To squeeze or not to squeeze

[tscott9330]

Alright, I hve a question. Actually, a few questions.

1. What is the highest compression ratio you can run a set of Iron heads at on pump gas with out detonation?

2. whats the highest compression ratio you can run a set of aftermarket aluminum heads on pump gas?

3. If I am shooting for a compression ratio of around 10:1 (possibly a little higher) could I go with a set of aluminum corvette heads on pump gas?

The reason for question 3, is i just read a book on getting power on a budget and it mentions corvette aluminum heads as a good high compression, and comparitivly low cost alternative to a set of aftermarket aluminum heads, is this true?

Is there anyone out there running this set up? If so how does it work in a daily driven application?

I JUST WANT TO DO MY HOME WORK BEFORE I SPRING FOR A SET OF PRICEY AFTERMARKET HEADS.

Tom Scott

 

[tscott9330]

Another dumb question. what years aluminum corvette heads will fit a 1978 block that wont require some major machine work to get the job done? I have heard talk of a reverse flow cooling system on later model cars. Will this negate the use of such heads on a 78 block? I basically want to know which parts will go where.

Also on a side note I have the opportunity to get a 1995 corvette fuel injection system for around $300 when I say "system" I mean the aluminum heads and the tpi intake that go with them. I dont think they come with a wiring harness but i'm sure i could scrounge one up. will this stuff fit my 78 block?

Tom Scott

 

[MaineShark]

What affects your risk of detonation is your dynamic compression ratio, not your static compression ratio. You could run 12:1 compression, with a cam that has an extremely long duration, and a large chunk of the air would get pushed back up into the manifold, before the valve closed, so the dynamic compression would be low enough to run safely.

But, since you want to keep the valve duration in a fairly reasonable range, there are certain limits. Generally, 9:1 - 9.5:1 is fine for iron heads. Even a bit higher, depending on the cam. And, also generally, 10:1 - 10.5:1 is fine for aluminum heads. Higher or lower, depending on cam, as before.

I'm shooting for right around 10:1 when I do my 406.

As far as year-to-year compatibility, there were some changes in the late 80's. Either '86-87, or '87-88 (I'm not sure which is the changeover). I'm not sure how much of that involves the underside of the heads. The top is a little different, though, since the newer heads were designed for the Vortec intake manifolds. All that will mean is that you'd need a Vortec-compatible manifold.

I don't believe it matters if the heads were for a reverse-flow motor (LT1, LT4), since I think they used the same heads. Reverse flow just means that the heads are cooled before the block, and I don't see how that would matter to the head design. Plus, I can't recall seeing any heads advertized as being specifically for the LTx motors, so I think they use the same heads as the rest of us.

I'm not sure how cheap Corvette aluminum heads are, but you can get decent aftermarket heads for around $700-850 per pair, if you shop around.

As far as fuel injection, I believe the last year for the TPI system was '92. I'm not sure if the systems newer than that will bolt on.

Joe