mt-engines wrote: ↑Wed Sep 08, 2021 3:26 pm
So what you are saying is static compression ratio is meaningless?
How can you get a dynamic without a static? What if the static ratio is 1.0000000001:1 vs say 18:1 you mean to tell me a simple cam swap will make the two equal in power and brake specifics?
Atmospheric pressure can only do so much
I think compression ratio is such a misnomer. Ultimately, cylinder pressure is all that matters, it's just a number engine builders can reference and have an idea of how it works due to looking at other similar examples. I look at it like setting tire pressure on a road race car. 18psi cold doesn't mean you're going to get where you want hot pressure wise, and ultimately, tire temperature is what matters. It's something you can set, sure, and have an idea of where you're going to end up, but it all relies on so many other factors.
Like I said, look at a modern NA Porsche GT3 engine or the modern Superbikes. 13:1 compression ratios, some still only have port injection, and they run all day on 93 octane. Everything I listed before effects it, and they're not doing it with late IVC timing.
Another example, NASCAR pre-compression rules, builders used static compression ratios in excess of 16:1 in restrictor plate engines. On pit road, and pace laps, engine speeds were low enough so that the cylinder saw the full weight of the atmosphere and they had a propensity to detonate and blow up if the throttle opening was too much. At higher rpm's, when the restrictor was in play, the engine didn't see the full weight of the atmosphere, so the higher static compression ratio was about trying to build back more cylinder pressure, by squeezing it tighter, to make up for having less "atmospheres".
In that case, compression ratio is pretty much meaningless as the dynamic compression ratio, as most understand it from magazines, changes due to the effects of the restrictor (then there is the whole wave dynamics aspect).
Replace restrictor with some other "choke" in the system, and the same thing is happening.
I think dynamic cylinder pressure is a way better way of thinking about it.