Slightly strange head flow question.

(Sorry if I repeated something here....)

The port size also has alot to do with power and the given size of an engine. In example, a set of RPM heads on a 318 (Or smaller engine) would be a very large head. You can make great power from a 318 with ported 302 heads. These ported 302 heads may only flow 230 cfm, give or take a few cfm on ethier side. (Ported by who? is the question. Reguardless)
If you put a 360 head on top, you may make more power, but it will be in the top end of the scale.

Velocity!

One of the reasons is port velocity. If the air is moving slowly through the port, the fuel is not atomizing well which creates a poor burn of the fuel. 2, even though the port is larger and will flow more air, the smaller engine can not take advantage of this possible potentail until the rpm's go up to get some velocity going in the big port.

Velocity is key. There is a point that there can be to much velocity, but at what point I do not know. This point also changes with engine size and the head port size, valve size, cam being used, etc......

A large port window will contribute to a lazy sluggish feel on the street at a low RPM. While tihs RPM area is ignored by a drag racer since there making use of a high stall converter and gear toluanch at, a street - street/strip machine will probably not want to do this (Very) high stall and rear gear.

Now if you can get a small port window, like a 302 head on a 318 to flow 300 cfm without createing problems, then performance should go up. (Even more so if low lift doesn't change, but theres allways a trade off.)


This example I gave above also translates to any engine. For a street engine, a certain port size is recomended. Then, for a street strip engine, the size will go up. etc... This size will be determines bythe whole package. And I mean everything. Car weight, intended performance level, cam and rpm's intended wth the build. I mean everything.

This is a key factor that is missed alot. Alot of builds here on the board are decent, but not a well thought out sci. project.

There are some good minds answering this question here today.

Ooo, soorrry, I should say everybody here today. I noticed after I worte it, everyone has great answers and have a great understanding how crap works.