I'm sure someone can explain it better, but as far as I'm aware it's all to do with the pulses being reflected back into the ports from the manifold - the step tends to moderate this effect by acting as a one way valve of sorts - matching it would allow the reflections back into the head and disrupt the cylinder filling effect of the outgoing gas on overlap.
Ok - but enlarging the port will allow the gases to escape quicker. I guess the is a balance point here were you compromise on both to get the best of both worlds.
i think its called reversion, but i might have made that up the last time it was discussed. but yeah, what gary said
The flow down the exhaust acts as if to draw out the gases itself, a sort of gas version of siphoning, this is why long cars have more efficient exhaust system. If you let the exhaust gas turn back on itself, which it tries to do because of the pulse from the valves, youll get semi static pockets of gas. As gary said the step helps stop the gases turning back in. Everywhere ive ever read about it, its a definate tuning faux pas tp match the exhaust ports.
As already stated the step is there as an anit-reversion device, it should help the engines torque when off-cam and on light throttle.
I have a spreadsheet here which can calculate the ideal exhaust and inlet port size from the valve size. It also gives seat dimensions, throat size, pocket size etc.etc.
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