As the title says, whats more important in a naturally aspriated and forced induction engine? Intake volume or port velocity?

I was reading in Super Chevy magazine that huge ports arn't always the best, but the article probably was talking about more big block carburated cars, instead of something fuel injected.

I was probably going to get heads and a different intake manifold and run NA for awhile, then build the bottom end and slap a single turbo on it down the line. I was just wondering what to look for in a good set of heads.

Thanks!

 

"Volume" is ambiguous. Where is the extra volume in a comparison between two heads? The bowl? The runner? The difference in the runner taper? It's hard to say.

I think it's alot more complicated when you add fuel to the mix because the port has to have very uniform velocity or the fuel will fall out of suspension. Area changes along the port are less tolerable.

On the actual port, I think it's a combination of things but the "volume" of the runner I feel ranks pretty low in the scheme. You want enough volume to feed the cylinder without restriction for that period of time that the intake valve is open. The higher the rpm, the more velocity through the port. The bigger the cylinder volume, the greater the air requirements.... all things considered equal.
The velocity, as I understand things, is a function of the pressure differentials on either side of the minimum cross section area within the port. So it would seem that this cross section area carries a bit of importance... probably even more than the overall volume since it tells us the max velocity the port can handle.
Why do you see CNC ported heads from Chapman and Weld Tech with very similiar "port volumes" but with different cross section areas?
Weld Tech offers quite a few Hutter (Ron Hutter) program 18* heads. Just to give you an idea let's look at a few that are real similiar in port "volume", within 3 ccs

HUT 921.1
249cc
2.519 in^2 (cross section area)

HUT 921A
246cc
2.637 in^2

HUT 1A
248cc
2.751 in^2

All similar in volume but with very different cross section areas. Vizard lists a few formulas in his book on small block chevy cylinder heads and that should give you some idea of what's going on here.
We want a port with a volume large enough to feed the engine to a given rpm limit without causing too much loss at the throttle... (we should take the plenum volume into account I think). We want the cross section area of adequate size to reach maximum velocity in the port at greatest airflow demand. You can't always say that one velocity is the highest that'll work though... it's been found that some of the high tech race heads out there can actually create power at even higher velocities but that's probably because they have straighter shots into the chamber. Same reason why you can see higher velocities in high port heads without the power drop out like on a factory low-port head.

Just some thoughts which don't account to much in the way of science and numbers but I think that much of this holds true from what I've seen and read over the years.

-Mindgame

 

Coolness, thanks for the info!