Considering The Combo
The Overall Picture
There's just no way to break component selection down on a part-by-part basis without having a good handle on the overall picture. Before buying anything, the goals of the build-up have to be clearly in mind. Will it be a mild, pump-gas cruiser built to rack the miles up while delivering a satisfying punch? A low-buck time bomb built for max power at a minimum outlay (for as long as it holds together)? A cost-is-no-object dragstrip crusher? A big-inch, moderate-rpm torque churner? A gas-sipping econo mill? The possibilities are limited only by your imagination, but the plan has to be locked down to make the right decisions in parts selection.

Defining the goals sets the constraints of the build, and the parts selected that will work best within these constraints. Before moving on, you should have a handle on the engines displacement, a rough idea of the budget, the upper-and-lower working rpm limits, the fuel requirements, and the targeted power output.

Ratio Rationale
The compression ratio should be dialed in to the optimal level for the buildup, but this isn't always the case. If budget is one of the key constraints, sometimes the compression ratio is already pretty much set by the pistons and heads at hand. Ideally, the ratio choice should be set by the fuel requirements of the completed engine. Compression ratio is a key component in both power and efficiency, so it pays to spend the time to get it right.

Compression ratio is directly related to cylinder pressure, and typically more pressure will result in more power. While the ratio is a big part of making pressure, it is actually only half the picture. The other half is the camshaft, since the compressing doesn't begin until the valves are shut. The cam and compression are closely related, and really should be considered together when working out a combination. The rule of thumb is simple: the bigger the cam, the more ratio that can and should be used. There are numerous other factors that also affect the amount of ratio and, ultimately, cylinder pressure the engine will tolerate. These fall into two basic categories: thermal management and combustion efficiency. Basically, these are the details that separate a well scienced-out engine from a regular bolt-together job.

Combustion efficiency includes chamber design, squish/quench, swirl/tumble, ring placement, and plug location. There's no way for the average Joe to evaluate this stuff, but he can pick a head with good quench, a compact chamber, and a deep plug location. Thermal management is easier to picture and has a significant affect on how much ratio you can run. Compressing a gas creates heat, so more ratio means more heat and pressure in the chamber. If we could subtract some of the heat in the gas, some more pressure can be added in. Simple, huh? Steps here include blocking the exhaust heat crossover, adding a cold air induction, and, finally, employing thermal barrier coatings. Coatings are most effective on the valves, particularly the exhaust; other areas to consider are the manifold, chambers, piston tops, and even the ports themselves. A good gauge of the balance of compression ratio and cam timing is in the cranking compression of the assembled engine. A good, strong, street/strip, pump-gas engine will typically have a cranking compression of 180 psi; a highly scienced-out engine may approach 200 psi while still swilling pump brew.

Which do you think will make more power?