Why is ther no formula to figure out port volume?

True, port volume is somewhat secondary to port flow. I always look at it like this, if I can get a head to flow what is needed for the engine to do what I want it to then I don't really worry to much about the port volume. But the smaller port volume will make more Tq. and HP in a given range as long as the cam and intake manifold and exhaust is compatiable with the other modifications.

What I've found happens is that if a engine needs a certain amount of port volume and the heads are a bit smaller is that the top end charge won't be there and the MPH will be down some also. But it still will perform nicely, it's just that it could be better. Another thing that I've found is that when you have a small runner head such as the 302/318 heads or the 906/452 type heads for the BB that they will respond very very well to lots of duration at .050 style cams with low centerlines to medium centerlines. Meaning 104 or 106 straight pattern cams. With the centerlines this low the smaller runner heads come alive with at least 9.5:1 or higher compression ratios and loose converters and high numerical gears.