Originally posted by 250rforlife
ok i understand 3/4ths of it. but the dynamic and static comprestion. i wanna no what each one means.and witch on is the one that makes u have to run race gas or not.
Simple explanation. First off, we're not talking about compression readings, we're talking about the actual calculations.

The compression ratio as you normally see it, like 9.4:1, is the STATIC compression ratio. Let's make up easy numbers. The "size" of the engine is the actually volume the piston moves, let's say it's 360cc. Let's also pretend this engine has a 40cc combustion chamber (the volume of space when the piston is at Top Dead Center, the volume when you're actually burning). The compression ratio is calculated as the TOTAL (360cc+40cc=400cc) divided by the combustion chamber (40cc). This engine would have a 10:1 compression ratio.

However, the valves don't actually close when the piston is at absolute bottom, believe it or not, the air is still moving into the cylinder. The valve actually closes a little later, the piston has already started it's upward travel on the compression stroke. This is the Dynamic Compression. It's not when the piston is at the bottom, but the compression ratio when the valve actually closes.

Let's pretend, in our same 360cc engine, that the piston has started to move up and takes up 20cc of that space before the valve closes. Now we have 380cc (340cc plus the 40cc combustion chamber) divided by our 40cc combustion chamber volume.

380/40=9.5

This is a far cry from our static of 10:1.


As you can see, the static compression is based on the engine design alone. You can change it with pistons that take up more of the combustion chamber, but the dynamic compression actually changes based on the camshaft specifications, because different camshafts close the valves at different times.