I recently bought a 3W 70i with a canister type muffler. The header pipe for my model installation needs to be lengthened by about 6-7 inches.

Can anyone tell me if this will have any adverse effect on engine power.
Will it cause damage to the engine.

Thanks,

Airtourer

It will have influence on the sweet spot rpm location of your engine. A longer header brings it down to lower rpm, so throttle response may be improved. OTOH, it may not, much depending on muffler design.