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Thread: Compression vs octane
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02-21-2021, 06:53 PM #1
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 fuelLast edited by Merc 2.5; 02-21-2021 at 06:57 PM.
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Baker343 liked this post
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02-21-2021, 08:35 PM #2
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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02-21-2021, 08:39 PM #3
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?
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02-21-2021, 09:14 PM #4Screaming And Flying!
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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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Merc 2.5 thanked for this postMerc 2.5, 25two.stroke liked this post
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02-21-2021, 10:12 PM #5
What is your exhaust port height?
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02-22-2021, 12:07 PM #65000 RPM
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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.
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03-03-2021, 05:03 PM #7
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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Michael J Giesler liked this post
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03-07-2021, 04:19 PM #8
Here is a good mixing site for fuel.
http://www.bazellracefuels.com/Calcs/OC1.htm
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03-12-2021, 06:58 AM #95000 RPM
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Check fuel densities also
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03-16-2021, 12:02 AM #10
My opinion, ave gas and premium is better than ave gas alone. I had better luck with a 50/50 mixture than straight ave.
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03-17-2021, 07:58 AM #11
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 ..
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03-18-2021, 04:58 PM #12
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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