Octane Requirements
Melting Slicks
Joined: Aug 2003
Posts: 2,135
Likes: 2
From: Dayton Oh
The rule of thumb is to use the lowest grade of fuel that does not cause you engine to ping. I'm thinking that if you engine is in good mechanical condition (ie no carbon build up,etc) , at 9:1 then you can use 87. If you get a ping then try 89. 
Instructor
Joined: Aug 2005
Posts: 107
Likes: 0
From: Mishawaka Indiana
My understanding is, if you dont have the compression or modifications to warrant the higher octane, than yes, you are wasting money.
The trend years ago, was to offer other "gimmicks" to get folks to buy the higher priced premium fuels, by adding detergents and other addatives that claim to prolong the life of modern FI cars and keep injectors clean, etc. Nowadays, about all the gas out there has some form or another of these addatives. So again, there really isn't a need to run the higher octane stuff unless you have the compression or modifications to warrant it.
My 70 is 10.5:1 and I ran 95 octane in it when I first got it because I thought that was closest to what the leaded gas was back at that time. (70 was the last year before fuel crunching started and GM dropped compression). A few months ago I actually put 87 in it when the prices started going up bad, and I figured I would just see how it did. If it pinged I would try 89 or go back to 92+. Well, suprisingly, it ran fine on 87. I have my timing set to factory and it didn't ping at all. Not sure why it ran ok on low octane. Theoretically it shouldn't have at my compression. But I was also told by an old mechanic, that even though the gas in the old days had lead and higher octanes, that the quality of modern fuels are much better than they were back then, and that some of those addatives even aid in reducing detonation and run on, and maybe thats why my car ran ok on the low grade. I've been using 89, just to ease my conscience, but I dont have a problem putting 87 in it again. Which the way prices are going, may be on the next fill up.
I used Citgo when I tried this. But from what he said, the addatives may be different in other regions. California and pollution laws, Winter addatives are different than summer etc.
Just try a 1/4 tank of the cheap stuff and drive it.
The trend years ago, was to offer other "gimmicks" to get folks to buy the higher priced premium fuels, by adding detergents and other addatives that claim to prolong the life of modern FI cars and keep injectors clean, etc. Nowadays, about all the gas out there has some form or another of these addatives. So again, there really isn't a need to run the higher octane stuff unless you have the compression or modifications to warrant it.
My 70 is 10.5:1 and I ran 95 octane in it when I first got it because I thought that was closest to what the leaded gas was back at that time. (70 was the last year before fuel crunching started and GM dropped compression). A few months ago I actually put 87 in it when the prices started going up bad, and I figured I would just see how it did. If it pinged I would try 89 or go back to 92+. Well, suprisingly, it ran fine on 87. I have my timing set to factory and it didn't ping at all. Not sure why it ran ok on low octane. Theoretically it shouldn't have at my compression. But I was also told by an old mechanic, that even though the gas in the old days had lead and higher octanes, that the quality of modern fuels are much better than they were back then, and that some of those addatives even aid in reducing detonation and run on, and maybe thats why my car ran ok on the low grade. I've been using 89, just to ease my conscience, but I dont have a problem putting 87 in it again. Which the way prices are going, may be on the next fill up.
I used Citgo when I tried this. But from what he said, the addatives may be different in other regions. California and pollution laws, Winter addatives are different than summer etc.
Just try a 1/4 tank of the cheap stuff and drive it.
Last edited by 70inSBIn; Aug 19, 2005 at 02:24 PM.
Race Director
Joined: Apr 2000
Posts: 17,649
Likes: 189
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Just a small note on this that I noticed recently. Mine will run on 91 octane with no detonation but it seems to run smoother on 94 
Instructor
Joined: Jan 2005
Posts: 159
Likes: 0
From: Chardon Ohio
Originally Posted by MotorHead
Just a small note on this that I noticed recently. Mine will run on 91 octane with no detonation but it seems to run smoother on 94
I tried my 71 LT-1 on 91 Octane but went immediately back to 94 and there is a very discernable difference.
