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Or the octane is too low.

A 10.8:1 compression nominally requires a 98 pump octane gas. It will always ping on 93. You can fudge the octane a little with timing, etc., but not 5 octane numbers. If you could, then no one would really need premium in a new car.

Adding about 40% ethanol to 93 octane gas might be about right. Ethanol by itself is about 115 octane.
I disagree. My 429 has 10.8:1 compression and runs through the rpm range on 93 octane with no additive and no ping whatsoever. There are many variables in different engines that can and do cause this condition. It would be incorrect to make a sweeping statement that just because an engine has a certain compression ratio, it will always ping with 93 octane.
 

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429ragtop,

I agree, there are a lot of variables. What is your altitude? That makes a big difference. Eastern Washington could make that a lot easier than in Boston. The first 1000' reduces the octane needed by about 1.5. At our 6150' altitude its reduced 9 octane numbers so 93 would be overkill which is why they don't even sell 93 around here.
 

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I disagree. My 429 has 10.8:1 compression and runs through the rpm range on 93 octane with no additive and no ping whatsoever. There are many variables in different engines that can and do cause this condition. It would be incorrect to make a sweeping statement that just because an engine has a certain compression ratio, it will always ping with 93 octane.

I agree, my 351w is 10.6:1 and i run on 93 pump gas, every now and then i will add a little something, but not everytime.
 

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Discussion Starter · #24 ·
Actually I need to make a correction. My static compression ratio is 10.4:1. I'm thinking it's related to the high cylinder pressure do to the stroke and the 236* intake duration not bleeding off enough. I installed a set of NGK 5820 racing plugs, colder than the Autolites and have a recessed tip. No difference, so I'm ruling out ignition at this time and leaning towards an excessively high cylinder pressure. I'm going to get a new compression tester, mine only reads to 150psi, and test the pressure this week. Maybe I can find a shop around that sells race fuel and see if it helps. The octane booster, Lucas Oil, didn't help at all.
 
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