static versus dynamic compression ratio

OK guys, help me out here. Can someone explain the difference and the significance of static and dynamic compression ratios? Thanks!

Take a ballon

put a hole in one side and hook up an air pump on the other.

As long as more air is going in than bleeding out, the pressure will stay the same or rise. If less air is going in than is escaping the pressure will get lower, until it stabilizes at a lower pressure. If you vary the air pump pressure based on how fast air escapes, so that it keeps the pressure stable. at some point regardless of the hole, it will stablize.

An engine is basically an air pump. Your cam allows a variable "hole" in the cylinder, at lower rpm more air bleeds off. As the rpm gets higher, the amount of time that air/fuel has to get through the valves is shorter.Which has the effect of making the "hole" smaller. At some point the pressure is enough to overcome the leak of the virtual leak, "hole"...thats where the pressure stablizes and you have your full compression.

Thanks for your response. Is static compression just the mathematical ratio of bore and stroke volume to cylinder chamber volume? From your description, it sounds as if the dynamic compression increases with rpm, where as static compression is just that, static. Is the dynamic compression linear, or does it level off at some point?

static does not include the camshaft timing events. It is simply the volume at TDC as compared to the volume at BDC.

Dynamic includes the effects of the cam and other kinetic effects of the intake and exhaust charge. (It's what the engine sees)
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