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Author Topic:   Octane VS Compression Ratio
Dirt Newbie

Total posts: 0
posted August 11, 2002 12:49 PM
Been away for a while, but I know someone in here's done the research. What's the best octane gas to run with a 9.5:1 motor? It's going in what's known as a Wissota B-Mod, but the powertrain's all street stock, so I figured this would be the place to find out.
7 or 8 years ago when I was running street stocks, I'd found a chart in one of the race magazines that had the info I'm looking for but, moron that I am, I didn't save the chart after getting the info I wanted off of it. Any help would be greatly appreciated.


Dirt Forum Racer

Total posts: 79
posted August 20, 2002 06:31 PM
OOOHHH Goody maybe I found what you ar looking for.
Hey Jammin might be good for the tool kit!

Which fuels have more oxygen? They are listed below, in order of oxygen
content (most to least):
- Nitromethane
- Alcohol
- Unleaded racing gas
- Unleaded pump gas
- Leaded racing gas
So, if you’re racing on gasoline the easiest thing to do is just mix unleaded
with leaded gas, or run straight unleaded.
7:1 comp ratio ---------- ~ 89 octane
8:1 comp ratio ---------- ~ 91 octane
9:1 comp ratio ---------- ~ 94 octane
10:1 comp ratio --------- ~ 98 octane
11:1 comp ratio --------- ~ 104 octane
12:1 comp ratio --------- ~ 110 octane
13:1 comp ratio --------- ~ 113 octane

Dirt Newbie

Total posts: 0
posted August 20, 2002 07:10 PM
I knew someone in here would come up with something sooner or later..
Thanks, dude..


steamatic racer
Dirt Full Roller

Total posts: 35
posted August 22, 2002 08:22 AM
Confused When I ordered my carb. from (C&S Specialty) they asked all kinds of ?'s about the motor and the one thing they said was, I did not need over a 100 octane for a 13:1 motor! So who do I listen to? THANKS TERP

Dirt Maniac

Total posts: 128
posted August 22, 2002 07:55 PM
Try this internet address, it is from phillips 66 racing fuels. Some people use "research" octane number and some use "motor" octane number. Maybe this will help.

Dirt Newbie

Total posts: 0
posted August 22, 2002 08:51 PM
Steam.. You need way more than 100 octane for a 13:1 motor. I might not know the exact octane, but I know that isn't even in the same ballpark. One thing I found out for sure during all this research is this: You'll never hurt a motor's life by running too much octane, only it's performance. But, by running too little, you stand to risk a meltdown due to excessive heat from preignition. It'll do the same thing as leaning it out.
What I'm trying to do here is to find the optimum octane rating for the compression I'm running, because I know that probably 90% of the guys we'll be running against are running race gas at 108 octane, and I gotta think there's at least a good 10-20 HP to be gained by getting the fuel burn at it's most efficient. I'm gonna spend some time on the dyno this winter, and that's gonna be something I play a lot with, given that the right dyno will give you a hard number for the fuel burn's efficiency.

Good Luck,

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