Originally Posted by alamostation

The 5 cylinder engine has a displacement of 3000 cc's. Each cylinder is 600 cc's. Compression ratio from what I have read is 22.5:1.

The compression ratio being the difference in volume from the cylinder at bottom dead center to the volume at top dead center, the displacement of the cylinder must be 21.5 times the volume of the combustion chamber.

Divide 600cc by 21.5, the combustion chamber is 27.9cc. I just happened to have a glow plug in the trunk of my car at work tonight. Filled a 25 cc graduated cylinder up to 20 cc's, stuck the glow plug in the water until it reached the threading. It displaced 1.5 cc's.

Using the compression meter plug instead of the glow plug gives us a combustion chamber volume of 29.4. The resulting combustion ratio is 21.4 (600+29.4/29.4). A 5% difference. Theoretically at sea level with an air pressure of 14.7 psi, the engine should have a compression gage reading of 315 (14.7x21.4) psi. Slightly less at higher altitudes.

In practice, carbon buildup reduces the size of the combustion chamber and higher readings can result.

A better way to figure the problem out is to read a previous thread which quoted the shop manual. 350-400 is excellent. 275 to 350 is good. 225 to 275 is acceptable. The key being that there should not be more than 30 psi difference between cylinders. If you are below 240, factor in the 5%.