I here a lot of people talking only about CFM. There is so much more to a head than the flow #s. Correct cross section and design. Then CFM.
And by the way I/Ex flow ratios are crap. Ex area vs HP and rpm and desirable Valve timing. Most of the good stuff on the exhaust side will happen when cyl pressures are so high the the air speed will be at the local speed of sound witch will react to area ,not flow #s. And as long as you can decrease the air speed to less than the local speed of sound by the time the piston is at BDC, there should not be much pumping losses. JMO
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