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Corrected Compression Ratio

Corrected Compression Ratio
"the most important number you should know when altering or building a motor"

by John Hildreth

Performance advertising tells us that the higher the compression ratio is, the more performance you receive. The compression ratio most people are familiar with should actually be termed “Static Compression.” The static compression ratio is simply the total volume of the cylinder at full stroke when compressed into the volume of the combustion chamber. 𠇌orrected Compression Ratio” on the other hand, is what the motor is experiencing at approximately 2,000 RPM. This RPM range is critical for a wide power band used for things like pulling away from the curb, passing a car on the Interstate, or climbing a hill. Too high of a corrected compression ratio results in detonation. Too low means no performance.

For example: A Twin Cam has a published static compression ratio of 9:1. But when you calculate the corrected compression ratio with a stock “B” cam with intake valve closing time of 24 degrees after bottom dead center, that ratio is actually 8.5:1. The reason the corrected compression is lower is because, at 2,000 RPM, the intake valve is closing after bottom dead center. The later you close the intake value, the lower the corrected compression ratio will be. At low RPM’s, when the piston is coming up from bottom dead center with the intake valve still open, a percentage of the new charge escapes back through the intake valve, thereby lowering the volume of air/fuel being compressed in the combustion chamber.

By installing an aggressive cam with a static ratio at 9:1 whose intake closes at 56 degrees after bottom dead center, the corrected compression ratio declines to 7.6:1. This corrected compression ratio barely has enough compression to light the load, much less do it in an efficient manner. A motorcycle with a low corrected compression ratio like this would likely not even be able to pull 5th gear. Not only that but, in 5th, this bike wouldn’t accelerate, nor could it maintain its speed. In addition, the bike would continue to slow until it began to misfire and experience other running abnormalities.

To correct this problem, raising the static compression ratio is required. You can do this by making the combustion chamber smaller by welding (or purchasing heads with smaller chambers), or by raising the dome on the piston. Either of these remedies will reduce the volume of the combustion chamber and raise the compression. Bumping the compression from 9:1 to 10.5:1 will bring the corrected ratio up to 8.8:1. This is considered to be within the optimal street range and will yield good power from 2,000 RPM up.

So far, we’ve discussed corrected compression ratios at low RPM’s. What happens as the RPM’s go up? The corrected compression ratio continues to change throughout the RPM range. In a well-designed motor, as the RPM’s increase, the corrected compression ratio also continues to rise. At some point, it becomes equal to and (in a well-balanced motor) will surpass the static ratio by a certain percentage. This is called “overstuffing.” Engine builders have been trying to overstuff cylinders since the combustion engine was first designed by using things like cam timing, turbo charging, blowers, nitrous, and ram air to push more air/fuel into the cylinder to produce higher horsepower in all RPM ranges.

While the static compression ratio provides you with a window into the capabilities of your motor, knowing your corrected compression ratio gives you a valuable number allows you to make sure you get all the performance you’ve paid for.

Have a specific performance question or a problem with your current configuration? I’d love talking with you about your performance needs. Contact me via email at jhfxr@aol.com.

© 2004 Hildreth Performance

Last Updated on 06/14/2004