# What Octane Rating can I use



## dedlund400 (May 17, 2020)

I am just about ready to start my engine after a complete rebuild. It is a 400 CI stroked to 461 CI. It has factory cast Iron heads with a Ram Air 3 cam. It has a Holley 750 CFM carb. I am running 9.75 :1 Compression Ratio.
I have three Big block Chevys with the same compression ratio and can run regular grade gasoline with no problems.
Can I do the same with this Pontiac engine ?


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## armyadarkness (Dec 7, 2020)

What climate? What timing? There are a few variables that you dont mention, but the logical answer would be "NO".


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## Baaad65 (Aug 29, 2019)

dedlund400 said:


> I am just about ready to start my engine after a complete rebuild. It is a 400 CI stroked to 461 CI. It has factory cast Iron heads with a Ram Air 3 cam. It has a Holley 750 CFM carb. I am running 9.75 :1 Compression Ratio.
> I have three Big block Chevys with the same compression ratio and can run regular grade gasoline with no problems.
> Can I do the same with this Pontiac engine ?


By "regular" do you mean 87 octane? Agree with army I wouldn't. I run nothing but 93 and I only have 9.4:1 compression, also agree we need more 411


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## chrisn (2 mo ago)

same here premium only no need for detonation breaks parts


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## Mike Honcho (10 mo ago)

My 400 will not even tolerate 91 octane. 93 is just enough.


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## lust4speed (Jul 5, 2019)

God no, and I doubt your Chevy's are that happy. Tough to get around physics. A lot of the problem is people are told what the specs are on the engines and usually what is said is a long ways away from what you would find with actual measurements. I know that here in California a true 9.75:1 compression ratio running on 91 octane would be ping city with a smallish cam like the RAIII.

Take Pontiac's advertised 10.75:1 compression ratio advertised for many years. That was overstated by at least a half point of compression and usually more. Those 72cc advertised heads were 76cc's and the ratio figured on zero deck when the pistons were at least .010" in the hole.

I get guys telling me that there engines never pinged and always ran good. Then I get into their engines and find rod and main bearings hammered down to the copper, and rod bearings just falling out of the cap.


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