My take on the guidelines were that DCR's could be a tad higher for aluminum heads, like 8.3 or so. Maybe the DCR's on JW's combos are just a by-product, and not fully maxed out like you would do if you were purposely starting from scratch and trying to optimize DCR. So your 8.0 number might be artificially low. Or maybe he's sly like the fox, and without actually knowing what the DCR is, has it optimized, and the 8.3 is a "chevy" number whereas the 8.0 is a buick number?
Hmmm, I could have sworn I read that over .5" lift and there would be problems. Of course the events are key, but I am not an expert on cam design by any means The block didn't need to be decked, so I must have more piston to deck clearance. Cutting back on my static compression...I dunno about that. Replacing my pistons as well as all these other things... Well, I'm doing an overhaul at this point it seems to me, just to use Aluminum heads. I can't do that. I'm still kinda hearing "don't use these heads". I am merely a competent amatuer engine builder; I follow the book and check diameters and so forth, but some of the finer points escape me. Such as- I run a cam smaller than the Ta 212 right now. I know I have crummy heads, but I had determined that my real compression ratio was more like 9.7:1 I have had a little detonation, just a little. This was with a rather bad distributor and 16* initial advance. With my new distributor and 13* initial (I have no timing tape as of now, so I am unsure of total) advance, I haven't heard any- yes I know that I could still have detonation and not hear it. If the 288-94H will let me get away with even higher compression anyway, why should this question of compression be an issue with the aluminum heads at all, using that cam, even though my deck is factory height? I really feel like there's a missing piece to the puzzle that I am not seeing here.
That might very well be true that you can run it higher. I would guess that he has optimized that number to be a good and safe area while still making excellent power. The question is, how far are you willing to push it? We're talking carburetors, for one thing. Your jetting may vary some with weather conditions. What happens if it is 110 degrees out, and your water temps are getting up there? There's a lot more to it than just that number, and it's better to err on the low side, in my opinion.
Chris, I think the missing piece is the smaller chamber size. Correct me if I'm wrong, but aren't the chambers 64cc, compared to 72cc with your iron heads? You're talking about going from 9.7 to 10.4. The aluminum will make up for some of this, but you will still need to choose a cam that will work well with the compression. Read this article: http://www.davemiranda.com/dmhotrods.htm It's by a Pontiac guy, but it's a good clear explanation of what to consider. some of the numbers won't equate to our engines, but the theory applies.
Here's another one to read: http://members.uia.net/pkelley2/DynamicCR.html This is where you can download the calculator.
Using the DCR calculator, with an Intake center line of say 108 deg, the piston is some 2.9 inches down the cylinder when the valve is at maximum lift. Or another way of saying that is the int valve is at max lift when the piston is 108 crank degrees below top dead center. So max lift has nothing to do with it, unless you have 2.91" of lift.
