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Jet Fuel...


Well-known member
Nov 23, 2001
San Diego, CA
Black Cherry 1969 Stingray
Hello to all,

I had a question about the correct octane use in the L71-L89. Is there a top end on what octane you should use? I've talked to a few people who have said that they've used airplane fuel (110 octane) in this engine. Is that harmful? What is the lowest level that I should use? And as far as gas stations to purchase which brand of gas and/or octane booster have ya'll had luck with? Thanks for the help.
Hot Rod did a very interesting test on Octane ratings and fuel boosters, last month. Timing plays the most intricate part in preventing detonation. To quote from the article:
By improving the quality of the base gasoline, octane boosters don't actually make power in and of themselves. Rather they allow the full utilization of what is already in place by helping to maintain controlled combustion. - Steve Magnate
In the test, done to a 10.4:1 compression ratio 360 Mopar, they made almost 2hp and 2 lbs.-ft more with 87 gas and 104+ Octane Booster @ 36 degrees. However, they made more power with straight 91 gas @ 36 degrees, than when adding Octane Booster to it or advancing the timing further. It slowed the burn speed and reduced cylinder pressure. Yes, they made the most power with race gas, but is it really woth the expense? In Hot rod's conclusion they said:
Knowing what we know now, we'll always experiment with ignition timing, both higher and lower settings, when we change fuels rather than presuming that more power can be made with more octane due to more timing.
Another conclusion that they made that the lower the quality of gas, adding Octane Boosters were cheap insurance in preventing detonation than none at all. Good advice, I think. :) --Bullitt

You are never going to get off the grounds if you start cyphoning JP4 from the chopper tanks. :D

........ Nut
awwww come on

Nut said:

You are never going to get off the grounds if you start cyphoning JP4 from the chopper tanks. :D

........ Nut

Hell, I figured I'd give it a shot. :D
Checked out your website,she's a beauty.I like the custom features.In my opinion,I would only use cam II in your baby mixed with some Sunoco 110.
Best of luck w/your car & your career.
I would want to use the highest octane that I could find. Just don't get Av gas mixed up with jet fuel. Av Gas is about 110 octane where Jet fuel is about 48, the same as kerosene...................ooooohh, that'd maker 'er smoke.....Steve
ssvett and others,

Being on the jet fuel topic and all...If there are any turbine powered corvettes out there (don't everyone raise their hands at once) They should be able to take AvGas in lieu of Jet A/JP4/Kerosene. This just decreases the time inbetween teardowns of the turbine.

Although hung starts, fuel scheduling, and hot starts could pose a problem...Ahh just keep an eye on the EGT and your good to go. ;)
I run a slow advance curve and keep the cam timing high and piston quench area @ .040 to run just under 11/1 CR It runs fine on 92/93 octane.

If i'm going out to raise some H I sometimes put in a blend. simple math if I have 5 gallons 90 octane and I add 5 gallons of 110 out of the local Sunoco pump I end up with 100 octane.

If you run more octane than your motor requires you loose HP and Money.
I think the point has been made that JET A or "jet fuel" ( also JP4 or JP5) is NOT "AVGAS" or aviation gasoline.

To correct another misnomer in this thread, AVGAS is not 110-octane. There are three types of AVGAS readily available in North America AVGAS 87, AVGAS 100 and AVGAS 100LL.

DO NOT use AVGAS 87 in a high compression automotive engine.

If you are going to use AVGAS, use 100LL which is a hair less than 100 octane when measured on the R+M/2 scale with which most car enthusiasts are familiar.

AVGAS 100LL and AVGAS 100 are both leaded gasolines so they cannot be used in any engine fitted with cataylitic converters or oxygen sensors.
Hib, I never responded because I thought they were joking about the CTAF and JP4/5/8, etc..

I said "about" 110 octane.:) ..................I never claimed to be a rocket scientist.........Steve
IMHO ... avgas is for planes

We use AVGAS in airplanes we work on & fly ... & little else. Occasionally I'll use it to clean car parts ... it leaves almost no residue ... but it sure does dry yer hands out! Since it leaves no residue, it might also be good to leave a vette's carb full of it during long storage ... dunno, it may be too aggressive for plastic floats etc. It does evaporate much more quickly than car gas. Just guessing, but it may have a buttload of benzene in it ... NOT good for humans. MANY years ago (when the Sunoco station closed up), I used to run it in Harleys ... along w/ AeroShell oil ... probably hammered my wallet more than anything else. How about napthalene mothballs??? ... or hydrazine ... wooohooo! BTW, Skydraul (aviation hydraulic fluid) will often burn/blister paint ... maybe it'd make a good paint remover? Though I have ready access to both AVGAS & Union 76 Race gas, I'll stick with car-pump gas ... building my DRIVER vette's stroker motor up with less than 9½:1 static CR ... just so I don't EVER have to hassle with gas issues. Just my .02

ps ... good to see Steve active on the forum again ... missed you pal!
Jack, We have been/ are really busy in the shop. And I have been bit by the "E-Bay" bug. So, now I have to share my computer time, if I have time to get on ot at all.........:)...........Steve
I checked out your webpage. The search for your 69, sounded remarkably like mine.:D

Higher Octane could boost the HP and TQ with your motor, considering the Tripower/435HP:cool.
Avgas is tops if you need it, otherwise 104+ works too and is still legal.

The octane factor becomes crucial , when compression ratios start getting up there, beginning Id say around, 10.7 : 1 on up. I didn't see what your CR was listed at?

You may know this allready?? but..
One example of looking at it is, With lower octane fuel, a motor with high compression pistons and increased lift cam profile, must have the timing retarded down from the optimal 36deg. total, to belay the dreaded engine knock(detonation). That makes for the motors poop to degrade, somewhat proportionally.
Bullitt's Post was right on. octane helps control the burn, so the piston is pushed down, not banged down, like hitting it with a hammer:eek , that'll ruin the motor quickly.

I used to put 104+ in when going out for a hundred on the twisties. I felt the motor was really humming with it in. this was Probably mostly my imagination. My 10 :1 330hp motor doesnt neet to be octane boosted..its pretty moot to do it:eyerole.

Awsome 69 Anthony :upthumbs

The car arrived today by the way; it is amazing!!! The gentleman I bought the car from told me that the compression was 11.5:1. I'm not sure if he had it tested or not because the Specifications page on this site says that the compression is 11:1 on this motor with these heads. I do know that there is an aftermarket cam from Comp Cams in the car. The information on the cam is found here ,but don't think that that makes any difference on the octane required. What I really need to know is if 93 Octane is too low for the car? Will I have to mix gas everytime I have to fill up?

Im not sure Anthony, Im thinking you would need to boost your octane rating UP every fillup, "IF" your wanting to run your car , with the total advance set to 36degrees. if you run with less advance curve, you may not need to.

Try it, fill up with premium, 91-92 octane, set your total advance to be 36degrees and to come all in at around 3k rpm. Drive it in all ranges, If you get any detonation at all, boost the octane until you get no detonation under any conditions, Walla..
If you still get detonation with a can of 104+ in 15 gals. of fuel, It would be necessary to begin retarding the timing gradually till you get no motor knock under any conditions.

Maybe gkull will come back with some finer tuning info, cause I know he's got a firebreathing vette..

Anthony, I recieved this month's issue (Febuary 2002) of Car Craft and they have an article about determining octane requirements for a high compression engine. They talk about intake valve closing, static compression and dynamic compression. The one thing that did catch my eye was the following statements:
Out in the field, an easy way of approximating an engine's octane requirement is to perform a cranking compression check using widely available hand-held gauges. According to engine builder Joe Sherman, small block cranking compression pressures as high as 175-185psi are compatible with 92-octane gas. You can even go up to 200psi on 92-octane, but the tune and calibration must be spot-on. If scraping by on 87-octane, observe a 160psi limit. Back off these numbers by 15-20psi on more detonation-prone big blocks." --Marlan Davis

One of the most extreme examples is a Joe Sherman built 400 small-block Chevy. The 12.35:1 motor made 632hp on pump gas with ported cast-iron cylinder heads, one carb and a failry big (280 degrees at .050) mechanical roller cam with a late closing intake lobe. This was with only 30 egrees of advance (it detonated at 34 degrees, scuffing four pistons in the process). The cranking compression was 220psi- about the highest we've eer seen for a pump gas engine." --Marlan Davis

Also that "aluminum heads can go 1/2 to 1 full ratio higher cushion." --Marlan Davis
The whole article is insightful, so pick it up, if you can. The last thing that I will mention is that there was a caption of someone filling up 4.204 gallons of 100-octane fuel. The cost? $21.02. :eek --Bulitt

I'll defenitely pick up that magazine. I was considering having a dyno done on the car, I wounder if I should have the same shop do a compression test and help me tune it for pump gas.
Raising the Subject Once Again

I was reading back through a couple emails with the previous owner and ran across something that said he used Leaded Fuel. This is something he wrote back to me in an email.

"Leaded gasoline is the best fuel to use, but not necessarily the only fuel to use. I used high octane unleaded gasoline with very good results. As long as the octane level is high enough (about 100 octane) burning unleaded gasoline should not be an issue."

I thought that an engine that required leaded fuel would not run at all on unleaded fuel. Can someone set me straight so I don't foul up this beast of an engine? I've refilled the car twice with Sunoco 94 octane and a bottle of octane boost and it has run fine, but if the engine needs leaded fuel then that's what I want to give it.

In my way of thinking , old motors that were built to run on leaded fuel, will eventually be damaged if you run unleaded fuel without a lead treatment in them. All new motors changed to new valve components and configurations when unleaded fuel came onto the scene.
What I would be looking to find out definitly , about your Motor is whether its had the valve seats upgraded , so the motor can run on unleaded. I guess Id just add lead treatment to all fuel until I knew for sure.
I would be confused by the previous owners remarks about it also??Sorry, I saw no conclusive answer to your very important question.

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