# Gasoline Ratings and Engine building



## Josh.AZ.GTO (Jul 11, 2009)

I am looking at possible combinations to build-up my 455. I have found a recipe close to what I would like for my car (strictly street) but would require 93 octane. I read that in my area "Mountainous West" that octane ratings are lower due to thin air. Our high grade is 91. 

Is this going to be a huge issue? Should I plan to build around the 91 octane? I live in Phoenix. 

Thank you.


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## Eric Animal (Oct 28, 2007)

Unless you have access to real octane booster, are using aluminum heads, or want to mix in some race gas....I would build the motor for the fuel available. Makes sense...you want to enjoy the car, not hunt for gas. eric


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## Roger that (Feb 6, 2010)

I would talk to Bruce at Rock and Roll Engineering. His website is Pontiac Engines : Rock and Roll Engineering


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## BearGFR (Aug 25, 2008)

"Rule of thumb" is to match static compression ratio to available octane rating. So for 91 octane your target should be 9.1:1 - generally - with iron heads. With aluminum you can get away with more. There are a lot of variables that can influence it so it's not a hard and fast rule by any means.

Here's the thing though - on my 461 the the difference between 9.0:1 and 9.5:1 is a whopping 8 horsepower. So, the lesson there is that you really don't give up much at all by erring on the 'safe' side. About the only time it's worth pushing things right to the limit is if you're building a race motor and looking for every last bit of power you can find.

Bear


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