Hi!
Remember that when using a tuned pipe you raise the compression inside the combustion chamber which mean you better lower the amount of nitro in the fuel if you use a "untouched" sport engine like an ASP,OS Magnum etc.
Most engines don't like more than 15% nitro and using a tuned pipe makes the engine "belive" the nitro content is higher...No good!
Use max 5-10% nitro!