Originally Posted by Element Tuning
Always worth testing something new but.....advancing the intake cam timing typically reduces the length of time for the fuel and air to burn. By retarding the exhaust cam you lengthen the amount of burn time. This is a good thing as it allows for more complete combustion. This is already a big problem with our motors and results in having to run lots of ignition timing. Earlier intake cam opening and later exhaust cam timing lengthens the burn window. Once you put boost into the mix then you have to be careful not to blow your intake charge out the exhaust valve and this can be tricky. This is typically why you can advance a lot at low boost but not so much at high boost or high rpm.
I think the interrelation between everything needs to be taken into account.
AVCS makes for better VE at low RPM which is agreed upon (at low rpm more duration and overlap generally means more pressure but less airflow). At higher rpm don't you generally get a faster more complete burn? One would think that more advance on the intake stroke (by using AVCS) even at higher RPM would yield more torque as long as the AFR stay in the right range. The problem being that as soon as you use advance, it would change your AFR, and MBT, correct? Phil might be getting a better VE by changing other values than the AVCS at higher RPM. I'd love to see some of Clark's and Phil's maps...
A lot of it might have to do with treading that fine line between blowing out your fuel on the change between exhaust and intake stroke. I think this is what AVCS would help the most by keeping all the other variables the same and then adding a little AVCS here and there lets you maximize your VE.
I really don't know more about my next question...
Is AVCS making a difference by changing intake pressure/compression?
I say this because I have read posts saying it helps low end and can make a larger difference around turbo spool/midrange torque. Would it be correct to say more duration results in higher pressure? which I *think* would be kind of like how having a higher compression motor can spool a turbo faster.
Adding to this...
Before a turbo starts or begins to make boost/flow how much of an intake restriction is it?
I don't want to get to far off topic, especially since I could be making myself look stupid, but a few more questions...
If a turbo is a restriction (pre boost) then could that be one reason why AVCS makes such a noticeable difference?
Would AVCS make less difference on a higher flowing turbo because of the fact that overlap or just increased intake duration increases pressure but reduces flow?
I'm not sure on all of my facts. I'm just trying to learn...
Hopefully my questions are constructive so that we can learn more about AVCS (This is my please don't flame me Post Script)...