A muffler is not only necessary to keep noise down. A 2 stroke engine like we commonly use with our heli's won't run worth a damn without one. In a 2 stroke engine the fuel/air mix first goes into the crankcase, then up through transfer ports into the combustion chamber. Without a muffler a good portion of it would go right out the exhaust port. The back pressure from the muffler is what stops this from happening. A "tuned pipe" or "tuned muffler" is one that is optimized for a particular operating condition and gives a substantial increase over a standard muffler.

If you don't need the extra power, any good muffler designed for your heli will do. If you are looking for a little extra power, contact whoever you intend to buy from and tell what heli and engine you have and they can recommend one.

Hope this helps,

Mike