The thing about how new cars w/o a lumpy cam can run higher CR than we can w/sbc carbed engines is mainly because we don't have electronic feed back from sensors to a computer that can retard timing when knock occurs, trim the fuel/air ratio, and change the cam timing just to name a few.
Now that's not to say we cannot use higher than 'normal' compression on a finely tuned engine. But w/only about a 4% (according to prevailing opinion) gain- all else being equal- to be had for each point of CR, I see no reason to run at the ragged edge of detonation. And I don't believe that's what TI is saying to do, either, but I do not presume to speak for him- he can do that just fine w/o my help.
Too much of ANYTHING is not good. Like the medical profession says, "The only difference between a poison and a cure is the dosage." I keep the CR for pump gas engine w/iron heads in the 9.5:1 range and aluminum a point higher, max. BUT- the Vortec (and other) modern fast burn chambers allows more CR w/o detonation than what could be safely used in the older heads. Obviously cam timing matters, as does ignition timing, air/fuel ratio, plug heat range, air inlet temps, engine coolant temps, etc. So there is no ONE right way. ALL the parameters have to be considered.
DCR vs. the real world:
The thing about DCR is (IMO) it DOES try to account for what an engine "sees" in in a running condition. That's the whole reason in calculating DCR in the first place. Now, if DCR used cranking pressure as a datum point, I could see how an argument could be made that it doesn't reflect what a running engine sees, but it doesn't.
DCR is not the end all, do all of engine parameters. But it IS a 'tool in the chest', so to speak. I think the more info the better, at least if that info is able to tell something useful. And DCR has- in the right hands- been shown to do just that.
Vortec heads vs. cam timing:
The following are my opinions.
The L31 Vortec head is probably the most over-cammed head GM has ever produced!
Guys think that because it's said to flow better than the old Bow Tie heads, they can run 240 degrees @ 0.050" duration, no problem. If it were only that easy...
The facts are the heads do not continue to flow well cammed much more than about 0.500" lift and 224 degrees on the intake side. And even there the exhaust is lagging well behind the intake, ratio-wise, and needs help. A split pattern cam will help but the exhaust port actually should be ported to gain the intake to exhaust ratio that's needed. And to use more cam effectively, the head needs help on both sides.
I say to use the Vortec head as-is (with the usual clean-up, valve job/guides, etc, as needed). If more head flow is needed, go to the aftermarket.
Last edited by cobalt327; 12-20-2012 at 09:25 PM.