72455

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

jaunty75

The '72 Cutlass owner's manual says to use 91 or higher octane.

Charlie Jones

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 .

72455

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 ?

Charlie Jones

Yes , the compression has been increased .

brownbomber77

72455

Hmmm ...the previous owner said the 750 that was on it was too much ...recommendations? I mean, it runs fine with the 670 and the plugs look good.
.

Allan R

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.

72455

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?

Allan R

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.

72455

Will do...but once this tank is empty I'm gonna drop the tank and look for my biuld sheet. Also, on the carb situation, how do I really know if the 670 is too small?

Allan R

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.

72455

He actually bought the 670 new, and he threw in the 750 as part of the deal when I bought the car. Since it seemed to run fine on the 670 and my plugs looked good, I figured I was good and sold the 750 to a buddy. Would it be worth the money, time and effort to go with a 750? I don't plan on winning any stop light wars...just enjoy cruising and right now I get around 10-12 mpg with the 670.

Allan R

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.