Corrected CompressionWhen the mechanical compression ratio is determined, it is assumed that the intake valve is closed when the piston reaches BDC at the beginning of the compression stroke. If this were the case, the total cylinder volume would be compressed on the compression stroke. In reality, however, intake valve closing is delayed beyond BDC, and this results in the piston being part way up on the compression stroke when the valve closes. Therefore, less than a complete cylinder volume is compressed, so the actual compression ratio is correspondingly less. Calculating compression based on intake valve closing is called corrected compression. Corrected compression is a more realistic way for determining an engine's compression ratio.
A dished or reverse-dome piston machined as a sphere results in the least amount of surfac
Since cam timing has a major influence on compression, cam events and compression should be coordinated when designing an engine. An engine with a long-duration cam will experience a significant loss of low-end torque unless the corrected compression is matched to the cam timing, specifically intake valve closing. Many street engines are deliberately built with a late-closing intake valve to bleed off cylinder pressure at low rpm to stave off detonation. However, that hurts low-end torque. Therefore, it is important to coordinate gasoline octane, mechanical compression ratio, and cam timing when designing an engine. All three variables should be determined as a coordinated combination instead of three separate variables.
For a well-running V-Twin engine, ideally the corrected compression ratio should be no less than 9.0:1. Experience has shown that with an optimized combustion chamber, a Twin Cam or Evo Big Twin engine running on 92-octane gas can support between 9.0:1 and 9.5:1 corrected compression ratio (calculated at 0.053 in. tappet lift) before encountering detonation. Therefore, to maximize performance with pump gas, it is important to design an engine having about a 9.2:1 corrected compression. However, keep in mind that many factors, such as ambient temperature, barometric pressure, altitude, humidity, dual spark plugs, gearing, and total bike weight can affect an engine's actual detonation limit. With dual spark plugs and optimum conditions, the limit can be as high as 9.5:1. But overall, 9.2:1 corrected compression is a reasonable guideline to use.
The most important points to take away from this discussion are not a specific corrected compression ratio to use, but the concept of corrected compression and a general range to shoot for when using pump gas. The mathematical formula for calculating corrected compression is somewhat complex and beyond the scope of this discussion. However, the easiest way to calculate corrected compression is to use the "Accelerator for Windows" engine simulator program or a program specially written for calculating corrected compression.