Thread: Figuring octane when mixing fuel.

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Hi Everyone, I hope everyone is doing well. My '32 Ford with a blown 392 Hemi in it is pinging. The instructions that came with the motor {Big Al's Toy Box motor} said to run 94 octane fuel but the best that is locally available at the pump is 91 octane {I'm in Calif}. I'm want to get 100 octane fuel to mix with the 91 but would like to know how much 100 octane I need to mix with 91 octane to get 94 octane. I have a GMC 8:71 on it with 10 pounds of boost. Also in the future I will probablly switch the pulleys so I have 15 pounds of boost. Is 94 octane enough for 15 pounds of boost? If not, how much octane will I need? {Also I was going to use aviation fuel}. THANKS , Bill

Dont think you can mix the 2 to get 94 octane. I could be wrong though.

Assuming the two grades use the same octane boosters, a 3:1 mix should do it. I can't see that small a change making much diff, though. Try a tank of straight 100. Check timing and air-fuel ratio carefully. Hemi's are probably the least prone to pinging of any engine out there (no squish area) and those thick heads muffle any that does occur, so yours must have other issues besides the gas. Fix the pinging before increasing the boost. 15 pounds doubles the static CR, you'll never find good enough fuel for that with the present setup. What is your static CR, BTW?

My understanding is that the relationship is linear. If you mix 87 and 93 50/50you get 90. In your case you have nine points difference, 100-91, and you're looking for three points improvement so a linear relationship says put in ten gallons of 91 and five gallons of 100 and you should have 94. Now the kicker is that you need to do that every time - if you're out cruising, get to 1/4 tank and add 91 you will "dilute" your base of mixed 94 in the same relationship.
Disclaimer: There may be someone on here who is more of a fuels chemist than I, but I believe that stations that offer three octane levels simply blend their high test and low test 50/50 in the pumping process to hit the middle number. To my knowledge they do not have three storage tanks.
(RPope and I were typing at the same time. Glad the math was the same!!)

I agree with rspears. I used to do this in my '68 vet which had 12:1 cr. Mixing is a real pain though. You could instead try water vapor injection.

Kitz

Try your local air port for sum 106

Thank you Big Truck Driver, R Pope, Rspears, Kitz & DA 34 Guy... Gee I have no idea what the static compression ratio is. I will call & ask Big Al, buy my guess is that he doesn't know either. I didn't hear a ping, I just figured that's what it was cuz it was running bad. I have a retart switch {msd} to retard timing up to 3 degree's per pound of boost & even at 3 degree setting it still runs bad. Also it back fires at high RPM. The local airport just had the 100 octane, nothing higher. thanks again, Bill

the air port LL 100 is not to bad you will fine this is a true 100 rating and will not very like what you may or may not get at the corner store gas pump . the 100LL i ran in my 14.8 bbc and many other hi pumpers

You might get into some legality issues running avgas, you do in Canada. No road tax on av, they can bust you.

Originally Posted by R Pope

You might get into some legality issues running avgas, you do in Canada. No road tax on av, they can bust you.

yep that is right how or if you do this is up to you but if you mix it or going to race it at the track and some is in the tank and your on the street and who going to wast there time busting you ? your going to tell evey one your going to get busted ? i do not think so. there just not going to sell to you not like your going to pull up to the pump with a fleet of cars ? your talking 5 or 10 gallons ? in you gas cans . i used it at the race track.but i did drive the car there. i did have to fill it with low grade pump gas driving it home so most all of the 100LL was burt at the track . i just use race gas now i can pull up and pump it in

I don't think you will be very successful mixing fuel. You have no idea what proportions of what are in the fuels. It will be a pure guess. Even with something like a hydrometer it will be hard. What value will you be looking for??

Just ask any old top fuel racer how hard it is to bring too low of percentage back up. Usually it means starting with fresh 'good stuff' and adding a very small amount of low stuff then carefully measuring with the hydrometer and taking temp into consideration.

You might try some octane booster.

My suggestion is to get an air fuel gage and start tuning. I have a blown SBC at 6 psi boost and run 89 octane with no ping or problems at all. I have over 1000 miles on this hotrod in the last 2 months including a bunch at less than 32 degrees F. Gas is purchased at the closest station at the time required.

By the way don't use octane booster with an air fuel gage. The O2 sensor won't last long.

Thank you Pat, R Pope & Bentwings. One of the reasons I was asking about mixing is because it will be impossible to not mix unless I always run the aviation fuel. And I don't want to do that cuz the airport is to far away from my house plus they won't pump it straight into my car so I'll have to use 5 gallon cans. I tried octane booster & it didn't make any difference. Don't know how true this is but the guy at the auto parts store told me that octane boost actually raised the octane level so little that it wouldn't make any difference.... Is this true? Thanks again to all!!! Bill

well they at one time sold a kit to check the fuel at the pumps at the boat docks. when i went to mercruiser they said more more warranty on burnt out pistons from poor fuel quick sliver did sell a booster they told us it would of been EZ just to sell something and put there name on the bottle and call it good but after testing evey thing out at that time just did not work so they made some thing that did ..so you could go to a marina and try some ? i use the stuff did work .there could be better out there now?or you could go to the paint store and buy some thinners this will work as well and cheaper there is some write up on this

Thanks Pat! did some checking & octane boost in the can raises the octane 1 point & 1 point is .1.... So when it says it will raise your octane 2 points it will make 91 octane fuel 91.2 octane. Can't possibly see how this will make a difference. I googled 'quick silver' but haven't found it yet... Thanks again, Bill