Highest safe compression ratio?

[formula]

I realize that this is going to have to be vague at best...but what's generally considered the highest safe compression ratio for 87, 89, and 91 octane on an iron block/iron head sbc before getting into knock?

[6'9"Witha69]

Depends on the head. I don't usually worry about 89, but 8.5:1 for 87 and 10:1 for 91. And yes, this is very vague.

[rocketman]

10.1 for safe pump gas.But more compression can be run with pump gas.

[formula]

Depends on the head. I don't usually worry about 89, but 8.5:1 for 87 and 10:1 for 91. And yes, this is very vague.

As I now have a little more time to type up the full story, here goes. A good friend of mine is building up a budget 350 for his 86 camaro. He's got an early-70's block, flat tops, and I think 882 casting heads (300 horse 327?). I haven't figured out what his comp ratio would be, but I believe the heads have either 76 or 78 cc chambers. I mentioned he might want to think about getting them milled just a little to give his compression a little boost. He was very averse to the idea, worried that he'd have to run 91, or worse, 110 in the car. He's just looking for something that will outrun his tired old 305 (and my car), so he'd rather have cheap gas than more power. I'm just trying to figure out how far we could push it and still run on cheapo or midgrade fuel.

[funcars]

There are a lot of variables to consider besides the static compression ratio. There are some dynamic compression ratio calculators that might help you pick an appropriate cam and cam timing. You can bleed off cylinder pressure down low with the right cam to help reduce detonation where it happens the most. Other things that help include maximizing quench, lower gearing, combustion chamber design, etc. Lots of trade-offs though...

[70GS455]

DCR info here:

http://www.empirenet.com/pkelley2/DynamicCR.html

HTH

[chicane67]

DCR and quench are your key players here.

I have built a couple of 9.8:1 350cid SBC's with alloy heads that made 425ish hp on 87. And then again, I have made 580ish hp on an iron head 355cid SBC with 10.8:1 on 91. Could have gone more, but the general rule of thumb on 'like' engines, with the only difference being head material, is to add roughly a point of compression to the alloy head for the heat rejection of the alloy head.

[Madspeed]

Yup what they said Its all about the combustion chambers and materials involved
If you take those heads you have now and toss them in the circular file then go get yourself some vortec truck heads or edlebrok vortec style alum heads you will outflow just about any early style head ported or unported and be able to up the compression

go do some research on the mid 90's vortec chev truck heads they are CHEAP and Baad Arse