What is compression ratio?

Question

im looking for a new motorcycle.and i keep seeing different compression ratios. what is a compression ratio

Answer 1

higher comp. ratios mean higher performance.

Answer 2

its the ratio of air and fuel mixed together in the cylinder.  A higher compression ratio(which means more air cramed into the cylinder) will allow the engine to burn and distribute fuel more effectively (this is why turbos works so well, because they raise the compression ratio of n engine)

Answer 3

an engine is basically a pump which squeezes in air/fuel mixture and then ignites it so that it expands back and produces mechanical energy.  the compression ratio is basically telling you how much the engine squeezes a certain volume of air that it intakes.  so a bike with a compression ratio of 12:1 means that for every 12 ounce volume of air that is sucked in, the piston squeezes that air to 1 ounce volume.  the more air that is squeezed into the piston the more energy that is produced per engine capacity on the expansion stroke.

why don't they just increase the compression ratio more to make more power?  well, one of the limiting factors in compression ratio is called detonation (this manifests as engine knocking or pinging) where instead of burning in a controlled fashion, the air/fuel mixture explodes, potential damaging the engine.  Also, a higher compression engine tends to have less clearance between the piston at top dead center and the valves fully opened, and running at a high rpm can lead to valve float which can lead to contact between the valves and piston which == bad news.

Answer 4

It is the ratio between the volume of the combustion chamber over the volume of the cylinder. In the old days it was common practice to mill the head ( flathead) to raise the compression ratio. Milling decreased the size of the combustion chamber thus raising the compression ratio.Did the run any faster? Of course not but that didn't matter.