The compression ratio for a piston internal combustion engine is computed by measuring the volume of the piston cylinder at it's greatest volume (when the piston is at it's maximum excursion) and comparing it to the lowest volume (which occurs when the piston is at top dead center, TDC).

Typical compression ratios for gasoline engines are in the neighborhood of about 9:1. A higher compression ratio gives more power out of an engine, but requires the use of a fuel with a higher octane rating. This increase in power (both horsepower and torque) causes engine components to wear more quickly and the engine must be kept in better operating condition.

Diesel engines have much higher compression ratios than gasoline engines, usually like 18:1 (sometimes as high as 24:1). The fuel/air mix must be compressed much more tightly in a diesel engine so it will combust on it's own. Diesel engines don't have spark plugs, making for a much simpler engine (They do have glow plugs which are used to preheat the cylinders to make starting easier in cold weather.) Because of this higher compression ratio, diesel engines are much more stout - their cylinder walls and piston heads are thicker and heavier than those in a gasoline engine.

danke, perdedor. Diesel engines are built to tighter tolerances than gasoline engines, but to a sloppier specification.