Head Flow Ratio Calculator

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Superflow determined a long time ago that the maximum ideal value for head flow ratio is 146 cfm/sq.in.
This number is for an 'ideal' frictionless orifice with no dump or expansion losses.
A good ratio would be 133 cfm/sq.in. for a single intake valve.
137 cfm/sq.in. is good for a 2 intake valve setup.
If your flow ratio is smaller than the 133 cfm/sq.in. then you are losing some power.
Talk to your head porter and see if he can get some better numbers.

(from Darin Morgan)

VALVE / BORE ratio
52-52.5% of the bore for Wedge heads.
53-53.5% for Canted valve, rotated canted vale (Mopar P/S hemi) and true Hemi.
If you decrease intake exhaust ratio down to 70-71% you can move both intake and exhaust
valves over to the exhaust side and increase intake valve size by .5-1%.

If you exceed the ratio in any way, your dead!
Anytime you increase valve size over the above stated Valve/Bore ratio you will see a small
gain in CFM but the discharge coefficient goes down the toilet and takes your power with it.
An over shrouded valve instills a host of nasty scenario's the two worst being
decreased discharge coefficient and increased reversion below and above intake tuned power band.
So, not only will your peak power suffer the width of your power band will as well.