Originally Posted by JjCoDeX75
Originally Posted by rapidcossie
I understand that cylinder filling changes due to different condtions, revs being one.
people think tuning an engine is easy
500 bhp= t4 turbo and bd cams...if only it was that easy
Glad to see that we are exposing exactly what goes on when tuning an engine! I think with better understanding of what goes on in an engine, mistakes and bad engines can be avoided!
Actually mate, its more likely to do the opposite, forums can be bad at empowering people to the point they *think* they know enough to build their own engine, then find out the hard way they didnt
Originally Posted by JjCoDeX75
Back to low compression a mo (sorry!). I'm not sure I understand what about the lower compression allows for greater advance. Does it relate to the heat generated in the combustion?
It kind of relates to the heat yes, if you try and ram 2 bar of boost into a high comp engine you end up with massive amounts of ignition retard (or tiny advance if you look at it that way) to avoid DET.
This becomes an issue more if the car is used hard for long periods, which is why you see phil and mike disagreeing on it, as they use their cars very differently, you can "get away with" more if the car isnt used at WOT for long
The basic effect of this retard (the timing, not phil) is to make the point at which peak cylinder pressure are greatest move later in the stroke.
This has several effects.
Firstly, it potentially loses you power if you go too high comp, as the point you want to be pushing hardest at you are still only halfway through the burn and secondly and more dramatically it results in a drastic rise in EGTs as some of the energy that would have been used to push the piston down is instead released so late that it instead just heats the gasses up, the third noteable effect therefore of course is another loss in power from that happening.
People always says "higher comp means more power from the same boost" but thats actually a sweeping generalisation that doesnt always hold true, and almost never holds true if you are talking big boost engines and looking at the figures near peak torque, so while it may increase power at the top end, it will often lose you torque even if the boost stays the same, something people's limited understanding doesnt allow for them grasping.
You will notice that I seem to have contradicted myself there, I just said that going high comp the negative effect is losing you power, but then I just stated that at the top end it can gain you power, the reason for this is that at the top end you have such massive advance figures that the peak cylinder pressures can actually fail to be as high on a low comp engine as they are on a high comp engine whilst the high comp still manages (at that point) to make them at a decent point in the cycle.
So again, its a case of compromise, generally speaking for a big boost engine, lower comp works better in the midrange, and higher comp works better at the top end.
But obviously "lower"and "higher" are relative terms