100LL av gas in an etype?

I run 100LL in a few different things around my house for a couple reasons 1. It does not tarnish so I run it in all my small engines for the last tank of the season so everything starts easy in the spring and 2. Because I’m literally 100 yards from our local airport and the fuel business will fill containers or cars for me anytime!
Now to the meat and potatoes. I’ve always struggled to find fuel that the etype liked. I have the 9:1 pistons so the manual recommends 97 octane or higher. The 100LL meets that and has some lead which is also good in my mind. I’ve been adding a lead substitute to my fuel previously. After doing some reading on JL it sounds like others do run the 100LL but the car will run too rich. Does anybody have advise on what changes need to be made in the SU carbs in order to run the av gas? Maybe just a starting point? I checked plugs and have confirmed it is quite rich looking initially but haven’t drove it with the fuel so can’t really tell yet. Thanks all you guys have helped a ton already what a great wealth of knowledge on this site!

This will be an interesting discussion. My understanding is that the only difference in the Av gas will be octane, its resistance to predetonation. I would think the air/fuel mix ratio will be unaffected and you would not run richer. ie I do not think there is more energy contained in AVgas per litre compared to car pump gas

Dennis 69 OTS

The manual recommendation for 97 octane or higher is based on the RON octane scale. This translates to 91-93 on the AKI or (R+M)/2 octane scale used in the US and Canada. Aviation gas uses yet another octane scale, though I understand that 100LL approximates to 100 on the MON scale, which would put it about 110 on the RON scale, or 105 on the AKI scale. All this and more about octane ratings can be found at: https://en.wikipedia.org/wiki/Octane_rating. As for what adjustments need to be made to your engine to use 100LL, I leave that to others more knowledgeable than myself.

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Aviation gasoline (AVGAS), 100LL, will have a higher energy content than an equal volume of Motor Gasoline, MOGAS if the MOGAS is a mixture of gasoline and ethanol. The ethanol in MOGAS has a lower energy content than the gasoline it replaces in a gallon of MOGAS. Of course, MOGAS without ethanol and AVGAS will have the same energy content. If I did the math right MOGAS with ethanol has about 97.5% the energy content of AVGAS.

One difference maybe worth considering is the vapor pressure of AVGAS vs. MOGAS. MOGAS has a higher vapor pressure than MOGAS. That means MOGAS would be more prone to causing vapor lock than AVGAS; good news if using AVGAS in a car. On the other hand the lower vapor pressure of AVGAS might make it harder to start the car. That’s theory, whether the difference is enough to be noticed in everyday usage is something else.

Because of differing specific gravities, 100LL will run richer, on the same needles.

At low altitudes, a mixture of the two should suffice to address pinging issues, if all else fails.

Wiggles after reading back it was your comment that got me thinking about this as it sounded like you had some experience with it. I like the idea of running a mix and calling it good. I don’t have scientific proof of ethanol causing issues but I have had to take carbs apart on a lot of motorcycles and small engines every spring because of bits sticking like someone glued them together and never had the problem with non ethanol fuel. The more I think about it I think making changes to adapt carbs to the richer mixture would be a bad idea as I do hope to travel some woth the car and won’t have access to 100LL once I’m away from home.

Significantly so? Or insignificantly?

FWIW…perhaps nothing…in the course of my work I often run 100LL in old Corvettes and the like and have noticed no mixture problems. But…these are cars that seldom driven. Just keeping them running at all is cause for celebration so a slight mixture problem would go unnoticed by me. :slight_smile:


Experience vary. I have no such problems with E10 with my yard tools, genertor, etc.

As time passes I really, really have to wonder if other variables are at play here. Perhaps different additive packages, varying from region to region?


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Quite significantly, in my experience.

When I’d run Tweety on the track, mainly in order to prevent any detonation, and to run a bit more advance, Id have to lean the SUs about a full turn, to get it to the track. Otherwise, at low loads and speed, it would run quite rich: at the track, I’d richen it back up about a quarter turn.

I also used it in my rental fleet of FVs. No “driving to the track”, in those, but in the Vees–a dead stock engine, in the main–would also exhibit the same characteristics.

Needless to say, at 7:1 compression ratio, Vees don’t require the octane, but rather, the slower flame front speed, and the better overall consistency of the 100LL.

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We have 7 pieces of yard equipment with 2 stroke engines. Like you I’ve had no problems with using E10 in them. They always start after a winter layup. The 2 stroke oil I use claims to also stabilize the gasoline.

I will say I haven’t had an issue in a 2 stroke but only 4 stroke motors but I’m sure that has to do with the oil mix. Not sure it has an effect but it often gets to -30 to -50f where I live ( I know I’m not sure why I live here either!) maybe the air is so dry at that point that it effects it but everything I own is indoors and heated at that point. I will say the car seems to start so fast with it that I half the time think I hadn’t even pushed the button yet! Just an observation. I haven’t driven this car in 10 years so no comparisons are valid based off seat of the pants.

FWIW…I an 5 gals of 100+ racing fuel in the E about 6 months ago. It started fine and I have to say, it seemed to me to rev easier. The exhaust pipes were the nice grey/gray that we used to see long ago, and I even posted a pic of them. I didn’t notice and need to adjust the mixture. The stuff is expensive though!!

I run avgas in all my old cars. These old cars like the lead for lube on valve guides. My 1965 E and Mk2 run perfect on it, do not run rich, exhaust pipes are nice grey color. One can smell the different smell of avgas, so no hiding it.
And it does not go bad from sitting nor gum up anything.
I live on a private airport, so makes it easy to fill up the cars.


Ah, yes!!! Looooove that smell!

When I drive the JeepType, at Bonneville, Ill use it just to please the onlookers!


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Love the smell as well! Especially when I’m mowing the grass! :joy:

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LL100: Delicious smell for a gearhead, second only to Aero Kroil :slight_smile:


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You guys who sniff tail pipes worry me a bit…

But why not go all the way and put some bean oil in there? Castor is gearhead perfume!
Great stuff, but like good whiskey prolonged use can have side effects. Stuck rings and heart valves…

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Ahead of ya, Davey: been using this in the Jag and the Rover, for years!

I ran across this and thought it might be of interest, especially Myth #5.

Luck that you have it because in New York it’s been illegal.
Check your timing!
You will notice a huge difference in burn and cleanliness out the pipe 10 btdc
After that you right side mixture screw
Two turns
Then reset your jets 2 1/2 turns down from flush
Your car will run like a Swiss watch on av fuel!
Just for giggles are you still on points?