gas octane question

Discussion in 'Small Block Tech' started by Brad W, Jun 12, 2018.

  1. Brad W

    Brad W Miles from nowhere...

    here in So. Cal. Whittier area, the gas is costing quite a bit lately so I was thing of changing from 91 octane to 89.

    The engine is a '72 350-4 stock but with all a/c and smog removed including EGR valve. PVC is still in place. Car is power timed and runs great no ping when wot and no run on when motor is shut off.

    What do you think, should I try a tank full of 89 octane?


  2. lemmy-67

    lemmy-67 Gold Level Contributor

    Depends on your compression ratio. If your engine compression is below 9.5:1, then 89 octane should be fine. I have 10.25:1 in my 430, so I have to run high-octane.
  3. Mark Demko

    Mark Demko Well-Known Member

    Try some, see if it rattles
    Dwayne B and BadBrad like this.
  4. alec296

    alec296 i need another buick

    72 is only 8.2 compression if it’s all original. So you could go 87 and should be ok. But you could try it first like mentioned, then advance timing alittle to get the most out of it , but obviously spark knock means you will have to either use higher grade or reduce timing back down.
    300sbb_overkill likes this.
  5. Brad W

    Brad W Miles from nowhere...

    Thanks guys, I think I'll try a tank of 89 and see what happens...
  6. Stevem

    Stevem Well-Known Member

    Any more Octane then what you need only cuts back on power and in turn mileage!
    Mark Demko likes this.
  7. MrSony

    MrSony Well-Known Member

    I wouldnt try a full tank right away, I'd start with maybe a 1/4 tank. Get a 5 gallon can, fill it up, drive around, run the car and bowl dry, fill up with the 89, save a mouthful to pour into the carb to aid in starting if you wish.
  8. Brad W

    Brad W Miles from nowhere...

    Thanks for the suggestion. I'll try that..

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