Expansion chambers and the RG500
By Randy Norian


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. I've owned an H2 750, RD350, RD400, 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 recently
made a pipe from scratch (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)

=09pipe temp 

=09back pressure

=09cone angles

=09multistage cones/ stalling

=09crankcase volume

=09transfer port angles

=09body waves

=09internal stingers

=09

=09



Yet more things non-pipe things that affect pipe/engine
performance

=09compression ratio

=09Ignition timing=09

=09blowdown

=09disc timing

=09Xo-Io crossover



Designing 2 stroke pipes (programs, websites, formulae)



widening the powerband

=09exhaust valves

=09adjustable pipes

=09ignition timing

=09exhaust throttling

=09water injection

=09



--------------RG500 case study-----------------



RG500 stock exhaust system (dimensions, analysis)

improving stock pipes



Other pipes I've tried

Wolf

Swarbrick

Power Pro

Nikon



Designing the best RG500 pipe Ever Known To Man 

(and why it won't fit on your bike)



dyno results and pipe experiments



Okay, here we go!



How expansion chambers work (basic theory)



=09I'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: 

=09







=09Waves 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. 



=09Back 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. 



=09A trip thru the exhaust/intake cycle=09

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)



=09Once 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.



=09Determining 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 =3D Eo x Vs / N



Lt in inches

Eo is exhaust open duration in degrees

Vs =3D speed of sound in fps (Jennings uses 1700 fps as an average)

N =3D engine speed in RPM



for Metric fans, Robinson uses Lt =3D Eo x 42525/ RPM , where Lt =3D
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. 



=09Wave 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. 



=09Body 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. 



=09Exhaust 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. 

 

=09Backpressure

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. 



=09Cone 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.   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. 



=09Crankcase 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.



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 RG500 has a case CR of
about 1.47 to 1. 



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.
 



=09Internal 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



=09A 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) 

=09

=09Compression 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 recently wrote an article on
this topic (month, journal) 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.



=09Ignition 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. 



=09Blowdown

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. 



=09Disc 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!



Designing Two Stroke Exhaust Systems



=09There 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-



(pipe prog links here)



SAE paper on pipe testing



randy's spreadsheet



Mota address and website



TSR address and website



links to small pipe design programs



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, but that's not something
I have much experience with. 



he stock 

=1A

--part0_900566646_boundary--

Rob Koopman ( Rob.Koopman@inter.NL.net ) ^{RG500 Index}