Fuel Facts: Why Not Avgas in Your Car?

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This isn’t a new idea: Why not run aviation fuel in your classic? Thanks to its high octane and lack of ethanol, this avgas sounds intriguing. 

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Summary: 

If you have a cat or an oxygen sensor on your car = maybe bad idea

If you don't = very good idea (other than the legalities / emissions of course) if your car can use the octane.

One aspect not covered is that Avgas is FAR more stable then road gas.  I had 100LL in my Ghia for many years, and it does not turn to turpentine like road gas does (at least not nearly as fast). If I had the option to put Avgas in my car, I certainly would. But of course, it's pretty much an aviation engine anyway.

Using leaded fuel is illegal on the road (and should be illegal everywhere) and irresponsible. The rest of the discussion is pointless, except to mention (as the article failed to) that unleaded racing fuel addresses any legitimate fuel needs for classic and competition vehicles.

Brian, lighten up.   Classic cars are hardly driven.  Same for drag cars.  They go 1/4 mile at a time, and some only 1/8 mile.  Your comment "should be illegal everywhere" is (as you yourself said) is irresponsible.  You can't just put whatever you want into the tank of an airplane.  If your Lycoming or Continental was certificated for 100LL, and you put in some ethanol-laced unleaded auto gas, burn a piston or valve, crash and kill somebody, you wil be sued.   If you survive the crash, you will lose the lawsuit.   The very first thing the NTSB does when they inspect a crash site, is to check the fuel tanks.

TorqueNRecoil said:

... Your comment "should be illegal everywhere" is (as you yourself said) is irresponsible.  You can't just put whatever you want into the tank of an airplane.  If your Lycoming or Continental was certificated for 100LL, and you put in some ethanol-laced unleaded auto gas, burn a piston or valve, crash and kill somebody, you wil be sued.   If you survive the crash, you will lose the lawsuit.   The very first thing the NTSB does when they inspect a crash site, is to check the fuel tanks.

I didn't suggest putting auto fuel - especially with ethanol - in an aircraft. Unleaded aviation gasoline is readily available, there are certified aviation engines to use it, and the general aviation industry has had decades to adapt to fuel without lead. Aircraft are operated for a very long time, but piston engines need rebuilds every few hundred operating hours so every one out there has been rebuilt since the 1970's and they could all be safely and reliably running unleaded. There is no excuse.

TorqueNRecoil said:

Brian, lighten up.   Classic cars are hardly driven.  Same for drag cars.  They go 1/4 mile at a time, and some only 1/8 mile.

That's like saying that I only drive on a highway one in a while, and there's only one of me, so none of the rules of the road should apply to me... after all, what effect could just my one car have? The fact that both classic road cars and any competition vehicle are completely unnecessary - just hobbies and entertainment - means that they have less excuse than real working vehicles for pollution. And if Formula 1 can run on unleaded gasoline (which it does), your obsolete drag car burning leaded gas looks almost as antiquated as a coal-fired steam engine.

Brian_13 said:

 Aircraft are operated for a very long time, but piston engines need rebuilds every few hundred operating hours ...

The most popular general avation engine for many many years is the Lycoming 0-360.  It has a recommended TBO (time between overhaul) of 1500 hours (I know someone who pushed his, with careful monitoring, over 2000).  A rebuild is about $16,000.  Most general aviation planes are not used that much.  It can take a LONG time to run a 360 out.

Now, a Merlin engine (P51), which is definitely a high strung engine, TBO is 240 hours, (rebuild $60,000+).

I just buy the 100 octane unleaded race gas for my vintage race car. This has way less ethanol the other pump fuels.  Driving to the airport to fill up seems like a pain.

Avgas also has a higher vapor pressure and lower BTU content

I store my car with 100LL and drive it with E10 pump gas.  Would use E0 for both if it were available locally.  Besides the environment, my AFR sensor doesn't like lead.

We had one idiot that paid for avgas in his 60s Corvette. Of course he got zip as far as any additional power - it had ignition with mechanical advance and adding higher octane fuel without increasing advance can actually yield lower output.  I guess it got him some bragging rights among his equally dim-witted friends  ("Wow - jet fuel!" - wrong of course. If he'd wanted jet fuel - kerosene - it wouldn't have run at all).

The Experimental Aicracft Association wrote several articles in their pursuit to get FAA approval to use car gas in aircraft.   The article I'm thinking about was written in the 1980's so it might superseded by now.   The EAA says that ICE fuel is comprise hundreds of various compounds.  I think they implied that the diffeneces in fuels might be as differnet as we might believe.

The vapor pressure for av gas is less, not more than car fuel.  This is because airplane fly at altitudes with lower air pressure which lowers the biling point of gas (even water).  The article mentioned that the car fuels they tested were better controlled regarding octane.    Aviation fuels are stable over longer periods of time than car fuel.  This doesn'r mean that aviation fuel can't go bad at some point. 

100 Low Lead has something like fours the lead that the old 80/97 octane would have.  In the 70's I ran av fuel in my Falcon and the plugs got leaded over.   Not that the Falcon would ever lknow the difference!

Alcohold is not used in aircraft fuel.  Possibly Rotax aircraft engines are capable of safely using fuel bleanded with alcohol, I don't know.   Alcohol entrtains water at warner temperature and when fuel is coold, some of it precipitates out of the fuel falling to the bottom of the fuel tank.   This has to be drained out because , as cheap as water is, it doens't work well in airplane engines.   

Worst of all, alcohold can detiroate certain componets in the fuel system, cause leaks, swelling. or shedding of the rubber or rubber like parts causing the fuel lines to be plugged.  I don't know why it's such a problem for aircraft when every modern car seems to work well otherwise. 

Overhaul times can range from 500 hours to 24 hours.  However, how an airplance is used and stored between flights is paramouint.   THe worst offencred to engien longevity is disuse.   I used to instruct at a flight school in the SFO bay are and the owner commonly got 3000, 4000 even 5000 hours on Lycomings (aside from cylinder changes) that had 200 recommended times before overhauls.  Why?  The planes flew 80 to 100 hours a month.   Many pilots are lucky to get 100 hours a year of flying done.

Hope that helps.  

Good chatter on the topic. Since it’s a popular question, we figured we’d ask the expert. (For the record, I don’t run avgas.)

In reply to Goluscombe :

Well, sort of correct....

Many small aircraft have Supplemental Type  Certificates (STC) allowing them to run (ethanol free) automobile gas (MoGas).  The source of water in the fuel is not a function of ethanol, but rather natural condensation in the atmosphere, which introduces water vapor into the fuel tank.  Hence, the preflight inspection of every light aircraft includes draining a small fuel sample to check for water or other impurities.

Alcohol is not a problem in aircraft, because no fuel that is put in aircraft has alcohol in it... 

Aircraft used "for hire" as in a flight school are also required to have an inspection by a certified mechanic every 100 hours of use.  The typical "Time Between Overhaul" (TBO) on Lycoming, etc small engines is typically 1,600-2,000 hours.  Jets? The Rolls Royce RB-Series powered jets I've flown can get upwards of 28,000 hours of use before needing an "overhaul".  That's around 14 million miles, for comparison to the measly 3 million mile Volvo...

Dad used to run a mix of unleaded premium and leaded regular in his XKE, when they still had leaded regular in the pumps.  He had a 5 gallon can of Avgas in the garage.  I can't recall the ratio, but he would mix that with unleaded premium, 1:10 or 1:20 mix I think.  The lead in the regular would boost the octane of the mixture, and was apparently friendlier to non-hardened valve seats.  Burning booze in it was not great, the carbs did not like that at all.

The effects of running unleaded on old style 'soft' valve seats seem to vary depending on seat material used by the factories and use of the car.

I have several MGs that have not shown recession with extended use on original seats but I have also owned another British car powered by a 383 Chrysler than showed significant recession. I think that at least part of the difference was due to me using that car for long sessions of high speed highway work.

For those with old cars that came with alloy heads, the issue is not as dire as the seat inserts are hard to start with, though not to the modern standards, and see to last quite well.

The big problem for old cars used for touring stems from their lack of advance based on knock sensing. Old style distributors sit wherever you set them and if you get some ping on a highway trip, and if you actually notice it, you have to re-time it to suit.   I was barred from having modern ignition on my vintage race cars so went to using a knock sensor combined with a cockpit read-out and adjustment of advance that I could set during practice sessions - run advance up under load until knock is detected and then back off a tad. Better than nothing.

I had an old corvair  with a much higher compression ratio than stock and the only fuel solution that would keep spark knock at bay and give decent performance and fuel economy was to mix 100LL with premium fuel. The combo made for a decent running car and reasonable fuel economy albeit at the expense of pricier fuel. Still i was happy with the result and where i lived no one was gonna be checking my fuel for compliance.

I used 100LL for years as ethanol gas was not really compatable with old primative carberators, but found that even the LL variant left heavy lead deposits on my valve guides and eventually neaded a  tear down and cleaning , then locally a fuel called 91 rec was made avaible at a gas station and while it's costlier than regualr it's cheaper than AVgas and also has no ethanol, no lead and is very stable and non hydroscopic , It's my prefered fuel for anything with a carberator, a electroinc fuel injected car digests normal pump gas just fine