10 Things You Always Wanted to Know About Engines

This article is copied from the September 1999 issue of Hot Rod Magazine

By Ray T. Bohacz

    A high compression ratio is synonymous with power. Common sense tends to dictate that the more air/fuel ratio is squeezed, the more powerful the burn will be. With this in mind, it would appear that a high compression ratio is required to produce race levels of power—until you investigate a 9.0:1 Winston Cup engine with 700-plus hp. How can a compression ratio that is considered low by today’s passenger car standards result in such prodigious output? Actually, the compression ratio is only part of the recipe.

    We must identify compression as dynamic or static. Dynamic compression is the cylinder pressure produced when the piston reaches TDC and the bore is filled with the incoming charge. Static compression is the difference in combined cylinder and combustion-chamber area with the piston at BDC and TDC. According to the first and second laws of thermodynamics, an increase in the static compression ratio positively impacts fuel efficiency more than it does specific output, providing that all other factors of the engine remain constant. If the static compression ratio is changed from 8.0:1 to 11.0:1, the gain in power will be approximately 5.2 percent, but fuel efficiency will increase by 20 percent. This is the reasoning behind the diesel’s stinginess; it operates with a 25:1 compression ratio, so it is very efficient thermodynamically. By nature of the combustion event, the process is very slow and does not allow high rpm, which in turn limits horsepower potential but not torque potential.

    Decades ago, General Motors confirmed that power increases in a gasoline engine are possible until the static rating reaches 17.0:1, assuming detonation is not present. Beyond that, the power curve begins to deteriorate because detonation becomes the limiting factor. The reason the Winston Cup motor makes high power with low static compression is because it has a high dynamic compression ratio, which is the byproduct of very high cylinder fill rates (identified as volumetric efficiency). Even thought the mathematically calculated change in area is not that great, the cylinder is full with charge, creating higher cylinder pressures during combustion. We’re not saying that compression ratio doesn’t have an impact on performance, but increased ratios usually pay dividends beyond the thermodynamic advantages when camshaft profile is part of the equation and when the rate at which the crankshaft accelerates is considered.