whwright said:I think you guys are right on the money when you say that 9:1 or so is optimum for a street engine. FWIW: my 460 runs 9.5:1 and I haven't noticed it detonating at all.
Wayne.
The piston clearance on the 331 is .003" head gasket thickness is .043. The 331 will run with 89, but you have to retard the timing some and it does ping slightly occasionally. It would be better to mix 87 and 93 in a 50/50 mix to get 90 octane, as it would cost the same as what 89 goes for here. That's what I've found to be how the oil companies make a few extra dollars off us, they sell midgrade ( 89) for ten cents more than 87 and ten cents less than 93, in effect you can get 90 octane for the same price by mixing it 50/50 at the pump yourself. -----------------------------------------------------------Now on my 68 Mercs 390, I don't recall what the piston to deck clearance was, but it's got the stock std bore cast flat tops in it and Felpro gaskets that are I believe .042" thick instead of the .020" stock steel shims it came with. That reduced the ratio down 1/4 point from 10.5 to 1. That and the Crane Energizer cam has allowed it to run on 89 octane where it required premium before.Route666 said:89 and 10.4 eh? What's your quench distance? (I've probably already asked you before but I've forgotten if I have.)
Would it be better to run 11:1 with a 98RON fuel (93AKI) or 10.5:1 with a 95RON fuel? (89AKI)
The 98 is more scarce, but burns more efficiently and cleans the engine apparently, but the 95 is more readily available and slightly cheaper, although if the motor is built for it, you should get better mileage from the 98 so the price basically doesn't come into it.
Well, I had actually intentionally used thicker gaskets to reduce the comp ratio when I built it.(the ratio was a tad more than I wanted to take a chance with at around 10.7 to 1, if I recall correctly) I had these heads on a 302 shortblock before and had them milled .060 to increase the comp ratio in it. The Canfields were supposed to be 58 cc chambers, but turned out to be 64ccs. That's the chance you take in buying on ebay. Still got good heads for a good price, just the woman I bought them from made a mistake in the chamber specs. Milling them .060 reduced the chambers to 57 ccs.Route666 said:Cool, it'd be interesting to see if you could run on 89 without retarded timing if you closed up the gap to .035 or so. Sure the CR would increase too, but I'd like to know just how handy squishing the mix can be.
Some sources to back up my apparent insanity:
Chevy High Performance, paragraph 2
One of many online compression calculators
Hot Rod, 2nd half of second paragraph and the best, simplest description IMO
HuKares said:At least two of those sources are chevy guys, and we all know chevy guys can't math.
I'm sorry, but I have to agree with the others, 11:1.
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