whwright said:
Have you pushed the tuning to the limits to get it to detonate? Like advancing the timing as far as possible? Then trying higher octane fuel? IMHO, 9.0 to 1 is about the lowest I'd comtemplate running on the street, with 87 octane gas, 9.5 to 1 should also be possible with 87. 10 to 1 or higher with higher octane fuel. My 68 Merc's 10.25 to 1 390 is fine with 89. My 331 has a 10.4 to 1 ratio and will also run 89 with the timing backed off, but really likes 93 with more timing. When I first fired it up, it had 87 in it. It didn't just ping, it HAMMERED with that stuff. That was the first time I've ever heard detonation that bad that I can recall. And I hope I never have to hear that again.
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:
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:
