i found this website because i wanted a quick way to see the maximum amount of hp i could make with a set of heads that flow these numbers: 62, 140, 203, 239, 259, 256. heres the website: http://www.bgsoflex.com/flowcalc1.html
its just that i dont see my hp coming up correct, what should i be typing for Test Pressure for CFM Measurement

 

Whatever those numbers were obtained using.

In general, with flow numbers at 28 in Hg, the MAXIMUM THEORETICAL HP you can get out of the heads will be about 2 times the flow numbers; or in that case, about 500 HP. The exact amount will depend on all sorts of other things; compression, BSFC, material the heads are cast from, cam properties, exhaust, etc. etc. etc.; but that will give you a rough idea of what to shoot for. Don't believe this "2.165 HP/CFM" type of crap, because there's WAY too many variables. Trying to "calculate" it down to that kind of precision is a "measure with micrometer, mark with chalk, cut with axe" situation.

 

This simplified formula is used and promoted by AFR.

.2575 X cfm@28" X number of cylinders = max hp.

To find how much power you can realistly get at a streetable compression ratio, I use the 90% rule. Multiply the AFR airflow/cfm formula result by .9.

So a head that flows 259cfm should be good for 480hp at a usable 9.5-10:1 compression ratio.
This assumes you have a correctly tuned motor with a big cam, good intake, headers etc.
You may exceed this on a very well built motor by employing all the dirty tricks, but, not by much.