Originally Posted by N61WP
Well, this might be a totally stupid question, like I've never done that before....but what is enough flow? I mean, I know it depends on displacement and compression ratio for what is needed, but I have to assume at some point the curve starts to go the other way once flow passes a certain point and velocity drops. Is this measurable?
Jc
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If you cannot support the additional airflow with appropriate camshaft choice and compression, as well as induction and exhaust, then there is certainly a point when you might have 'too much'.
So putting a fully ported big-valve 2.4 head on a 1.9 block with a Solex and a stock hydraulic camshaft won't gain you much.
But as far as the potential for a drop in velocity, it is more a matter of port size than port flow. If you can gain flow but not lose velocity, then by all means go for it! Porting is easy, but porting
effectively is not always so simple.
I like bigger valves because the curtain area is so high (more exposed valve seat area at low lift). In conjunction with modest porting, the big valve head can actually make a fantastic street head where you have no loss of power or torque...only gains throughout the powerband. Now for racing, you will want even more flow, and at some point you'll need more port volume...as you said dependent on compression ratio and displacement.
It's also dependent on the required powerband. A larger runner can store more of a charge between cylinder firings and improve VE on a smaller displacement engine too, however the trade-off might be a narrower power band requiring special close ratio transmission gear ratios and accurate induction/exhaust tuning for best results. I've had one Opel racing engine pull 112% VE for a 400 rpm powerband thanks to inertia tuning, most street engines would be lucky to break 85% at any point!
Bob