1991 Marathon Muffler

[bblover]

Is there any problems with fabricating a custom exhaust for a 1991 3PG? I don't know if there would be a problem with the change in back pressure. If anyone has tried this, comments or suggestions would be appreciated.

[bblover]

Anyone have some advice or comments on this? I am planning on getting things wrapped up on the long weekend. Thanks.

[caddymankc]

Im guessing 3pg is the 2 stroke, its best to stay with the stock pipe(so you dont end up siezing it), unless you have alot of time to put in a exspanision chamber otherwise you might just kill your engine and your horsepower.

[ruredy]

I remember getting this from someone a few years ago but for the life of me I can't remember who.......

Backpressure is most important in older 2 strokes with no exhaust valves. In that case, the 'pulse' from the expansion chamber is acting like an exhaust valve, increasing compression and keeping the fresh charge in the combustion chamber and not in the pipe.
Newer 2 strokes with exhaust valves benefit from this as well, but I think not as much as the old ones, which is why a lot of 'modern' 2 strokes, even high performance ones, use a single pipe, instead of doubles or tripples, which give a better timed 'pulse' for each exhaust cylinder.
4 strokes have valves, and many 4 strokes will run best with open pipes or no backpressure, but other 4 strokes still depend on backpressure, depending on what kind of cam is in it.
Diesels definately depend on backpressure, how else would you spool the turbo?
Thus
"back pressure" as per say is the resistance of flow of spent fuel/air leaving the engine through the exhaust system. Exhaust pressure in a turbo as per say is the result of the resistence to flow caused by the turbo unit. It is not a "thing" that is designed in or desired. "Flow" is what causes a turbo to spin, not pressure. In a standard 4 stroke engine, "back pressure" is totally unwanted as it restricts the engines ability to scavenge the exhaust out of the cylinder, thusly reducing hores power. Tuned lenght headers on a 4 stroke engine aid scavenging due to some simple laws of physics. The gases leaving one cylinder and passing through the collector( the point where all header pipes converge) causes a low pressure to be exerted on the other pipes in the collector, thusly "pulling" the exhaust from those pipes. On a standard V8 engine, 30 to 36 inches of primary tube before the collector is ideal to provive maximum scavenging.

Now a 2 stroke is a whole different kettle of fish. The pressure created in the exhaust system is allowed to dissapate in the larger area of the pipe( the expansion chamber). This causes the same low pressure scavenging effect as a header pipe on a 4 stroke but with a critical diffeence. The pipe radically narrows near the end of the expansion chamber so that as the "wave of exhaust pressure hits it, a pressure pulse or wave is sent in the opposite direction. This is done at a specific point in the pipe to cause the return wave to act as an exhaust valve by stopping the fresh fuel/air charge that enters the intake side of the combustion chamber from being "scavenged" and sucked out the exhaust port. Because of this, the design of expansion pipes is actually critical to engine performance. Too long a delay in creating the bounce back pressure wave and too much new fuel/air escapes the combustion chamber. too short of a pipe will cause the wave to happen too soon and not allow a full charge of fuel/air to enter the combustion chamber.

[turbosundance]

IMO the stock exhaust on a 3PG gives up a lot of power in exchange for a quiet motor and ease of mass production. A tuned pipe setup off a Polaris 250 quad (similar motor) has interested me for years. A 3PG definitely responds to a bigger carb, porting changes and more "tuned" exhaust. When making exhaust changed just ensure that the carb is setup to keep the motor from starving for fuel and going lean. I believe the polaris EC25-3PG makes 20HP at 6500RPM with a ~30MM carb and tuned exhaust. These can by found in Polaris 250 Quads although they are free air cooled motors. A Polaris EC25-2PG can be found in early eighties / late seventies Polaris snowmobiles which makes ~20 HP and is a fan cooled motor