ORIGINAL: Frost_

Eric, you have misunderstood someone or been badly advised on the following idea, this one at least. Adding a head shim DECREASES compression, which is how detonation is avoided. It works like this: [and bear with me as I do not know what the compressed volume of the avg. 0.15 engine is but that is not important here] Imagine here how this works, Your engine displaces 2.5 c^3(cc), a measure of volume. Now, as the piston reaches top dead center (TDC) that is the point of maximum compression acheived by the piston. Now assume with no shim that once the piston is at TDC there is .03 cc of volume left in the head when compressed. Now you add a shim (or more than one) and the compressed volume goes up from the height of the shim. Now with the shim, the total compressed volume has increased to .035cc from .03cc. As you can see, it takes less force (less compression) to shove 2.5cc into .035cc than it is .03, as there is more volume in the head with a shim. The same thing applies to real cars. When you increase the volume of the combustion chamber in cylinder heads, you lose compression. Different thicknesses of head gaskets can be used to slighty alter the compression ratio. Some people like to run thicker gaskets with nitrous to help curb a bit of the compression, the same goes for other forced induction systems as well. That's it. Increase the volume in the chamber at TDC and you lower the compression.