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  1. #1
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    Compression vs octane

    Me and a buddy just put together and ported promax , put stock heads on for now, but I have a set of oring heads that I can get the oring groove milled flat and i get around 170 ,180ish lb cranking compression roughly, what kinda fuel octane should I run with that kinda compression? I installed head studs so I can swap heads out little easier

    Can I do a mix of 93 and av fuel ? If so what kinda ratio. And what is the norm octane for av fuel
    Last edited by Merc 2.5; 02-21-2021 at 06:57 PM.

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  3. #2
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    Most av fuel is around 100 octane .. If your at 180psi, personally I’d stay close to 100. Your safe cutting it back some but I wouldn’t go much lower than 98 depending on how hard your turning the motor .

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  5. #3
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    Yep seems like gonna be 26cc. 180ish lb.

    Ill have 2 sets of heads and a dedicated tank to 26cc heads. Will the plugs ,pistons read like 93 fuel?

  6. #4
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    I’ve used AV gas +91 it works ok. If I run 175 to 180 compression I run 2 gallons 110. 3 gallons 91 pump gas. 98.6 Octane.

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  8. #5
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    What is your exhaust port height?

  9. #6
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    in a 15 gallon gas caddy, I mix 5 gallons of VP C-12 (108) with 10 gallons of 90 octane Recreation fuel (no ethanol), yields about 96 octane.

  10. #7
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    if you can get AVGAS near you then stick to that all the time. it is the most reliable fuel anywhere in the world. the octane rating will not change . even race fuels can have a change in octane , but NOT avgas..
    its got very expensive here int eh uk , or i would run it anyway..

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  12. #8
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    Here is a good mixing site for fuel.

    http://www.bazellracefuels.com/Calcs/OC1.htm

  13. #9
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    Check fuel densities also

  14. #10
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    My opinion, ave gas and premium is better than ave gas alone. I had better luck with a 50/50 mixture than straight ave.

  15. #11
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    Quote Originally Posted by rude tim View Post
    My opinion, ave gas and premium is better than ave gas alone. I had better luck with a 50/50 mixture than straight ave.
    Octane should follow compression ratio . Too much octane and not enough squeeze will be slower than a proper fuel for the job.

    I ran VP- NOS in my car. 120 octane / SG .709 / on motor it was a pooch. A pair of 300 HP kits on top .. no worries , no new pistons every other race ..

  16. #12
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    Sorry guys. I been down for while. Just got new phone and can see everything again as for my port height. It's nothing crazy. I can get a exact #bfrom machinist. I put head studs in so I can swap heads easier , just lookin for a good compression # vs octane # I have av fuel not far from me. Not even sure what that octane is

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