What do you think your heads flow right now? Or better yet, how much do you think your heads need to flow to make the power you want? Well, here's the ugly truth. Right now, your heads probably don't flow enough, but you still might need smaller heads anyway. Confused? I thought so. Let me explain.
On average, a properly set-up engine can make about 2 horsepower for every 1 cfm of dry intake port airflow. However, that does not automatically translate to "bolt on better-flowing heads and make more power." I've tested many different size engines with many different sets of cylinder heads and found that when switching to larger cylinder heads, the torque lost on the low end compared to the power gained at the top end might hurt overall performance. That's because our vehicles are usually heavy and they don't spend that much time running at the rpm where they make peak horsepower. They do, however, spend lots of time at or below peak torque, which is where I've seen great power gains come from running smaller heads. And what do I mean by smaller heads? I'm referring mostly to the intake ports.
A run-of-the-mill stock iron small-block's intake ports might measure around 150cc. A good stock head, such as GM's iron Vortec castings, will measure around 160cc, but can make more power. A well-designed aftermarket head will typically measure from 170-200cc and will be capable of even better gains. They all get bigger from here. There are small-block race heads out there with 280-plus-cc intake ports and bigblocks way over 400cc, but those are way out of our league for real street power.
CFM And CC
It's important to understand the difference between cfm and cc when talking heads. As on carbs, cfm is a measurement of dry airflow in cubic feet per minute, usually recorded at a standard 28 inches (water) vacuum (aka depression). It's important to mention the vacuum, the medium used for testing, and at which point the airflow is measured, because more vacuum or a different medium may alter flow. The industry standard for comparison has been a 28-inch depression of water, not mercury, so that's where you should look when shopping for heads.
This topic actually gets a lot more complicated, involving things like 45ccbarometric pressure, temperature, the specific gravity of the medium used in the tests, and even the test equipment itself. But don't worry about all those things for now. Just try to compare flow figures at equal depressions, and hopefully from the same flow bench, otherwise those results might not help.
The cc measurement is simply a volume measurement expressed in cubic centimeters as measured from a graduated burette. When comparing cylinder heads, it's important to understand these two figures, because one or the other can make or break power.
Think of a long 1-inch-diameter tube. Now take something like a leaf blower and try to push as much air as possible through that long, skinny tube. At the other end of the tube there's not going to be too much air pressure, is there? That's because most of it will get blown around the tube instead of going through it. Now take that tube, shorten it, and make it 2 inches wider. At triple the original width, you'll get a lot more air through it, and you'll feel much more pressure at the other end. Next, make that tube 100 times wider and blow the same amount of air into it. Guess what? You'll find the pressure drops at its other end. Weird, you say? No, because as the tube's cross-section increases, it gets harder to maintain a constant velocity through it. The smaller the tube, the harder it is to force a large volume of air through it. The bigger the tube, the less pressure it'll blow.