Calculating Octane Formula • The Dodge Challenger Message Board

by **Jon** » 10 Oct 2009 1:48

If there is 10 gallons of 91 Octane fuel in a tank, and one added 5 gallons of 100 octane AV gas, what would the resultant octane be?

by **drewcrane** » 10 Oct 2009 1:58

could be about 94 octane give or take 1 octane, you have alot of chemistry involved,due to the effect that both fuels are not made the same :biggrin:

by **Jon** » 10 Oct 2009 3:14

Thanks Drew, is there an algebraic formula or something to get the base figure.

by **drewcrane** » 10 Oct 2009 4:22

yea i hate math,i got seth the safety engineer on that :thumbsup:

by **christer** » 10 Oct 2009 9:29

drewcrane wrote:could be about 94 octane give or take 1 octane, you have alot of chemistry involved,due to the effect that both fuels are not made the same

It feels like you are spot on,drewcrane: 2/3*91+1/3*100=94
(This formula seems to be working. At least, I do not have any better suggestion.)

by **Jon** » 10 Oct 2009 13:19

That seems to be correct Chris/Drew, thanks.

The airport is conveniently located between my home and town. As part of the weekly trip in for supplies I decided to stop and buy 5 gallons of AV100 for my "off road dirt bike". :wink:

I'll mix it up and see how it works. :biggrin:

by **Jon** » 16 Oct 2009 20:43

Jon wrote: I'll mix it up and see how it works.

Not a hint of pinging with this ratio. In fact I added 6 gallons of 91 octane to top off the tank at the end of the test session, along with adding 2 degrees advance. Again no detonation.

I plan to add 2 more degrees initial advance (16) and settle the ratio to about 20% Avgas to 91 Pump Fuel, and see if that's what we can live with.

Of interest, the idle went down a little bit after mixing the brew. It was very humid today though so I may wait till the weather normalizes before resetting the fuel/air mixture.

The Oder is quite noticeable. I wonder what the people following me thought. Guess you would have to be involved with aircraft to know the smell. :s022: