Consider the simple concept of port velocity. Flowing 295 cfm through a small 205cc intake runner is not going to be as fast as flowing 295 cfm through a 235cc intake runner.

Additionally, there are differences in port shape that all the porting in the world can't change.

Ben T.

 

I think you got that backwards Ben. If you move a given volume through a smaller port, the velocity will be higher than through a larger port. At least that is the way I understand it.


TO the OP. Head porting is kind of like cam selection. There is a bit of a black art to it. I can't really explain it because I don't know exactly.

Let me put it this way, all of the numbers you could get from a head (flow numbers, port volume, chamber volume, etc) only tell part of the story, the rest is told by the shape of the port, localized velocities, and such. The best suggestion I could make if you want to buy used ported heads, is to find a set that have been running, find out what they made on the car they were on, what setup it had and adjust accordingly.

 

Stang's advise is sound regarding ways to evaluate heads.

Also, yes you're right, and I was wrong.

...and to think I took fluid dynamics. Dr. Gaunthier would be dissapointed. As cross-sectional area increases velocity will decrease.

Ben T.

 

It happens to the best of us(me more than others!!! )