Raising Compression

Very simply, compression is the amount the fuel and air mixture is squeezed in the combustion chamber by the movement of the piston. Compressing the mixture tighter leads to a greater explosive force when ignited by the spark plug.

1. Increase bore size.
2. Decrease combustion chamber size with aftermarket heads or by milling the stock head.
3. Install pistons with larger domes, but not so large that they rise out of the bores at TDC.

Remember to leave enough clearance above the valves and pistons. Checking with clay strips is much better than waiting for a loud, expensive noise when you start the engine. After milling the stock head the combustion chamber is redomed with a doming tool, and the area above the valves is machined with a flycutting tool. Adding new aluminum heads are preferred over milling the stock head since they have greater strength over the combustion chambers, in addition to increased cooling capacity.

Be careful that in your search for compression you do not restrict the flow around the valves. Making the chamber too small will shroud the valves and decrease breathing. Also, high compression can bring on damaging detonation (pre-ignition). Unless you like to replace head gaskets, pistons, etc. on a regular basis this is to be avoided! Aluminum heads run cooler then iron, so they do help avoid detonation. Obviously, the fuel you run will help determine your compression ratio. If you want to have a car that needs 110 octane CAM2 gasoline to stay alive, that is your choice.

Even as far back as the 1950s, racers were realizing that running pistons that “pop-up” out of the bore (are higher than the deck height at TDC) was not making the power they expected. From what I can discover, the best way to do it is to run combustion chambers that are small enough in volume to give good compression but shaped correctly so that the valves still can flow well. Each engine builder and head maker has his/her own ideas on the best way to do this.

Aftermarket heads usually have two compression ratios advertised (often stamped right on the head). For example, a head marked “8.25 8.75” will give 8.25:1 compression on a stock Ford bore, and 8.75:1 on a stock Mercury bore. Remember that changing the bore and stroke will change the compression ratio, so do your homework before buying or milling heads.

Recommended Compression Ratios
(on 1952's gasoline)
Compression RatioDescription
7.75 to 8.25 Use in coupes, sedans, trucks with premium gas
8.75 Use in light coupes, sedans on premium gas
9.25 to 10Use in roadsters, light coupes and sedans on fuel


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