Here's a definition from Motorcycles Forum moderator, Bill Wood: Think of Compression Ratio this way: When the piston is at the bottom of the cylinder, say you could pour 100cc of water into the sparkplug hole (both valves closed) and it would be full. When the piston is at the top of its stroke, you can only pour 10cc of water into the hole to fill it. The compression ratio would be 100 to 10 or 10 to 1. That's about as easy to understand as I can make it. Oops, forgot to say -- Higher compression ratios (in general) will let the engine make more power, require higher octane gas and be harder for the starter to turn over.