educate me. If XXX head flows 300 cfm and an intake flows 270 cfm doesn't the head flow drop to 270cfm when they are bolted together?

 

no. in that scenario, you have entered a state of diminishing returns. in essence, that intake would not allow the heads to perform at their optimum level, but i do beleive that a 280 cfm head would flow less than a 300 cfm head bolted to a 270 cfm intake, even at full lift. although the gains would be minimalistic. im not sure how the physics or science works behind it but here is my guess:
you have an intake that is shown to flow 270 cfm. this is a rating, BASED ON A MEASUREMENT TAKEN AT 28 inches of mercury. that rating is a measurement of how much vaccum was applied to the intake. the actual vacuum level the head/intake is subjected to on the engine is dependent on how large the motor is, as the vaccum is created when the piston is movig down the cylinder, creating a vaccum in the cylinder that the heads and intake try to equalize out by allowing air/fuel in. the amount of air it took in at that vaccum level was 270 cfm. more accurately, i beleive that this is a rating of how easily it flows air. therefore, if you bolt a head to an intake that flows less than the head, it will be harder for the head to "breathe" to its potential.
now,
the reason you have 300 cfm heads( I.E, trick flows or AFR's) used with a 280 cfm intake (I.E. touched up fast 90) is because of the way the head works in contrast with the position of the valve. with the valve fully open, it may flow 300 cfm, but at lower lifts, say the other 90% of the time when the valve is at anything but maximum lift(dictated by the camshaft), the idea is that a head that flows 300 cfm will flow more air at those lower lifts (when the valve is on the opening or closing portion of the cycle) and therefore allow the engine to pull in more air at all points of the camshaft/intake valve opening and closing events. make sense?


if someone can correct me, please do!
Nine-Eight

 

There is a good discussion about this in Patricks recipe for 500 rwhp thread.

 

Excellent reply nine-eight. Typically, a plastic intake will knock down the flow of a good cylinder head, but that does not mean it stops flowing at some magical number (like 280 cfm). It just means that once you approach that point of diminishing returns, head flow gains will be harder and harder to achieve past that number. This is where choosing a head with better flow at .300,.400, and .500" pays big dividends in power.

A 330 cfm head may get knocked down to 300 cfm by a plastic intake, but it should still make more power than a 290 cfm head that gets knocked down to 275 cfm by the same intake. Notice, you can still get more airflow than 280cfm, it's just that the gains in flow start to diminish after a certain point. This number varies by manifold. The stock LS1 has the lowest flow and a properly constructed sheet metal intake would have virtually no restriction to the head. Ported FAST intakes and ported single planes fall somewhere in between.

 

Patrick would a single plane be a victor jr or a gm carb intake? and can does be port matched to your heads?how much more power does the fast make over victor jr if any.

 

A single plane is both the GMPP manifold and a Victor. When they are properly ported and have a high flow throttle body, they will make more power than a ported FAST. You usually won't seem them pull ahead of the FAST intake until above 6000 rpm, but if you regularly run in the 7000+ rpm range, then they're worth a look. If you're the typical 6600 rpm and under street motor, then the FAST 90 makes more average power under the curve.