u know your engine small and require small piping to make adequate power to move the vehicle. So if u increase the diameter of the piping anywhere after the manifold, u are only changing the power band arrival. U know why? Because small engine can only pump and vent only so much air. So in order to keep the power constant, u need small diameter pipe to retain some back pressure for the next combustion cycle. Yes there are so called rally experts that say back pressure no good. But they are running turbo engines. This they never even thought about. NA engine different. Really u spend RM600-1000 on an exhaust system only to find the power changes between gears and not overall. And changing exhaust system does not give you any power increase unless u do something to the engine. Its always psychological factor