Original octane requirement
#1
Original octane requirement
Good morning Oldsmobilers!😀
As I have mentioned several times here on the board, my 72 Cutlass Supreme lived one of it's former lives as a drag car. I'm pretty sure work has been done to the engine, but what's been done, I'm not sure. It has 670 Holley Street Avenger with an Edelbrock Oldsmobile 455 intake, but beyond that remains a mystery to me. I was told by the previous owner that it will not run right on 87 octane unleaded. He recommended 93 premium, and I have been using only Exxon supreme, with no issues.
My question is that if it were stock (and the engine is numbers matching) would it run on 87 regular, or were the original octane ratings for this engine higher? I'm just asking to back up the claim that the engine isn't stock.
I haven't had it dyno'd yet...was gonna do that after I replace the exhaust, (right now there are some leaks and it's been bandaged and gooped) but now I'm thinking I may do it sooner so I can get a baseline and see what difference the new exhaust system will make.
Thanks,
Dave
As I have mentioned several times here on the board, my 72 Cutlass Supreme lived one of it's former lives as a drag car. I'm pretty sure work has been done to the engine, but what's been done, I'm not sure. It has 670 Holley Street Avenger with an Edelbrock Oldsmobile 455 intake, but beyond that remains a mystery to me. I was told by the previous owner that it will not run right on 87 octane unleaded. He recommended 93 premium, and I have been using only Exxon supreme, with no issues.
My question is that if it were stock (and the engine is numbers matching) would it run on 87 regular, or were the original octane ratings for this engine higher? I'm just asking to back up the claim that the engine isn't stock.
I haven't had it dyno'd yet...was gonna do that after I replace the exhaust, (right now there are some leaks and it's been bandaged and gooped) but now I'm thinking I may do it sooner so I can get a baseline and see what difference the new exhaust system will make.
Thanks,
Dave
#3
Jaunty , the number quoted is a research number.
Modern gasoline is rated by the motor plus research divided by two method .
This would equate to about 87 octane in today's gas .
Modern gasoline is rated by the motor plus research divided by two method .
This would equate to about 87 octane in today's gas .
Last edited by Charlie Jones; August 19th, 2018 at 03:20 PM.
#4
Aha...so the previous owner told me that on 87 octane the engine sounded like it was coming apart...so if it runs like it should on 93, then it probably has had some work done ?
#5
#7
#8
Just an Olds Guy
Join Date: Jul 2008
Location: Edmonton, AB. And "I am Can 'eh' jun - eh"
Posts: 24,525
The original carb on the 72 455 was a Quadrajet (7042251) and flowed 750 cfm.
You have stock GA heads IIRC, so in all honesty I don't know why you don't try running some lower octane to see if there's any noticeable performance or running issues.
You have stock GA heads IIRC, so in all honesty I don't know why you don't try running some lower octane to see if there's any noticeable performance or running issues.
#9
If I try lower octane, what (if any) damage will happen? Also, how would I know if my carb was too small? Performance, acceleration, fuel mileage...and what else?
#10
Just an Olds Guy
Join Date: Jul 2008
Location: Edmonton, AB. And "I am Can 'eh' jun - eh"
Posts: 24,525
If you try lower octane, start with mid grade and only use 1/2 tank or so. If the octane is too low you will notice pinging from the engine. Not dangerous, just a sign that you need to stick with higher octane, so go back to premium fuel. If you hear nothing and performance isn't affected, you can go down to regular fuel using the same process and see what happens.
#11
If you try lower octane, start with mid grade and only use 1/2 tank or so. If the octane is too low you will notice pinging from the engine. Not dangerous, just a sign that you need to stick with higher octane, so go back to premium fuel. If you hear nothing and performance isn't affected, you can go down to regular fuel using the same process and see what happens.
#12
Just an Olds Guy
Join Date: Jul 2008
Location: Edmonton, AB. And "I am Can 'eh' jun - eh"
Posts: 24,525
Under normal driving the 670 should be ok. But under heavy acceleration if the engine would likely not perform well and also likely an indicator you have too small a carb. The 670 would be a great carb on a 350 car, but in my opinion it's too small for that 455. There's a reason Olds installed a 7042251 (750 cfm) on your 455 straight out of the factory. I suspect the PO simply put on a smaller spare carb when he decided to sell the car and kept the performance carb that was on it for himself and another build.
#13
Under normal driving the 670 should be ok. But under heavy acceleration if the engine would likely not perform well and also likely an indicator you have too small a carb. The 670 would be a great carb on a 350 car, but in my opinion it's too small for that 455. There's a reason Olds installed a 7042251 (750 cfm) on your 455 straight out of the factory. I suspect the PO simply put on a smaller spare carb when he decided to sell the car and kept the performance carb that was on it for himself and another build.
#14
Just an Olds Guy
Join Date: Jul 2008
Location: Edmonton, AB. And "I am Can 'eh' jun - eh"
Posts: 24,525
If your buddy still has the carb and wants to swap for the 670 I'd go that way. 10-12 isn't great mileage but it's acceptable. I think the 750 would do better.
Thread
Thread Starter
Forum
Replies
Last Post
Christophe2m6
Small Blocks
14
November 19th, 2014 10:52 AM
russell-t
Small Blocks
10
September 2nd, 2012 07:10 AM