Editing Cylinder Head Choice

The runners are the ports in the heads, one intake and one exhaust port for each cylinder on conventional two-valve heads. The intake runner connects the intake manifold to the intake valve and the exhaust runner connects the exhaust valve to the header or exhaust manifold.

There are many ways to compare cylinder heads when doing an engine build, but the two main ones are runner volume and CFM of air flow through the runner at a certain valve lift. This is done on a flow bench where air is pulled through the runners at a 28" depression of water. In other words, more and more vacuum is applied to the runner until there is enough vacuum to pull a 28" high column of water up into a tube (manometer). The volume of air that is passing through the runner is then measured and recorded. You would start with the valve off its seat at 0.100", then test again with the valve off its seat at 0.200" and so forth through the range of valve lift that you're testing.

Comparing heads by flow can get very confusing if you don't set some kind of mean figure to look at on each head. A generally accepted figure is 0.400" valve lift. Any cam you use will pass through 0.400" valve lift twice on a cycle, once when opening and once when closing, whereas if you use some higher figure, the cam you're going to use might not even open the valve that far, so the numbers are meaningless.

In choosing heads for a high performance street motor where rpm's will be limited to somewhere around 6K, I like to use the formula (CID X 0.5 = intake runner volume). For instance, on a 0.030"-overbored 350, cubic inch displacement = 355. So, (355 X 0.5 = 177.5) Now if you were building for torque and lower rpm's you might back that down to 170 for runner choice, depending on flow. If you were building for power through the entire range, you might step up to 180 cc runners, again, depending on the flow figures.

VOLUME VS FLOW. You'll see in the chart I've included that there are some heads with smaller runners that will outflow heads with larger runners. What that tells you is that the heads with the larger runners don't have the VELOCITY that the smaller runners do, so you can't base your choice purely on the runner volume. If the heads don't provide sufficient velocity, then they will not contribute to good cylinder filling while the valve is open. This is demonstrated with the GM 12558060 L31 Vortec heads for instance. The intake runner is only 170 cc's, but it flows 227 CFM @ 0.400" valve lift. Notice the head beside it in the chart, the GM Fast Burn. With a 210 cc intake runner it only flows another 5 CFM over the Vortec at 0.400" valve lift. That tells you that the VELOCITY in the Vortec is much higher at that point and it would make a much better street head on a 355. Of course, the Fast Burn is moving quite a bit more air at higher valve lifts and so would make more power in a bigger motor with a more aggressive cam or a smaller motor that will be revved much higher than 6K and will pass enough air to support the larger runners.

I'm just trying to get you to understand the relationships between heads and provide you with enough information to make an intelligent decision. We see builders all the time on here who think they are going to put together a world beater 355 by using 230 cc heads. Well, it just ain't gonna happen and now you know why. Bigger is not necessarily better in this case.

http://www.purplesagetradingpost.com/sumner/techinfo/heads1.html