Cool,

Most tuned pipe systems really aren't pretuned. They may be close, but you'll still have to do some work.

Start out with about a 1/2-3/4 inch gap at the silicone joint. Start the engine, lean it to peak and tach it. Shorten by 1/8-1/4 inch. Start, lean, and tach again.

Keep doing the cycle until you get no change in RPM. Back the pipe out the amount you just shortened it and clamp it all down.

If you find that you need to trim the header, do it in 1/4 inch increments. You want to wind up with no more than about 1/4-3/8 inch gap in the silicone joiner. Too much gap will cause the joiner to pulse with the engine, and that will mess up the pipe's tuning.

If you change props or fuels, you'll have to re-tune the exhaust system.

Listen carefully to the model in flight, since you'll likely have to adjust the pipe system to account for RPM changes in flight.