In this month's Power Principles installment, we conclude our series on engine building basics. In the Feb. '12 issue of MM&F, we stressed the importance of developing a clear, underlying goal as far as your engine needs are concerned. The second issue dealt with block basics and starting with the correct foundation for whatever engine project you have planned. In this issue, we will talk about what is probably the most important aspects of an engine build: the top end. The "top end" usually refers to everything above the short block, i.e. the heads, cam, and intake--basically everything that's needed to make horsepower. Our goal here is to offer some information to help you make better, more informed choices when it comes to picking the top end components. Let's get down to business.
The simple fact is that you really can't put a finger on which engine part is the "key" to making horsepower. You need a strong short-block to contain the horsepower. You need the correct camshaft to put the horsepower range where you need it to be. You need the intake to bring the air/fuel mixture in at the velocity/volume that you need it. Every part in an engine works together.
That being said, it's hard to say that the cylinder head isn't the glue that holds everything together. The ports bring the air/fuel mixture in, the valves allow it into the chamber, and then the chamber provides the venue for a complete combustion. Important role isn't it? When selecting the right head, you will need to keep your eyes on several factors: port volume, flow numbers, valvetrain allowances, header flange bolt patterns, and exhaust port locations.
Most heads are advertised in "sizes," which refers to the intake port volume. If you thumb through your favorite aftermarket parts supplier catalog or magazine advertisements, you'll see SBF heads listed in sizes such as 185cc, 205cc, 225cc, and so on. Those are the intake port volumes. This is a fairly critical variable in sizing up the correct cylinder head for your application. If you pick a head that's too small, you may actually create a bottleneck. If you pick a head that's too large, you could make the engine unfriendly at lower rpm.
An air/fuel charge has its own inertia and a larger volume of air/fuel would be harder to get moving than a smaller volume. In Part 1 of our Power Principles, we talked about how important it is for the engine to fit the application. The '70 Boss 302 Mustang is often an example in this. Here we have a car that's not really a lightweight, but combined with a small engine and enormous (comparatively speaking) 351C 4V heads. In street cruising mode, the engine wasn't really a performer (again, comparatively speaking), due to the fact that the engine needed high rpm to create the velocity necessary to move the larger volume of intake charge. If you fitted the car with a 4.56 gear and kept the engine screaming, then you had something that could take advantage of the larger head volume. So the underlying theme here is to choose the cylinder head for the application. Lots of guys tend to get overexcited with larger spec numbers, but it can often shoot you in the foot. Bigger is not always better.