were designed with high crankcase compression ratios, as
high as 1.7 to 1. "Stuffing the cases" was a common practice,
where you would fill every nook and cranny of the case with
epoxy or filler, in order to eliminate any wasted space. Thus,
when the piston descended, the charge would really squirt up
through the transfers at terrific speed. Often the stream had
such speed that it would quickly zip up the cylinder, loop
around, and shoot out the exhaust port.

Transfer Port Angles
Remember, back in the old days, transfer ports were angled up
towards the back of the combustion chamber.
Eventually ideas changed and transfer ports became flatter at
their entry into the cylinder. The idea was to let the incoming
mixture streams collide gently in the center of the bore, and
rise up, slowly and completely filling the cylinder. Crankcase
CR dropped as they slowed down the transfer streams. This hurt
flow after BDC, so we began to rely more on the pipe to pull
mixture after BDC. As it happens, it's easier to suck a big
breath of air from a big room than a thimble, and crankcase
volume increased so that the pipe could more easily draw mixture
through the transfers. Modern motors have large cases, with
crankcase CRs as low as 1.2 to 1. The stock RG500 has a case CR of
about 1.33 to 1.

Case Compression diatribe:
Case Compression is a critical element! Here are two key phases to keep in mind as we try to tune
up our two strokes. I'll list them in reverse order, because this is how we often do our tuning.

1) Get more mixture into the cylinder! We port the cylinder for more airflow. We design a pipe to suck more
mixture up from the case, and into the cylinder. Typically we overscavenge the case, and draw mixture out into the
exhaust pipe, then we stuff it back into the cylinder before the exhaust port closes. Fatter pipes can draw even
more mixture from a larger case, so there is a temptation to make the case volume larger to facilitate this.

2) But.. we have to also FILL that case very efficiently. Increasing the case volume decreases the amount of vacuum that the engine will create, as it tries to suck air in through the inlet tract. Imagine a 125cc piston atop a 50 cubic foot case. There would be hardly any vacuum at all, under the piston. As you increase case volume, you must find a way to fill the case well- the simplest tactic is to reduce restriction in the inlet tract. Shorter, larger intakes with larger carbs, will allow you to keep the air flowing in even as you reduce case compression. IF you can still fill the case well, you can come out ahead with a larger case volume and lower case CR. Reducing case CR and keeping your intake tract unchanged is pretty much a guaranteed way to reduce your engine's ability to fill its case. Lower case CR also reduces the strength of the vacuum signal to your carb. Combat this by using carbs with vacuum-boosting design and moving the carbs as close to the case as possible. Every extra cubic centimeter of volume in the inlet tract, dilutes strength from the metering signal that makes it to the carb.
Sometimes you have so much vacuum, it's not a problem. ie: 560cc engine with 35mm carbs that are set away from the engine on rubber manifolds.
Sometimes it becomes a problem: ie: 498cc engine with 43cc adaptor plates (that add to case volume) in addition to 35mm carbs
on rubber manifolds.

 

Back to pipes!
A fatter pipe can extract more energy from the exhaust pulse,
and can generate stronger vacuum to pull mixture. However this
only works to an extent, as a small case will frustrate this
approach. You simply cannot take a deep breath from a small
bottle. Fat, high-suction pipes will also pull more mixture out
the exhaust port, and with older-style transfer ports (upward
aimed transfers) an excessive amount of mixture will escape into
the exhaust pipe--more than the stuffing pulse can push back in.
So.. it all needs to mesh- each factor affects the others!

Internal stingers/ side exit pipes
Here's the basic idea. In a normal 2 stroke exhaust, the
pressure pulse heads down the pipe, making its reflections, and
traveling until it gets to the back of the pipe...at which point
what's left of the pulse escapes into the stinger. Why should
we waste this last little bit of pulse? Why not let the baffle
cone continue down until it comes to a point, and extract all
the energy out of that pulse where the gases are at their
hottest and most highly compressed? Let all of the pulse bounce
back up the pipe, and simply let the gases come out someplace
else, like from the side of the pipe in the center. That's the
premise for a side exit pipe. For some reason these have not
really caught on, but rear-mounted internals stingers are not
uncommon. On these pipes, the stinger simply extends into rear
of the pipe a few inches. The exhaust pulse is entirely
reflected back up the pipe, and pressure is bled off the pipe
from a point further up the exhaust, in this case, maybe 4 or 5
inches before the end of the baffle. I believe Spec II uses
them on their RD350/400 pipes. Internal stinger pipes are
generally acknowledged to be a little quiter than conventional
stinger setups; proponents of internal stingers say this is
because more of the pulse is being put to work in the motor, and
less exhaust energy is escaping out the back of the pipe.
Yet more things in the engine that affect pipe operation & design
A very experienced tuner told me to think of the expansion
chamber like a turbocharger. The more heat and energy you put
into it, the more you will get back from it (in terms of
stronger wave action)
 

More Things To Consider

Compression Ratio
Increasing the compression ratio generally increases the energy
released during combustion, because it's a good thing to squeeze
the mixture very tightly before igniton. On the other hand, a
side effect of higher CR is that more of the energy released is
taken against the piston crown, and less escapes into the
exhaust pipe. Less energy going into the pipe =3D weaker wave
action. weaker waves =3D less effective movement of fuel/air
through the ports. So now what do we do? Our new, high
compresion heads are a double-edged sword. If the pulse
heading into the exhaust is weaker, we can switch to a fatter
pipe-- in an attempt to extract more energy from the weaker
pulse we now have. Kevin Cameron wrote an article on
this topic (Feb 1998) and cited examples of race engines
where compression ratio and pipe diameter are linked, a
mysterious reduction in CR for a new model is accompanied by a
reduction in pipe diameter, and vice versa. It's common
knowledge that higher compression heads will give better
midrange, while lower compression heads will often let a GP bike
rev out harder on higher speed tracks, can it be because the LC
heads allow the pipes to work better? There are cylinder heads
on the market (by Polini) that feature a floating combustion
chamber that recedes at high RPM, this may not only address MSV
(max squish velocity) but probably allows the motor to pull
crisply in the midrange with a high compression setup, while
retaining low compression rev-out characteristics at peak RPMs.

Ignition timing
Advance affects the expansion chamber primarily by altering
exhaust gas temps. More advance generally reduces EGT, to a
point. Retarding the timing makes the mixture burn later, and
more heat escapes into the exhaust pipe. Higher EGTs raise Vs,
and remembering the equation for Lt, the peaking RPM of the
motor varies directly with Vs.
Simple version: retarding the timing at high RPMs will give you
more overrev. This is no big secret, and my motor will pull
hard for an extra 500 rpm or more simply by retarding the timing
4 degrees. The trick is to keep a good amount of advance
through the upper-mid rpms, and retard timing after peak power
to extend the rpm range of the pipe. Being able to rev another
500 rpms may save a racer several shifts per lap, so retard is
an easy way to tune the exhaust system on the fly.

Blowdown
Blowdown refers to the interval between the opening of the
exhaust port, and when the transfers open. Usually this is
31-35 degrees of crankshaft rotation. Blowdown is important
because the high pressure in the cylinder has to bleed off
before fresh charge can flow up through the transfer ports. 35
degrees of rotation doesn't seem like a long time for this to
occur, yet 35 degrees is a very generous figure for blowdown.
30 degrees would be considered insufficient for a high rpm
motor. Generally, we'd like as much blowdown as we can get!
Why not let the pressure in the cylinder fall as much as
possible before trying to pump in that new charge? Well, have
to move the exhaust and transfer ports further apart to increase
blowdown (sort of), and there is a limit to just how far these
ports can be relocated.

A modest engine may have exhaust opening at 86 degrees ATDC, and
transfers opening at 118 degrees ATDC. That gives 32 degrees
of blowdown, not a whole lot, especially for a high rpm motor.
The only ways to increase that figure are to raise the exhaust
port (reduces the power stroke) or lower the transfers (not the
best idea for making more high rpm power). However, these are
not hard and fast rules. One tuner said that raising the
exhaust port on a stock RG500 will help even in the midrange,
because the stock porting setup is short on blowdown. Raising
the port allows the exhaust to vent more completely before the
intake cycle begins, and can result in better cylinder filling.
By that same token, too-high transfer ports will look racy on
the spec sheet, but will drastically reduce blowdown timing, and
will hurt top end performance. I have ridden motors with
transfer ports too high, and they revved really high, but never
seemed to pull very crisply. Just never came on the pipe really
hard.

That's a bit of an oversimplification, though, because it
disregards the SIZE of the exhaust port. A very wide exhaust
port with 30 degrees of blowdown may well outperform a motor
with a tiny exhaust port and 35 degrees of blowdown. We need to
consider the exhaust Time-area, which factors in exhaust port
size AND exhaust duration. In general, you want as much exhaust
port area as possible, and that's why we see large bridged
exhausts or triple exhaust arrangements. These are tricks to get
as much exhaust area as possible without overstressing the
rings. In an ideal motor, we would have tons of exhaust area
but a low port height, so we could have excellent low and mid
rpm power, while retaining plenty of exhaust time area to
support good top end power. This doesn't impact the pipe
directly, but the exhaust design needs to be in agreement with
what the porting is trying to do. Bolting a super high rpm pipe
onto a motor with mild porting will result in an engine that
doesn't know if its supposed to be coming or going. Everything
needs to be in synch and in step with each other in order for a
2 stroke to really get that terrific power peak.

Disc timing
On disc valve motors, the disc timing can interact with the
pipe. One parameter especially strikes me. I mentioned earlier
the problems with a small crankcase volume- high transfer
velocity, and also a limited volume for the pipe to draw from.
Well, that second one isn't strictly true- there is a way to
trick a little more air into moving through the cases. On an
Rg500, the disc valves open about 147 degrees BTDC. The
transfer ports, meanwhile, do not close (as the piston rises)
until about 119 degrees BTDC. This gives us 28 degrees of
crankshaft rotation where the engine is open all the way from
the exhaust port to the carburetor!! An exhaust pipe with good
vacuum late in the transfer event can pull mixture all the way
from the carburetors- basically from a crankcase with no volume
limit! I have heard speculation that the Aprilia 250 GP bike
uses this tactic to good effect. You never know!

Reed valve engines can allow air into the crankcase any time there is a
vacuum on the transfer port. So, this overlap timing bit does not need to consider
disc timing. Basically anytime the transfers are open, air can enter the case if the engine asks for it.

Designing Two Stroke Exhaust Systems
There are all sorts of programs available that will help you to
design an expansion chamber. Most of them do not worry too much
about ignition timing, compression ratio, crankcase volume, disc
timing, blowdown, port timings, exhaust size, projected exhaust
and pipe temperatures, etc etc etc. All of these things have a
bearing on how the pipe/ engine package will function, but are
often overlooked. It's fun to plug in a few numbers and have a
little program shoot out a pipe design, but there is no
substitute for a good analysis beforehand and some trial and
error testing afterward!

Having said that, here are a few of those little programs-

SAE paper on pipe testing: 942515 is very interesting and I made much use of their info.
Robert Fleck, QUB

Randy's spreadsheet: I used this before I had any other software, it's a spreadsheet I wrote to
analyse the timing of the pressure waves inside the pipe. Been so long since I used it, I can barely
remember how it works. But, I'm posting it anyway. You plug in your port timings, and it tells you
when the pressure and vacuum waves should arrive back at the cylinder. I then analyzed those wave
timings against when I needed them to arrive, at certain RPM.

I used that spreadsheet, and a lot of time spent chewing pencils, to develop the 113mm pipe that
eventually got built - the Darcy pipe! it is probably of no use whatsoever to you, but while I'm making
link, I might as well add that one.

here are a few other spreadsheets
calculate case compression
wave speed versus temperature

 

These are DOS programs:
a simple DOS program to design a pipe
A few 2 stroke programs (includes the above)
A whole bunch of 2 stroke programs (includes both of the above)

A good Excel spreadsheet with tons of calculators (I have not verified it)

A page of engine formulae

designing a pipe
I am not sure what would happen if I tried to design a pipe
totally from scratch. It's certainly easier to take an existing
design and build from there. In my case, I have measured a few
different RG500 pipes and the ones I like all had specs very
similar to each other. The stock pipes, Wolf, and swarbrick are
all pretty close dimensionally. The tuned length is within a few
cm. I was happy with the RPM range of my motor using these
pipes, so I decided to keep the tuned length in that same range
and focus on trying to get more air to move through the motor,
not just increase RPMs endlessly.
The pipe design programs certainly can give you a starting
point, if you're starting from scratch.

I've done a lot of chopping up pipes to see what happens, if you're into this sort of silliness, pick something with one or 2 very simple pipes! laboriously fitting pipes with exotic shapes gets old in a big hurry, especially if welding is not your second nature. My RD350 was a perfect test mule for a lot of pipe testing.

The first thing I do is check the tuned length of the pipe. even if the equation for tuned length doesn't agree exactly with what you're seeing, changes will probably be relative to predicted numbers. so, if you want to raise peaking RPM by 10%, check the equation and see what sort of change shows a 10% increase in RPM.

TSR's software will spit out a complete pipe design, based on only a few simple input parameters. A lot of guys swear by TSR's software, so maybe you can save some grief and start there. If you really want to see what's happening inside the motor, with respect to pressure and vacuum waves, I have to recommend Ian Williams Tuning- “MOTA” . This software is more fun than a barrel of monkeys if you're into analyzing 2 stroke engines. It will give all the data you could want (both graphically and in numbers) with respect to flow direction, velocity, temperature, charge purity, etc etc etc at almost any point inside the engine. But, it does not design a pipe for you.


MOTA can be found at http://members.ozemail.com.au/~iwt/

TSR software can be found at http://www.tsrsoftware.com/

One tuner listed a set of relationships between certain parts of the pipe, and sure enough, in a wide variety of the pipes I have measured (TZ250 pipes, RG500 pipes, and ROC500 pipes) those relationships held. The only pipe that deviated from those relationships is my current pipe (aagh!) which frankly works very poorly and has been adapted to its current application from my old motor. It is all wrong for the current engine and the torque curve shows it.

Next I try to set the diameter of the pipe's belly section. There appear to be many rules of thumb you can follow, my suggestion is find a pipe that works well in a similar application and measure it! if that motor uses a 40mm exhaust port, and sports a 125mm pipe, give it a shot. In general, the belly section on most HiPo pipes is greater than 3x as large as the exhaust port diameter.

If you're into experimentation, there are all sorts of baffle sections to try. 1-2-and 3-stage baffles are all in widesprad use. We had good success with a 2-stage baffle on the darcy pipes (steep, then steeper). That was also used on my Swarbrick pipes. TZ250 uses a 1 stage baffle. Multistage diffusers can be used to ‘ease' the pressure wave into steeper and steeper cones without stalling.

When you do get a pipe whipped up, try to incorporate a variable-length center section, and then get to a dyno and do a bit of testing. there WILL be one optimum length, you can sort it with a few tests and then lock things down from there. It is unlikely that your first guess will be exactly correct, there is no substitute for testing!

A few things to remember, which I am mostly reciting from yet another excellent article by Kevin Cameron:
-very large diameter pipes suck hard, and are not really intended for use with older cylinders that feature upward-sloping transfers.
-too steep of a diffuser cone can have stalling problems internally- when the advancing wave cannot ‘stick' to the rapidly-diverging walls of the cone. This can be staved off to some extent by making the stinger smaller and increasing backpressure in the pipe.
-large header pipes tend to favor top rpm power at the expense of lower RPM torque
-an increase in compression ratio may call for a fatter pipe. Lower CR motors dump more heat and energy into the pipe and can generate more vigorous pumping action. Yamaha's TZ250 pipe got fatter in years when compression went up, and got skinnier in years when compression went down.
-shorter, steeper baffle cones = more powerful stuffing pulse, but a narrower powerband.
-pipes that pump harder (fat, steep pipes) tend to have even worse off-pipe behavior when they are out of step with the motor.

Widening the effective range of a tuned pipe

Exhaust valves

Remember, before the exhaust pipe comes into tune with the rest of the engine, there is usually a preceeding phase where it is at odds with what the engine needs.. the “pre-pipe hole” .

Exhaust valves are a way to reduce or eliminate that effect.

There are all sorts of exhaust valves in use, although 2 general types come to mind. Some vary the height of the exhaust port (YPVS barrel valve, TZ-type guillotine valves, etc) This has the obvious effect of changing the exhaust tuning to try and get around that bad combo of exhaust timing, and off-tune pipe resonance.

The other type of exhaust valve I have experience with uses a small chamber (Honda ATAC, Suzuki AEC) on or near the exhaust header. This chamber greatly reduces the amplitude of the exhaust pulse, so even when the pipe is out-of-tune with the rest of the engine, it has little effect. There are SAE papers on this topic, and that's how they work, so forget what you heard about changing the volume of the pipe so it resonates at a lower RPM.

Adjustable pipes-

Physically adjustable pipes exist! They're in use on some racing go karts- the pipes have a slip-section in the middle, like a trombone, and can be adjusted shorter or longer to change the tuned RPM as needed. I suspect this is done in the pits!

There are ways, however, to alter how the pipe behaves, without changing its physical dimensions.

Water injection

My full article on water injection can be found here
The short version is, injecting water into the headpipe cools the gases in the pipe. This slows the speed of sound. waves take longer to move up and down the pipe.. and so the pipe acts like it's longer. you can tune the peaking rpm of a pipe downward by several thousand RPM by cooling it with water injection. Then, just shut the water off and let it rev out normally on top.
A dyno chart of runs with water injection can be found here (link)

Ignition timing for overrev

Retarding the timing puts a lot more heat into the pipe. Hotter internal temps increase the speed of sound, so the pipe acts like it's shorter. After the power peak, pulling back ignition advance can add generous overrev- it works like a charm. A good pipe in conjunction with water injection, and well-set-up ignition advance curves, can have a tremendously wide power plateau.

Exhaust throttling

This has been tried in years past as a way to increase backpressure, and also, internal pipe temps. I was told that kawasaki had tried it on their roadracers years ago, with modest success, but the systems were not mechanically reliable enough so it was dropped. Still, potential exists for yet more pipe control, if one wanted to pursue it.

Various Pipes and the RG500

Pipe Analysis: Stock RG500 exhaust pipe
The stock pipe on the RG, despite its lumps and bumps, took my engine to 106 Hp. So, if you've
laid out thousands of $ for zooty chambers and find you're making 95 HP, you have my sympathy.
There are all sorts of bottlenecks on the RG engine, and the pipe is one of them, but the stock pipe
can be made to work quite well - as well as most aftermarket pipes.
Here is a little tip on how to clean up the stock pipes for better performance.

here are the dimensions of stock pipes, as well as some other popular pipes.
Pipe dimensions from Marc Alexander

Like I said, the stock pipe on an RG500 isn't a bad one.
Stock and swarbrick (drawing & dimensions)
Nikon and GT Thunder
TC RPM and PowerPro
Wolf and TC Torque
JollyMoto knockoff

Lets go back to the equation for tuned length.
If we consider a stock RG500, this distance is about 84cm, (33
inches) and Eo is 188 degrees. Using Vs of 1700 fps, this
formula predicts a peak power RPM of 9684 RPM. This is a
pretty good estimate, as my bike peaked at 9500 rpm in stock
form. The Stock pipes are narrow, as befits a bike with low compression.

Other pipes I've tried:

And I have to say this first- even if I paid a lot of cash for something, if it doesnt work, I'll be the first to admit it. A lot of guys will spend $$$ on a pipe or whatever, and by god, that pipe's just the greatest thing ever. For me, if it doesn't work, that's pretty important!

the first aftermarket pipe I used was borrowed from a friend.

? mystery pipe. short, with tiny mufflers and earshattering sound level. I could not really discern any performance improvement from stock. The problem here may be that the pipe HAS to work in conjunction with your porting. dropping a 12,000 rpm pipe on top of a motor with 9500 rpm porting will just make a wheezy, revvy, gutless setup.

Next up, I bought a set of Nikon pipes from the UK. These were beautiful, yet the upper and lower pipes were drastically different in length. I suspect the upper pipes were short, to gain rear tire clearance. The result? my motor didn't know if it was supposed to be coming or going. They would have worked well enough if one equalized the pipe lengths, I suspect. It pulled smoothly down low but was slower up top with less overrev than my stock pipes. nice sound though. Sold em in less than 2 months.

Somewhere in here I opened up and gutted the stock pipes and removed the screening.

There are also aftermarket silencers available, I have no experience with these. Ask around! Carron sells a muffler for the Gamma.
http://home.cogeco.ca/~vapats/cans.htm

MFactory west sells beautiful carbon cans that can be fitted to many expansion chambers
www.mfactorywest.com

In my stock pipes, just inside the headpipe there was a very obvious and large ridge of weld, restricting flow out of the pipe. grind that as flush as you can! What you now have is a set of pipes that functions every bit as well as Wolf pipes. How do I know?

I dynoed a set of wolf pipes back to back with the gutted stock pipes (and stock mufflers), with exactly the same power across the board. Not bad for stock stuff, eh?

Stock (gutted) pipe vs Wolf pipe- back to back, same day, same dyno
Swarbrick pipes are also on this chart.

here's a little snippet on Wolf pipes, by Rick Lance

>>A brief history lesson on the origin of Wolf pipes/

In 1985, a 2stroke fanatic by the name of Howard Jacobs jumpstarted the
whole Gamma craze in North America by attending the Koln show in
Germany, where he made arrangements to have an RG shipped home along
with a brand new pipe design from Jolly Moto (they had to be new, since
the bike they were designed for was only two weeks old at the time).
Howard's Gamma was the first one in the New World to my knowledge, since
it arrived at his house before the bikes were released for sale in
Canada. Howard set up about six of his friends with RG's who eventually
all wanted pipes as well. In 1988, Jolly Moto stopped supplying the
pipes for Howard. He then took his personal set to Rocket Racing in
Canada to see what could be done to have them duplicated. Robby
Micklejohn and Gary Wolf said "Sure, we can make anything!" and promptly
sent them to Stuart Toomey, who in turn sent them to his stamping maker
to cut them up and make stamping patterns. Stuart supplied the
stampings allong with flanges, stingers and mufflers unassembled to Wolf
at Rocket Racing where the final assembly was done. Some early teething
problems in assembly were eventually ironed out but took the fun out of
making more than one production run and they were discontinued. They
perform just like the Jollys they were patterned after; i.e. soft on the
bottom and midrange, come on nicely at 7500 rpm, pull well to just past
10 grand where they plateau until 11,200 or so and then pull to 12k and
sign off. The Jolly/Wolfs are my second favorite chambers for the Gamma
(TC's being the best, which I sell, but that's another story, and it's
past my bedtime)<<

I measured the Wolf pipes and dimensionally, they are nearly identical to stock, which would explain why the run pretty much exactly the same as the gutted stockers. Anyhow, it seems Rick likes them, so run out and gut your stock pipes! I also talked with Dammy Walker when he was at SWMS, and he mentioned that they also had found the stock pipes to be excellent.

I also bought a set of Power Pro pipes for my Gamma. These came with carbon fiber cans that looked cool but were extremely heavy, due to the massive steel caps at each end. Oh well- The Power pro pipes had an ENORMOUS flat spot, due to the powervalve opening too early for their state of tune, and they revved to the moon, probably 12,000 rpm. A lot of drama and excitement accompanied a modest but measureable increase in acceleration. (see testing chart) . The power pros crossed over each other- they later became guinea pigs which I would modify several times. There was no way you could get around a racetrack faster with these pipes- the powerband was too spikey. Stockers were still best.

At this point I had spent thousands of dollars on pipes and not really found anything much better than stock. Rick Lance kept telling me I had to ride his black bike, but it was just too far to really get a chance to stop by. Besides, the TC pipes cost a fortune. By this time, I was measuring every pipe I could get my hands on, and numerous people had sent me dimensions, as well.

My next set of pipes, I keep calling swarbrick pipes. It's been so long I can't remember who actually makes them, swarbrick, or Stan Stephens. I think Stephens sells them as completed pipes, and Swarbrick sells them complete or in kit form. But they're both the same pipe. I bought mine with assembled lower pipes, and I put the upper pipes together myself, so I could fit them to the bike. Overall length is very similar to stock, around 86 cm, but slightly fatter and with a 2 stage baffle cone. They worked GREAT, similar power to stock, but with a bit of added punch on top. no stratospheric revs. Nice build quality and nice sound, pretty light, just good solid, strong-running pipes. And they bore that out on the dyno.

People offered to send me TC pipes to test back to back against the swarbrick pipes, but since Lance Gamma always insisted his parts were part of an overall tuned package, it just didn't seem like a fair shot to plop them onto my bike and run em in front of the world. In retrospect, I wish I had! Ha!

Darcy Pipes- the beginning
Somewhere in here, I met Greg Lewis, an incredibly nice and sharp fellow who, like myself, suspected that there had to be something that would work better on our bikes. Greg sent me an SAE paper detailing testing by QUB (Queen's University at Belfast) with a variety of different pipes, on a TZ250. There was a LOT of data. pressure plots, you name it. Finally something to analyze, a solid starting point!

At this time I was using my own system of graphically analyzing pipes, which was something I had sorted out while studying Computer Engineering at ISU. Actually, I probably developed it while skipping classes, while taking Cpr E at ISU, which might explain why I never finished my degree. Anyhow, analyzing pipes this way seemed to work to some extent but it was laborious. Here's my posting on the technique: (by the way, it is MUCH easier to do this with MOTA!)

Graphical pipe analysis

here's more stuff to cause disruption- awhile back, I was doing a "graphical" analysis of expansion chambers. 2 axes, X= time, from zero to 4.5 milliseconds (Tzero = the moment of exhaust opening), Y= distance (cm). across the top.

don't actually do this, I'm just running on here. My wave analysis spreadsheet does the same thing, with no rulers or paper needed- however, it's not as obvious to the eyeball, what's happening.

Plot out when the transfers, and also exhaust, are open at say, 6k, 7k... up to 12KRPM or so. do this up above the entire graph, at the top of the sheet, as a bunch of horizontal lines. so now you have a bunch of bars representing the open period of the ports. The exhaust bars reach all the way to the Y axis, since it opens at Tzero. The transfers open whenever they do, and close whenever,and laid out like this you can visually SEE where they overlap.

Next, I draw out the pipe along the Y axis, (cm) and show the cones.
draw rules across, horizontally, at the junction of each cone.
Now draw a line along Y=X, a 45 degree angle, 1:1 is 50cm (Y) = 1 mS (x) this is for a wave speed of approx 1640 fps.

This is going to graph where the pressure pulses reflect back to the ports.
at Tzero the exhaust port opens. a pulse travels down the pipe. as it hits the first diffuser, the neg pulse begins to reflect back, so find the start of the diffuser, go across horizontally till you hit the 45 degree axis, from the point of intersection then draw a line back down to the X axis, at slope = (-1). This shows when, in mS, the vacuum pulse begins to arrive at the port.

do the same at the end of the diffuser, this will show when vacuum at the ports ends.

" " for the start of the baffle, the start of the pressure pulse
" " for the end of the baffle, the end of the pressure pulse.

Now you're looking at a graph of when your ports are open, over a range of RPMs, versus when the vacuum and pressure pulses arrive at the exhaust port. Of course, that will not vary with RPM.

This seems like a lot of work, but you can just look at it and SEE when the exhaust is too early, or too late, sure it's not exact and it doesn't take into account the body waves in the pipe (that gets way too complicated! it takes a few reflections to get in step, and they're very spread out and weak.)

The longer cones produce a reflection which is longer (weaker), right, 2X as long in duration as the time it takes the pulse to travel thru that cone. and the short baffle makes a short reflection (stronger), if you plot a body wave its a reflection of a reflection, and it's very Lo-o-o-o-o-ng and weak, it overlaps everything and it's just hard to tell how it interacts with anything. I suppose this is where a computer is better, as long as the model is a good one!!

I used this VS my adjustable RD350 pipes and did a little empirical observation of how it behaved vs the what showed on the graph. I also did this vs the RG pipes I used.
I haven't done this in awhile, mind you! I was just cleaning the closet and found my big board w/ overlays on it for 4 different pipes.

I'll use stock pipes as an example.

Anyhow, some reference points I used are:

some vacuum is available at the transfers AFTER BDC. This shows when the pipe is (starting to) helping to suck extra mixture into the cylinder after it stops being displaced by the descending piston. Not sure how much of a biggie this is. On stock pipes this occurs at 6500 RPM. [usually this number is a little before the motor begins to wake up]

"transfers immune to + pressure" the RPM beyond which the transfers are closed BEFORE the + pulse arrives at the exhaust port. Otherwise it may be trying to blow mixture back down the transfers? Stock pipes this happens at 9500. [does this explain the little extra hump of torque at the top of the powerband?]

"vacuum wasted" RPM beyond which transfers are closed, but vacuum pulse is still arriving at the Xh port. So the "sucking" is losing effect after this point. Stock pipes, 10200 RPM. This RPM has the entire vacuum pulse doing work at the trasfer port, after this the effect is diminished.

"Exhaust losing effect" rpm where the + pulse is ending JUST as the exhaust closes. Perhaps the "ideal" RPM? after this RPM, the exhaust port is closed while there is still + pressure arriving at the port window. power peak usually close after this RPM. stock pipes this occurs at 9000.

"Exhaust effect done" RPM after which the exhaust port is CLOSED before the + pulse can begin to arrive. There is little or no exhaust stuffing after this RPM. Stock pipes this occurs at 11400 RPM. This is usually the very end of the powerband, upper limit of RPMs.

just from the eyeball analysis, most good things are just starting to happen at 6500, things really get into maximum synch around 9000, and the pipe is done at 11400 rpm.

the exhaust/transfer open plots depend on port timing, of course.

Here's a comparative Power Pro analysis:

Stock P-Pro

some vac. ABDC : 6500 7000 rpm
Transfers immune: 9500 11000 rpm
Vacuum wasted: 10200 11500
Exh losing effect 9000 9500
Exh effect done 11400 13500

the P-pro is a very short pipe, with a Lo-o-ong baffle cone, maybe an attempt to spread the power out.

transfers are fighting the exhaust (+) pulse till 11000 rpm.
optimum ( -) pressure at transfers at 11500
best exhaust + pulse from 9500- 13500. The bike can't possibly begin to rev that high, much of the exhaust + wave is wasted.

basically it's not really ready to rock till 11000 rpm, (transfers + wave timing) but the exhaust pulse is hitting its best starting at 9500. Confusion! then the pipes wants to hit 13500 but the motor won't breathe at that RPM. So you get a pipe that can't decide to start running till it's almost time to stop running! The result on the road, a VERY weak on-pipe transition, things don't all come into step at the same time, and a fluffy powerband that just peters out weakly as the pipes are still placing a weak + wave at the port till 13500 rpm.

They suck!

That belly section serves to get the exhaust + pulse "off of the transfers' back" and give a period where the transfer ports do NOT have a + wave hitting them. On the P-Pros, the + pulse just starts too soon.

Anyhow, that's my theory.

This does not take anything into account for crankcase volume, pipe diameter, transfer efficiency, etc. just my little homemade analysis. The sad thing is that I figured an "ideal" pipe, and it came out just like the wolf pipes. Just going by where you'd like to have things sucking and blowing, the wolfs appear to be right on the money, where the comes are positioned. It's really a pretty simple set of constraints, especially for the baffle cone.

Of course, all this is part of the whole, I still dunno why my bike revs short ( 2 degrees otside? no thanks) but hoping for a 40 degree day soon, maybe to test? we'll see.

So the only thing to come out of this is that apparently, in the distant past, I decided that Wolfs (or Swarbrick, their twin brothers as it turns out) are a pretty good pipe for the RG! *sigh* at least the cones are in the right spot, from my graph.

BTW I measured and wolf has identical upper and lower pipe dimensions. so does / Swarbrick/ Stock, for that matter. They're all symmetric. So that's an argument for that
approach!

Darcy pipes- serious work

OK. So far we hadn't found any aftermarket pipes that were really much better than stock. We were going on the assumption that there SHOULD be some major improvements available. So, why aren't aftermarket pipes that much better? Or at least, why weren't the stock pipes even better?

This was my thinking: Suzuki must have had some basic constraints which limited what they were able to do with the stock pipes.

1) noise requirements
2) cost requirements (encompassing materials , fabrication techniques, assembly, etc)
3) matching up with the stock intake, porting, compression, etc. RPM limits, what have you.
4) fitting the pipe onto the bike


as far as (1) went, we could use any muffler we wanted, as could other aftermarket pipes. I didn't see any silver bullet there.

2) cost? with some pipes costing well over $1000 USD, I think they weren't skimping on construction or welding.

3) The stock pipe/port combo is fairly well matched, with the exhaust port being a bit on the conservative side. The transfer porting is right there for 10,000 rpm operation. The carbs are definitely on the small side to begin with. Dropping a pipe designed for operation at 11500 to 12000 rpm on top of this motor is just ridiculous. The whole engine is a system. It needs to work together- resonance tuning works that way! At any rate, without massive reworking of the ports, especially the transfers, it appeared that a stock-barreled engine would run most efficiently if we kept revs to a 10,000 to 10,500 RPM limit.

4) fitting onto the bike... aha!.... aftermarket pipes have to fit onto a stock gamma. They need to leave room for a rear tire. They need to fit under that slender bodywork. All these things limit maximum diameter. That was the missing ingredient (so we hoped).

I took my dimensions for the best-performing pipes I had used (Stock/Wolf, and Swarbricks) and gave them all my most thorough analysis with the old graphical technique. I scoped out the SAE article on TZ pipe design. I spent about 6 months fiddling with all this stuff, in between calls to Greg to bounce ideas off of him. The pipe I eventually came up with was similar in length to the stock-wolf-swarbrick pipes, but fatter, and with different diffuser and baffle sections. I had read that the 2 step baffle could give certain characteristics (high torque with less overrev) and was willing to try it. After discussions with some real tuners on the 2strokes list, I wanted to incorporate internal stingers, as well.

About this time, I began shopping around on the lists for someone who could make cones for this project. It happened that Darcy Rosentreter was on the 2strokes list and saw my queries.

To my incredible good fortune, Darcy contacted me out of the blue and offered his assistance. For those of you who don't know Darcy, I won't toot his horn too much. There are a lot of 2stroke tuners out there. There are also a LOT of snowmobile tuners and racers. Every year the sled racers gather for a North American drag racing championship- this means the biggest names, the top outfits in the business. Darcy has built and raced sleds in this championship, and has won the most prestigious class- and when he's not winning, he's not far off. And behind the scenes, his pipes are on a lot of the other top sleds. I suspect he's about as good as anybody out there- his 750cc Vmax (basically a TZ750 engine) made somewhere in the neighborhood of 250 Bhp- and that's normally aspirated. That's some serious power. So if you want to discount what I have to say about pipes, go ahead! I'm just a backyard tuner. Darcy knows his stuff, though. And that's a bit of an understatement. He has a combination if creativeness, ingenuity, open-mindedness, technical and fabrication skill and know-how, and dogged perseverance, that make for a very tough competitor.

Anyhow, let's just say that he was a good man to have on board when it came to building a pipe.

Darcy reviewed the pipe and engine specs and made a few suggestions- he altered the belly diameter, based on his experience with exhaust port-to-pipe diameter ratios, and adjusted the header pipe angle. We settled on a configuration and he sent me a set of test pipes for the upper cylinders- with adjustable center sections, so I could get them dialled in just right on the dyno. I figured I could just dyno-test 2 pipes; if they worked better, it should show even with just pipes on 2 cylinders.

So, I went off to the dyno armed with my new pipes. First run, we baselined the bike with swarbrick pipes on it, and it pulled ~111 Hp. The I bolted on just two Darcy pipes, on the rear cylinders. With a few adjustments for length, we got ~ 116 Hp.
Here are results of 3 different belly section lengths

That was encouraging so I went ahead and made a full set of 4 pipes. Next trip to the dyno, with 4 darcy pipes: 127 Hp.
Here are Swarbrick baseline, vs 2 Darcy pipes, vs 4 Darcy pipes

The Darcy pipes worked great, made just as much power as stockers in mid rpms but way more at high rpms. They really came into their own with a few degrees of ignition advance, but that of course makes the power fall off rapidly after peak. Darcy pipes + a programmable ignition give a really stellar improvement over other pipes. The biggest drawback is, they are quite fat at 113mm, and need to be carefully fitted to go under stock bodywork. Darcy built several sets of these pipes, later on, he was too busy so if you want to make a set, you need to do it yourself. I have good luck ordering cones from Aircone, however, once you get past a certain size (I forget, 115mm or so) the price goes up substantially and my Delta pipes cost me pretty dearly to order cones.

Here is a spreadsheet for plotting pipe shape, or figuring cone sizes. It comes pre-loaded with darcy pipe specs.
PAC_Delta pipes.xls

Delta Pipes
When I changed to TZ barrels, it was clearly time for a new pipe. Greater case volume and higher compression both dictated a larger diameter pipe. We had to start somewhere, so I started off by using the old darcy pipe as it was. Oddly enough the engine peaked at a reamkably low 10,200 RPM, despite the very aggressive TZ port timings. The RG timing map probably did not help things, either, as it went hand-in-hand with the 10,200 RPM pipe.

So, I shortened the pipe, and torque fell, and HP stayed the same but at higher RPM. I shortened the pipe again, and torque fell even more, and HP stayed the same, but at an even higher RPM. Clearly, this was going nowhere. I needed to keep torque around 70 ft-lbs and do that at 11500 RPM if I wanted to hit my target of 150Hp.

I researched TZ pipes and got dimensions on several model year pipes. I decided to stay with the more conservative 123mm pipe instead of the one-year-only 130+mm pipe. We added a bit of length to take the edge off of RPMs, and basically, I am running a 1994 TZ250-spec pipe on my RG.

Here is a PDF of Darcy pipe spec, vs TZ pipes and the pipe Mark Dent helped me design (it is very close to the final design)
Delta pipe designs.pdf

In retrospect, I think I have more work to do on the intake side of the engine, than the exhaust side- but that's a topic for another page.