Engine Compression, Compression Ratio

Measuring your compression is done by removing a spark plug (or injector in a diesel), inserting a compression measuring gage, then spinning over but not starting the engine. Because of this, compression can vary as to how fast the engine turns over. You look more for even compression between all cylinders than a certain number on the gage, although a gas engine needs at least 100 pounds of compression per square inch (PSI) to fire.
Most modern cars turning over at a normal starter speed generate 150 PSI or more.

Compression which is too low can result in poor running, high emissions, and bad fuel mileage.
Low compression can be caused by cylinder head problems or bad piston rings (or apex seals in rotarys).

A more accurate test is the leakdown test. This blows compressed air into each cylinder. Not only can the percentage of compression loss be measured, you can determine the cause of the low compression by seeing where the air leaks out. Bad rings will leak into the crankcase and you can hear the air blow out when you remove the oil filler cap. A bad exhaust valve will blow out the exhaust pipe, and a bad intake valve will blow back through the air cleaner.

WHAT IS COMPRESSION RATIO?
The compression ratio of an engine is determined like this: the volume of the combustion chamber with the piston all the way down (bottom dead center) is divided by the volume of the combustion chamber with the piston all the way at the top (top dead center). The lowest compression ratios are around 8 to 1. The highest stock compressions (found on 60's muscle cars) are around 12 to 1. Higher compression makes more horsepower. It can also make you burn high octane, premium gas. Few modern cars exceed 10 to 1 compression ratios.