Please Explain: SCR & DCR vs Timing vs RPM vs Boost

[dingo16123]

As the title indicates, I'm looking for some help understanding the relationship between compression ratio (SCR & DCR), ignition timing, boost, and engine speed.

It's clear that the higher the compression ratios, the lower the timing, in general. The part I get hung up on is the DCR vs ignition timing vs rpm. I've seen more than once a generalized statement about ignition timing in relation to 'cam bleed off', which I interpret as referring directly to DCR. The statement goes something like '...with a big cam, there is more cylinder bleed so could use more timing.' Now, at low RPM I can understand how a low DCR would bleed off cylinder pressure, lower torque output, etc. In this case, the engine can probably use a little more ignition timing. Got it.

Now, at high RPM things start to get fuzzy for me. With a lower DCR (big cam), the IVC closes later, which seems to correlate with a higher RPM peak power. How can cylinder pressure be bleed off at higher RPM, with a lower DCR cam, yet make more power? It seems to me the later IVC is allowing better cylinder filling at higher RPM. Better cylinder filling = higher cylinder pressure, no?

When boost is added to the equation, do same rules apply? I assume it gets slightly more complicated with overlap, SC vs TC etc...?

[Higgs Boson]

a big cam also opens the exhaust earlier

[dingo16123]

Understood. However, I was looking more for the cylinder pressure (detonation) relationships with the above mentioned factors. I know all of the valve events work together, but EVO seems like a secondary factor in regards to cylinder pressure and detonation. Excessive cylinder pressure and detonation are long over by the time EVO comes into play. What am I missing? Thanks in advance!

[Higgs Boson]

think of it more along the lines of the cam isn't what makes power, it just has to do with how much RPM you need to make power.

more RPMs are more power so the power isn't coming from "the cam" but from being able to flow more air at a higher RPM and you need the valves open longer to do that.

at some point a bigger cam isn't going to make more power at the same RPM as a smaller cam, let's call it 6000 RPMs. yes compared to super small stock cams you can just put bigger cams in but at some point the engine is going to have to spin higher to realize the benefit compared to the next smaller size.

cylinder pressure peaks in the mid RPMs (torque) and then get's multiplied by RPMs to make horsepower so no it's not necessarily keeping cylinder pressure up at high RPMs, it's about moving air.