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Discussion Starter #1
I need some experienced input here guys. I've come up with two different static compression ratio figures for my 400

The first method I used was with several different compression ratio calculators, yielding the higher of the two figures. I input everything necessary to yield the factory stated comp ratio of a 75' 400(my year) I then adjusted the input to reflect the changes to the best of my knowledge. Those changes, from stock are, .010 off the block deck, .030 over bore, and 60 cc combustion chamber edelbrock heads. This method yields about 10.5:1 comp ratio.

The second method was a calculator on the Keith black website that was much more involved, required me to do some research, and gather many more figures. It yeilded a 9.75:1 comp ratio.

I've seen it said that simply switching to Aussie heads with the same cc chambers as the edelbrocks I'm running, would yield the higher comp ratio figure.

If anyone is willing, here are my specs off the top of my head.
75' Ford 400
Keith black oe style replacement pistons, .030 oversized 4.030 total bore.
Stock crank, .010 under
Block decked .010
Regular felpro head gaskets
Edelbrock Cleveland heads, 60cc combustion chambers.
[email protected] 108lsa .544 lift 4 deg ground in, +2 deg installed
 

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What is the deck height? I see .010 off the deck, but that doesn't say what the remaining deck height is.

David
 

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Discussion Starter #3
Sorry, 10.297 stock minus .010=10.287
 

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Sorry, 10.297 stock minus .010=10.287
Nic - is that a measured deck height, or one given in spec's? The reason I ask is the spec number is often far off in actual production, and blocks are not square either. Most blocks square-up with .010, but not always. My last block needed .014 on one deck and .008 on the other to square them lengthwise and and even them to each other so all bores were even deck clearance. Don't bet on anything until you measure. But...

Here's the big whopper for 400s - they had HUGE deck clearance. Those puppies are so far down in the hole they echo. If (and that's a big if) your block started at 10.297 and squared with .010, then here's the numbers:
10.297 - .010 = 10.287 deck height
rod + compression height + 1/2 stroke - deck height = deck clearance
6.580 + 1.63 + 2 - 10.287 = .077 deck clearance

That deserves a round of frowns and head-shaking. :frown: For reasonable det control, you're after .035-.045. Maybe .050 max with the Edel chambers. Also note the Edel chambers are nearly 18CCs smaller than the specially large (78CC) 400 chambers. Yikes.

In any case, that may be the difference you are seeing, as some calculators assume zero deck, 4.100 gasket bore, .040 compressed thickness, etc. Try the KB calculator again with 12cc piston dish and the other numbers above. Hold on to your panties, because it gives a soft number (a bit above 9.6:1) with all that space above the piston. Personally, I'd be tempted deck the crap out of it mill the hell out of the intake for better quench and controlled det, even with about 11:1 comp and use a wider LSA on your cam and more duration if those are the only piston choice. It's that or finding pistons with more compression height (around 1.71) to solve the issue. Maybe something like KB282 Holden pistons and have the eyebrows added and a touch of dish. Yeah, that's custom. I'd start shopping and asking 400 gurus for leads on solutions and piston sources. Who knows? Good luck dude!

David
 

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Discussion Starter #5
Tmeyer has pistons that bring it up to zero deck. But I have to bush the small end of the rod. Which means they're likely Cleveland pistons. I mentioned a piston up grade in another thread, and those are the pistons I was after. As you mentioned, ill likely end up with aroumd 11:1 comp ratio. With which i think I could swing pump gas using the edelbrock heads. Thoughts on that?
 

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You can do 11:1 with better quench (zero deck and Edel heads), picking a cam with lower DCR at low revs, and a properly matched converter to avoid tip-in and lugging det. Bushing rods is no biggie, but I doubt they are C pistons, as their stock compression height is no better. They're probably modified Holden-based pistons I mentioned earlier. If you have a buddy with a lathe, it's simple to put a tiny dish in them to drop the CR a bit if you're concerned with the grade of pump gas in your area.
:tup:
David
 
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