Compression equals horsepower.
The reason is simple: The more you squeeze the air and fuel mixture in the combustion chamber, the more power you can create. In theory, forcing air and fuel in as small a combustion chamber as possible (measured in volume) will yield maximum engine compression and horsepower. There are a number of ways to accomplish the task. You can buy cylinder heads with small volume chambers already cast or CNC-machined in. You can use dome pistons to effectively reduce the volume of a large combustion chamber or even choose thinner head gaskets.
Or you can mill the heads.
Milling involves removing material from the cylinder head (or block deck surface where the heads and block meet) to effectively reduce the volume of the combustion chambers. This method, which is also used to correct warped or imperfect deck surfaces, allows you to control the size of the chamber so you can get the volume needed to achieve a desired compression ratio. There are a number of calculators like this one available on the Internet to help you determine how much material needs removed to achieve a desired compression.
Milling allows you to build higher compression using flat top pistons, avoiding the potential detonation problems associated with dome pistons. As with any method of increasing compression, you will have to reduce total ignition timing and possibly use higher octane fuel with milled heads to prevent detonation. Proper tuning will tell you how much timing and octane your engine will need.
Keep in mind, milling changes the relationship between the heads and/or block and the intake manifold. Depending on the amount of material removed, the intake may also require milling to compensate. To help you determine the proper milling specifications for factory heads on popular classic V8s, we’ve included the chart below. When working with aftermarket cylinder heads, it’s best to consult with the manufacturer to determine how much milling can be done. Some aftermarket heads are made with extra deck material to accommodate future milling.
Remember, these specs apply to flat milling and are intended as general guidelines. Before you start milling, consult with a reputable machine shop that specializes in this type of work and follow their recommendations.