[Crazyrabbit]

Moonmonkey,

Smaller is not a good way to go in the exhaust system. Back pressure is not good for FE. The engine performs useless additional work forcing the exhaust out on the exhaust stroke. Also, the increased combustion cylinder pressure at exhaust valve closing, will cause dilution of the incoming charge and will upset the tuning without a closed loop system. That adds exhaust gas recirculation and lowers the maximum combustion temperature, which lowers the efficiency. So both power and efficiency drop.

On the flip side, increasing the exhaust system size may not neccessarily reduce back-pressure on the engine. The transition from the original flowpath to the new larger one must be handled carefully because a sudden stepchange without a well designed diffusion section, could result in a Vena Contracta, that can cause increased flow resistance. Conical diffusers are typically limited to less than 7 degrees included angle. Swirling flow can stand a little more before flow separation occurs and a smaller angle works better. Gases have mass, and smooth attached flow in a diffusing or expanding situation requires very smooth bends, and expansions free of steps.