[LeeT]

Originally Posted by arghx7

Sigh. Dynamic compression ratio. Don't take this the wrong way, but that's really not a metric that engineers use at all in practice. It's just stuff that people talk about on internet forums.

It is the brake mean effective pressure and indicated mean effective pressure, and indicator diagrams that are used in practice. Those require cylinder pressure transducers to actually use.

Don't worry, I didn't take it the wrong way. I don't mind technical discussion (bickering or not) :happy0180:.

I was referring to dynamic compression ratio (DCR) because I thought this thread was about boosting the engine. Looking back a few pages, I see that this thread has sort of turned into a discussion of power output and might be the reason behind your post, but I digress.

Solely looking at the static compression ratio (SCR) of 12.5:1 isn't the best way of determining if an engine can avoid detonation under boost. It simply is the ratio of cylinder volume at bottom dead center (sweep volume) divided by top dead center. However, this isn't the compression ratio the engine sees when its running. DCR is the compression ratio when you take into consideration the crank's position when the intake valve closes. The piston will already be traveling upwards when the intake valve closes, resulting in a smaller sweep volume. Because of this, DCR is always less than SCR. Simply put, there is less "squeeze" on the mixture, and therefore less heat, and subsequently less risk of detonation.