http://enginebuildermag.com/2014/05/...ylinder-heads/

 

This is where you may get different opinions. In my understanding, flowbench is static depression. A stable port continues to flow as valve is opened more. Depending on head port shape and valve angle, there can be a velocity limit in the flowpath beore heads choke in flow. It is said the 23 deg sbc is especially velocity limited. Air doesnt like to turn at speed and going over the short side radius into the chamber is difficult to do and keep stable unless you slow it down enough. Slower air on bench will not show bigger flow numbers at those lower mid lift ranges but tend to help the high end lifts.
Now the issue is if cam is only .500, what good is .800 lift numbers? Answer is depressions on running motors exceed flowbenches depressions of 28" h2o. This means air can choke in flow sooner as high depression differential the air speed can increase. So a head that backs up in flow on bench at .800 may show backing up sooner at say .600-.700" on running motor. This seems to be a complex issue that is effected by many things including fuel mixture, manifolds bolted to heads and valve motion from the cam.

So you would think you'd want to pay alittle attention to head flow way above your cams lift number... But ask some builders or head guys, sometimes motor combo still works and makes power even if port looks bad on bench. Again i think thats where cam and manifold effects alter performance of head port to make it work. There was an excellent example and discussion of this at chevelles.com called lessons learned.

Now forced induction cars in high boost absolutely want stable port to high lift ranges. Sacrifice low and mid lift flow numbers for increased stability. Dense air doesnt turn any better than atmospheric air. Infact it should be worse.

 

Ask a Stock Eliminator racer how they can make so much power with unported heads.

 

Great example! Theres so much involved

 

Good article, can't disagree with a single word he says. Pretty basic stuff mostly, alot of just common sense, good for somebody that doesn't know hardly anything to start out with, that will help keep a n00b out of the weeds.