Originally posted by f8.@Jul 21 2004, 01:46 AM
this is my own theory, so it is probably wrong. but i am a believer of small ports for higher flow velocities. i suspect the reason mitsubishi reduced the sizes of the port for evo commpared to vr4 is that it was too big to begin with.
we always think the bigger the better but i personally feel a smaller port with higher flow velocities encourage more momentum energy in the intake charge to enable you to cram more air in over a shorter period of time.
so with the evo3 head, just leave it as it is. port and polishing gives you the most gain on old american v8s where their castings are so poor and unfinished anyway. japanese heads are quite well finished and so the gain, if any is minimal. and besides, some roughness on the port walls, if it encourages a more homogenous mix of air and fuel by tumbling, would encourage a higher flame speed and more thorough combustion.
you see, when people do port polishing properly, they use a flow bench to verify the gains. to me, the problem with a flow bench is that it only gives you the results for steady flow. ie you just apply a constant pressure of air through the head for a given valve opening and measure the volume flow.
the air flow in a real engine is hardly steady. the valves are opening and closing all the time. instead of measuring how much air is flowing in by volume, i think a more useful metric is the velocity of the air that goes in. of course this doesn't mean the smaller the ports the better. there is a compromise between flow velocity and volume rate. and i think the manufacturers got it pretty spot on when they designed it. so don't mess with it unless you really know what you are doing.
but if you have already ported and polished it, then of course it is much better hehehe. ;)
just my 2p!
