Tuned Pipes

All modern two stroke engines use a tuned exhaust system. The exhaust system is a very integral and important part of the whole performance parameter of the engine. The exhaust shape, length and other factors concentrate on two performance parameters, engine power and power band. Engine power is the highest horsepower the engine will deliver, the power band is the RPM range that the engine will deliver usable horsepower. One of the characteristics of the two stroke engine (as with almost any engine) is that it will not deliver it's best available horsepower over a very wide power band. A designer can design the exhaust system to deliver useable power over a very wide power band but not have lots of power anywhere, or he can design the system to deliver lots of horsepower over a very narrow power band, he simply cannot do both.

If for instance, you are building a racing engine where lots of horsepower is desired, you design the whole machine to use all this power in a very narrow power band. However, in an aircraft, we need usable power over a moderately wide power band. We need a good bit of power for full throttle operations such as take off but we only need it for a short while. Most of our use is at a more moderate RPM such as cruise where we spend lots of time. The exhaust system provided with the Rotax engine that we use on the Challenger is a very good compromise between the power we need and the RPM range we need power in.

Most any two stroke engine can be 'hopped up' to deliver more power and the exhaust system is one of the easiest and most effective ways to do it. There are after market exhaust systems out there advertised for our aircraft and some actually do increase the power (some do not) but always at the sacrifice of intermediate power which is what we use the most. You will get the most satisfactory use and long life out of your engine with the exhaust system provided with the engine so resist the temptation for more power and use it as it comes out of the box.

Author:   George Hurt