View Full Version : Exhaust Pipe "Theory"

02-19-2007, 02:12 AM
Exhaust Pipe "Theory"

With all the questions about expansion chambers going on, I thought that I
would pass this on again. Mind you it's out of the January 1997 issue of
DIRT BIKE, but it's the basics on 2 stroke pipes. I've copied it word for
word, so don't blame me for any odd, funny or generally weird remarks!
BTW...for those of you asking about mufflers and diameters, check out the
last paragraph!

Expansion chambers are fascinating. The look cool. They sound great. They
are mysterious. Everybody knows that, somehow, these non-moving parts are
responsible for quite a bit of the mind bending power today's two-strokes
crank out - but hardly anyone knows how they work.

So, for readers with a healthy curiosity about bulbous sheet steel two-stroke
exhaust systems, we present the following pipe primer.

Basically, and we mean basically, expansion chambers are shaped as they are
so that they reflect sound waves back at the exhaust port to hold the
burnable charge in the cylinder. Without the expansion chamber, a large
amount of power producing fuel and air would escape from the exhaust port
because the exhaust port must be open when the fresh fuel/air charge rushes
into the combustion chamber. Four-strokes don't need two stroke type
expansion chambers because they have valves that seal their exhaust ports
during the intake cycle.
Though each section of an expansion chamber has its own areas of influence on
power delivery, it is important to point that no section of an expansion
chamber works entirely independently. Of the others. Any change in length,
shape or volume in any part of the pipe will bring about changes in the way
the pipe affects performance. Generally, changes that hurt performance in
one area will boost performance in another, but it is possible to make
changes that only hurt or help performance. Skilled pipe manufactures seek
out changes that are most beneficial through time-consuming track and dyno

Perhaps you have noticed that the first sections of today's expansion
chambers are tapered and that some pipes use straight-tube head pipes.
Tapered head pipes are relatively more difficult and costly to manufacture,
so they are rare on non-race machines. Tapered head pipes have proven to
boost performance and ease pipe tuning in their main area of influence - low
to mid rpm power - has proven to be best for most racing applications.
In general, a relatively longer head pipe will bring about more bottom-end
power at the expense of peak power. A short head pipe generally brings on
stronger peak power and subtracts bottom-end. That's why pipes on trials
bikes have very long head pipes and those on 80cc motocross bikes are short.

The length, volume and taper of the first cone strongly influences the amount
of peak power the engine will produce. A relatively short, steeply tapered
first cone, as on 80cc motocrossers, creates high peak power with sacrifices
at other engine speeds. Pipes on Open-class bikes usually have gradually
tapered first cones because smoothness, rather than peak power, is of more

A pipe's midsection is where length or volume adjustments are make to
compensate for less than "ideal" head pipe, first cone, final cone and
stinger/silencer dimensions that can't be used due to the size and shape of
the bike. The pipe's midsection or "belly" can be enlarged, shortened or
lengthened to bring about the same results as most "ideal" designs.

Anyone who rides will tell you that what happens after and engine's power
peaks is nearly as important as the peak itself. Controlling power after the
peak, the overrev or overrun is the final cone's job. A relatively longer,
gently tapered final cone will give you more overrev. A short, steep final
cone gives you less. Why not go for lots of overrev? You will lose too much
top-end. Getting the picture? It's pretty much all give and take.

The tailpipe, or stinger, is as important as any part of the pipe. Its size
and length influences peak power and bottom-end, and can even affect an
engine's resistance to holing pistons.
In general, smaller stinger diameters create more peak horsepower but
increase the likelihood of melted pistons because they bottle up the exhaust
heat. Big stinger diameters boost bottom-end at the expense of peak power.
Excessively large stinger diameters can hurt performance at all engine
speeds due to insufficient back pressure.
Stingers length is important, too, because it's part of the total pipe length
and volume. Generally, longer stingers help low and midrange power. Why not
run a long, large diameter stinger? The pipe has tofit on the bike!!!


Tuned Pipes

What's with all the fuss?

Unlike four stroke engines, in which intake and exhaust valves retain fuel in the combustion chamber, a two strok engine depends on the header and tuned pipe to retain fuel in the combustion chamber.

It has been said that the single most performance gain that one can achieve on a two stroke is made by strapping on a tuned pipe. This is very true if it is done properly. Don't just go down to your hobby shop and purchase a pipe marked ".15 Tuned Pipe" or ".21 Tuned Pipe". It's not that easy unless of course the pipe happens to be manufactured by the same company that made your engine. Which still doesn't guarantee that you will achieve optimum performance for your application. What are the sections of a tuned pipe called? What does each section of the pipe do? What are negative and positive sound waves? What won't a pipe do? These are some of the questions we'll try to answer here.

Sections of a Tuned Pipe

Header - Although not part of the tuned pipe, the header plays an important role in the overall tuning of your engine. The header attaches to the engine and is the straight or slightly divergent (opens up 2-3 degrees) section of the pipe. It helps to suck the exhaust gases out of the engine. The header pipe cross-sectional area should be 10-15% greater than the exhaust port window for when maximum output at maximum RPM's is desired. In some cases the area of the header pipe may have a cross-sectional area 150% of the exhaust port area. The length should be 6-8 of its diameters for maximum horsepower, for a broader power curve 11 times pipe diameter may be used. This is the part you trim lentght to tune the header.

Divergent (Diffuser) Cone - The section of the pipe that attaches to the header and opens up at an angle like a megaphone. It intensifies and lengthens the returning sound waves thus broadening the power curve. The steeper the angle the more intense the negative wave returns, but also the shorter the duration. The lesser the angle, of course, returns a less intense wave, but for a longer period of time (duration). The outlet area should be 6.25 times the inlet area. 7-10 degree taper angle.

Belly - Located between the divergent and convergent cones, it's length determines the relative timing of the negative and positive waves. The shorter the belly the shorter the distance positive waves travel and the narrower the RPM range. This is good for operating at HIGH RPM only. The longer the belly the broader the RPM range. The diameter of the belly has little or no effect.

Convergent (Baffle) Cone - Located after the belly and before the stinger, reflects the positive waves back to the open exhaust port and forces the fresh fuel mixture back into the combustion chamber as the exhaust port closes. The steeper the angle the more intense the positive wave and the gentler the angle the less intense. 14-20 degree taper angle. The taper angle primarily influences the shape of the power curve past the point at which maximum power is obtained.

Stinger - Located at the opposite end of the pipe from the header and after the convergent cone, it is the "pressure relief valve" of the pipe where the exhaust gasses eventually leave the pipe. The back pressure in the pipe is caused by the size (diameter) or length of the stinger. A smaller stinger causes more back pressure and thus a denser medium for the sound waves to travel in. Sound waves love denser mediums and thus travel better. A draw back to a small stinger is heat build up in the pipe and engine. DO NOT USE TOO SMALL A STINGER! The stinger diameter should be .58-.62 times that of the header pipe and a length equal to 12 of it's own diameters.

When your engine fires it detonates the fuel mixture in the combustion chamber, pushes the piston down, opens the exhaust port and allows the burnt gases to escape along with the sound wave produced when the engine fired. The negative sound waves pull the exhaust gasses out of the exhaust port. The positive sound waves, reflected back from the convergent (baffle) cone, force the fresh fuel mixture back into the combustion chamber through the exhaust port thus super-charging your engine.

Here are some formulas from Gordon Jennings' "2-stroke Tuners Handbook" (1973)

***See Pic At Bottom of Page

When using the formulas above for designing or calculating what parameters the pipe to buy should have, the first step is to calculate D1. When you calculate D1 with the D1 formula above, remember that the number is in sq-in and must be converted to the diameter of the header pipe. Do this by dividing your calculation by Pi and then taking the square root. This will give you the radius of the header. Just multiply it by 2 to get the diameter. What you are doing is working the formula for the area of a circle backwards (Area of a circle = Pi r^2). From this point on, no other conversion should be necessary unless you use metric (Multiply numerator of LT formula by 83.3 and use 518.16 m/s for VS to get mm instead of inches) instead of English.

To compare, here's another set of formulas from Martin Hepperle (1997):

***See Pic At Bottom of Page

If you use both formulas, you'll see a slight difference, this may be due to new research between the dates of publishing (1973 Vs. 1997).

Selecting a Tuned Pipe

An ideal tuned pipe is thought to have a gently divergent header pipe to keep exhaust gases at a high velocity near the exhaust port opening, then a second medium diverging cone and a third high diverging cone attached to the belly. In reality it is what works for you. So how do you determine all these things? One at a time. Let's look at setting up an engine for course racing.

What do we want?

1) Quick acceleration

2) Broad RPM range

3) Broad to lower power range

This means we are probably not going to turn the maximum RPM's that the engine is capable of anywhere on the course. If our engine is capable of turning 25,000 RPM's, we will probably only use up to 20,000 RPM's. Look at each section of the pipe in the above descriptions. The Header cross-sectional area should be at least 10-15% greater than the area of the exhaust port. Length at this point doesn't really matter (at least 8 diameters), but make sure it is long enough to work with. The divergent cone would be at a medium angle for a broad power curve at lower RPM's. The belly would be medium to long for a broad RPM range. The convergent cone would be at a gentle angle because we want the duration of the positive wave to be longer.

How long is the pipe? If we review the formulas and get the formulas for Exhaust Systems Tuned Length and Length of Curved Pipe (if you need to calculate a curved pipe) we can calculate closely the pipe length. The formula for determining the length is:

Lt = (Eo x Vs) / N English OR (83.3(Eo x Vs)) / N Metric


Lt = tuned pipe length, in inches OR millimeters

Eo = exhaust open period, in degrees

Vs = wave speed (1700 ft/sec OR 518.16 Meters/sec at sea level)

N = crankshaft speed, in RPM

Let's say, for example, we have an engine that will turn 25,000 RPM. We calculate that we will only use 20,000 of those RPM's and our exhaust duration is 180 degrees. Then we substitute in the formula:

Lt = (180 x 1700) / 20,000 OR (83.3(180 x518.16))/ 20000

Lt = 15.3 inches OR 388.46 mm

Now this is where you need to make a personal decision. Some people say that this distance is measured from the exhaust port opening and some say that the distance is from the center of the cylinder. The choice is yours, but I take the longer distance, which is from the exhaust port opening. Remember that this is not the total length of the pipe. This is the length from the (in my choice) face of the piston at the exhaust port to the center of the convergent cone including the invisible intersection of the convergent points not just what you see. Go back and reviews the formulas for the Baffle Cones to determine this point.

Tuning that Pipe

Now comes the fun part! We get to go to the track again, unless of course we have our very own dyno. Not! So we have set the pipe up so that we have an optimum length. Well take it off! That's right, take it off. First we want to get the right prop, right fuel and right needle before we even mess with that pipe. You see this is where the "What a pipe can't do?" comes in. A pipe cannot make up for poor engine setups and crappy gear ratios. A pipe also cannot make up for bad engine timing and some engines are timed so poorly that no pipe will increase performance.

Ok, we make a few (2-3) passes without the pipe. We have the right gear ratio, the right needle setting and this is the fuel we are going to be racing with. Put the pipe back on richen the needle a little (1/4 turn) and make a run. We pay close attention to what the engine is doing. If the engine turns slower, something is wrong. If the mixture is correct the pipe is too long. Shorten it by 1/8" at a time until the revs start to rise (this can be done at the exhaust coupler). If the pipe is too short the motor will run harshly and the needle setting will be unstable and critical. Add 1/8" to the length at a time (again, at the coupler). When the pipe is at the proper length you will experience the thrill of a lifetime. You will hear the engine and pipe become one in resonance. You will see your car accelerate like you walked behind it and gave it a kick in the rear. This as known as being "on the pipe".

02-19-2007, 12:21 PM
thanks Rustytinhorn for copying that.. it was useful... and was easy to understand... i'm sure alot of people will find this useful, including myself....

02-19-2007, 02:05 PM
I read it at 3:00 am this morning, went out into the storage room, and was able to get a pretty good idea what power range my Tecate pipe of unknown origin was designed for.


02-19-2007, 11:54 PM
My god, how long did that take to copy.....

05-06-2008, 08:33 PM
I have a knot on my forehead when i fell asleep and hit my head on the screen. Spec II's Tuned pipe.http://i166.photobucket.com/albums/u104/beets442/6-27-2007-20-1.jpg

05-06-2008, 11:56 PM
great info, thanks!

05-07-2008, 10:10 AM
beets442... that's a NICE Tripple you have there !! have you ever rode the H2 or H1 ?
also ... thats a COOL Artical Rusty !!

05-07-2008, 08:05 PM
Thats sticky material right there !!! awesome find!!!:Bounce :w00t:

05-07-2008, 09:01 PM
I had a blue 72' H2-750 bored, lectrons, Fast by Gast pro pipes. It would do High 10's in the 1/4. The bike in the pic does mid 13's. Only did it a couple times. Just rebuilt it last year, don't want to burn her up!! It's fast for a H1-500. I have more fun riding this, than my wife....

The Goat
12-15-2008, 05:40 PM
Bump bump for those who missed this

12-15-2008, 05:56 PM
I had a blue 72' H2-750 bored, lectrons, Fast by Gast pro pipes. It would do High 10's in the 1/4. The bike in the pic does mid 13's. Only did it a couple times. Just rebuilt it last year, don't want to burn her up!! It's fast for a H1-500. I have more fun riding this, than my wife....

Damn thats quick. I really love the kawi tripples, they are so damn cool. I have a It490, and there's a guy who used to race a stock one at the track, he would run 13's in the 1/4 also.

02-20-2009, 05:51 AM
anybody here ever alter the length of their header pipe?