This example can be translated directly to cylinder heads. A head that's too small will not let enough air into the cylinders to make power. On the other hand, a head that has too much port volume will cause the intake air/fuel velocity to drop and not "ram" into the cylinders fast enough for the most efficient filling, which will also cost power. A very famous Pro Stock engine builder once told me, "Give me a set of heads with the smallest intake port cross-section that'll flow all the air I need." What he means by that is he already knows how much airflow he'll need based on his entire engine package. Now all he needs are the right heads to make the number.
Port Size Makes the Difference
I can think of a great example that I saw in action once. It was on a bigblock, but regardless of engine size, the principle still applies. A guy takes his heads to a local inexperienced head porter and lets him go to town on them. They were some older castings, and he really took the grinder to them. He produced a set of heads that he was very proud of and handed them back to his customer. Now, $1,500 later, the guy clamped his freshly ported heads onto a flow bench in another shop to show off the awesome porting job. The numbers read something like this:
|INTAKE PORT: ||410CC |
|LIFT: ||.800 INCH |
|INTAKE FLOW: ||411 CFM |
|EXHAUST FLOW: ||315 CFM |
|I/E FLOW RATIO: ||77 PERCENT |
Not too bad on paper. But then the shop pulls out a flow sheet from similar, new, off-the-shelf CNC'd cylinder head made by one of the bigname manufacturers, and our buddy just about croaked:
|INTAKE PORT: ||355CC |
|LIFT: ||.800 INCH |
|INTAKE FLOW: ||422 CFM |
|EXHAUST FLOW: ||354 CFM |
|I/E FLOW RATIO: ||84 PERCENT |
After his heart had been thoroughly shattered, he looked deep into the issue and just about passed out. You see, not only did the out-of-the-box CNC heads beat his ported castings, but they also did so with a 45ccbarometric smaller intake port. This is a prime example of how the right port size and shape can affect airflow. Needless to say, the guy who owned the heads wasn't too thrilled about this, but regardless, he'd spent a lot of money on them, so he bolted them on, added a big nitrous kit, and drove off a happy customer anyway.
But all this is straying away from where we need to go. Unless your engine is big, like over 500ci, street heads won't need more than about 300 cfm max. And even that's too much for most engines. One of the best combinations I've ever tested used Edelbrock's E-Tec 170cc heads on the infamous Super Chevy 355-cid "Danger Mouse." That little pump-gas Mouse made almost 460 lb-ft of torque and more than 485 hp. And it idled decently, too, but I bet it would be one helluva kick to drive.
As a follow-up test, I bolted on Edelbrock's bigger E-Tec 200cc heads and got my butt handed back to me along with all the torque I'd just lost. Overall, torque fell off by 24 lb-ft. But interestingly, peak horsepower stayed right at 485, although it moved 200 rpm further up in the powerband. That'd be nice if I were planning on racing in a tirelimited class and didn't care so much about torque, but I wanted this to exemplify the best combinations for the street.
That day's testing really taught me a lesson about proper head management. I learned that the E-Tec heads are a great design overall, capable of making good power. But the smaller of the two, the 170cc, was better suited to overall running around, at least on that little 355-inch Mouse.