Compression Ratio Effects When Adding a Supercharger or Turbocharger

by Joe Curry

If you are considering adding a supercharger or turbocharger, it is important to consider not only the current compression ratio of the engine but also what it will be when the boost is applied. The effective compression ratio will go up considerably depending on how much boost is applied. This effective ratio will determine timing changes as well as the octane rating to prevent detonation. There are sources of controllers that will vary timing as boost increases in an effort to reduce detonation.

It is a good idea to actually reduce the compression ratio of an engine prior to adding a blower or turbo unit, either by replacing the pistons with dished ones or by adding combustion chamber volume to the head. Also, cam timing is very critical and in this case, having a radical cam is not desirable. Therefore, if one has "hopped-up" his engine, it may be necessary to undo these things before adding boost.

The following chart shows the effect that various amounts of boost will have on the compression ratio. It is obvious that if your car is already pushing 9:1 compression, adding just 8 pounds of boost will cause some extreme effective compression ratios that will need to be addressed if the engine is to obtain any degree of reliability, not to mention the need for extreme measures to increase the fuel octane.

Compression Ratio 2 pounds of boost 4 pounds of boost 6 pounds of boost 8 pounds of boost 10 pounds of boost 12 pounds of boost
7.0:1 8.0:1 8.9:1 9.9:1 10.9:1 11.8:1 12.7:1
7.5:1 8.5:1 9.5:1 10.6:1 11.6:1 12.6:1 13.6:1
8.0:1 9.1:1 10.2:1 11.3:1 12.4:1 13.4:1 14.5:1
8.5:1 9.7:1 10.8:1 12.0:1 13.1:1 14.3:1 15.4:1
9.0:1 10.2:1 11.4:1 12.7:1 13.9:1 15.1:1 16.3:1
9.5:1 10.8:1 12.1:1 13.4:1 14.7:1 16.0:1 17.3:1

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