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Originally posted by Dave A
copy and pasted from another thread I posted on about backpressure:

exhaust design is a lot more complicated than people realise. most off the shelf exhausts are more for the look and sound rather than perfomance really.

what some people term as 'backpressure' does exist but its not strictly backpressure as such. Making an exhaust that is well designed isnt easy, to do it properly everything from inlet design, cylinder pressure, valve overlap values need to be taken into account (sometimes refered to as 'pulse tuning' )

the Idea behind 'backpressure' is this: as exhaust gasses exit the combustion cylinder into the exhaust posrt and into the manifold they are at a high speed. once the exhaust valve closes the gasses create a virtual vaccum behind them, this helps to suck the next load of waste gasses out and so on... if the exhaust bore is too big or too free flowing or the primaries of the manifold are too short or long then the amount of pressure pulling the next lot of waste gasses out of that cylinder is reduced therefore the gasses escape slower and the speed of the engine is reduced and the inlet gasses enter slower and so on....

there is a lot more to exhaust design than you might think......

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