Quote:
Originally Posted by 67Lemons
So for discussion I wanted to post something that I read on one of the FB Pontiac groups. I find it interesting because it goes against very common knowledge on this forum & is closer to what most folks think or say when it comes to high compression engines. Note, I am not agreeing or disagreeing with this persons post, I am merely looking to educate myself & have a discussion that hopefully helps others as well. Here is the post in quotations below:
“FYI: Just a notice or reminder to all of you 1970 and ealier 455, 400 engine owners that those engines were made for 98 octane minimum. If you're trying to get away with 91 or 92, even 93, you're looking down the barrel of future problems that most guys will try to blame it on something else. Know this, inaudible detonation can break a crank in two. Seen it. Because the detonation isn't so severe to where you hear a "ping" most guys don't think they're having a problem. All the while they're chasing heat problems or increased oil consumption. I bring this up because there's no education anywhere about this problem. And I want everyone to be enjoying their Pontiacs, not pouring money into them. You do not want to rebuild your engine stock. That's the worst idea. If you're going to run 93 octane or less you must drop the compression to be compatible with that octane. So listen up - the first reply which no doubt is coming will be from some guy trying to defend his high compression engine by saying "I've been running mine for ten years on 92 octane and it runs great!!!" As if that means his combo is safe. Understand that because of the plus and minus allowable factory tolerances I've measured a stock factory rated 10.75Cr. 400 with # 13 heads at 9.75Cr. Still too high for 93 but will last a lot longer before the signs manifest themselves. Here's a pic of inaudible detonation. Good luck everyone and if you have questions“
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Its not black and white like that.
You can run a 400 with 13s or 62s all day long with the right cam. You can build a 400 that runs PERFECTLY well on 93 with zero detonation.