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Thread: COMPRESSION RATIO / OCTANE NEEDED?

  1. #1
    Join Date
    Jan 2002
    Location
    USA
    Posts
    190

    Compression And Gas

    What is the highest compression piston you can run on pump gas(93 octane)?

  2. #2
    Join Date
    Oct 2001
    Location
    N.J.
    Posts
    4,403
    it depends on the compression once the piston is installed.

    they vary a little bit.


    best way to make sure of it is use a compression tester.

    after that youll know if you need to be running straight 94 or 50/50 mix of 93/108-(vpc12) or straight race gas

    im running a 50/50 mix with 204 psi.
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  3. #3
    Join Date
    Sep 2001
    Location
    Ohio
    Posts
    130

    Lightbulb

    Rule of thumb for octane on different compression setups?

    Air-cooled engines 125cc - 450cc - up to 10.5:1, premium pump fuel. 10.75:1 to 11.75:1, mix race and pump 1/2 and 1/2. 13:1 and higher, race fuel or alcohol.

    Water-cooled 125cc - 450cc - up to 11.5:1 (12:1 on some engines), pump fuel. Anything higher, race gas.

    Smaller engines can deal with higher compression without detonation (less surface area). Really big bore engines (like an XR600) won't deal with compression well at all, and will require higher octane fuel than the compression ratio would seem to necessitate.

    Hope this helps. Don't sue me if it doesn't work for ya.
    "Before you criticize someone, you should walk a mile in their shoes. That way, when you criticize them, you're a mile away and you have their shoes."

  4. #4
    Join Date
    Jul 2001
    Location
    Michigan, 1 1/2 hours from Silver Lake
    Posts
    864
    Put this in the F.A.Q.
    Nate, 1987 250R
    Need an '86-'87 stock swing arm. Reckless@drenthe.net

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