How to prepare for the end of 100LL

How should aircraft owners prepare for the switchover from 100LL to its replacement? In a previous post, I stated that if — or when — 100LL goes away, the only fuel that will be universally available for general aviation will be one that is chemically very similar to 100LL, but without the lead, and an octane rating of around 94.

If you have a low compression ratio engine qualified on 80/87 or mid-90 octane fuels, you really do not need to change anything. The only concern is that when the engine is overhauled and broken in on straight unleaded fuel, there maybe a problem with exhaust valve recession. If I owned one of these aircraft, I would wait until 100LL is about to go away and then buy a barrel. Then after I had my engine overhauled, I would add a gallon or so of 100LL to each fill-up during the break-in process. The only problem here is that it may not be legal to store a drum of fuel in your garage, so you will need to look into the legal and insurance aspects of storage or find a place where it can be stored.

If you have a high compression ratio engine, you have a more difficult decision to make. This is like trying to solve an algebra problem with three equations and four or more unknowns. The good news is that if you have a common Lycoming or TCM engine, you can be confident they will develop a system to qualify most of their engines on the new fuel. This will probably also be true of common engines like the P&W R-2000, R-2800 engines and several others. The bad news is that the new systems may be expensive and may require some de-rating of the engine.

Another factor to consider is that we do not know when — or if — 100LL will go away (remember there is an election this year and another big one in 2012). So what is the best plan? A lot depends on how many hours are on your engine, what kind of condition it is in, and how much you fly. If you have a low time engine, just keep flying and see what happens. The longer you can delay the decision, the more information you will have when you finally need to do something.

If you have a mid or high time engine, I would baby it as long as possible. If you do need an overhaul, check with the manufacturer or your rebuilder to see if there is a system or modification that can be fitted to allow operation on a 94 octane fuel.

If you have a rare aircraft with a high compression ratio engine or an engine where the manufacturer is no longer in business, you have the same concerns as the previous owners, but with another variable to consider. For low and mid time engines, just keep flying. For high time engines, I would start doing some homework and looking for STCs to replace your engine with a more common model or brand that will probably be able to be modified to run on a new fuel. However, there will be a few models or cases for which there is no good option. For example if you own a DC-3 with an 1820 CW engine, you may need to consider replacing the aircraft.

The best advice is to procrastinate. Wait as long as possible. During the wait, spend time learning about your options and the cost and advantages and disadvantages of each option. The more you know, the better your chances of making a good decision when the time comes.

But remember, when 100LL is finally done away with, there is going to be a mad scramble for new engines and systems, so timing is going to be critical.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at


  1. says

    Kudos to you for being one of the first to recognize what the REAL concern is about switching to unleaded fuel – exhaust valve recession. This IS the real problem with running air cooled engines on unleaded fuel that nobody wants to mention. Worse – there simply is no fix. Without a sufficient and regular supply of lead in the fuel the exhaust valve will beat the crap of the seat – causing rapid diffusion and micro welding due to the high (~900F) temps that the exhaust valve and seat run at in an air cooled engine. There is no known material other than a dissimilar metal (lead in the fuel) that can prevent this problem when operating at the temps of air cooled engines. This is why there all air cooled engines on the road went away right around the time that auto fuel went unleaded.

    The ONLY solution is to run the seat and valve much cooler (~350F) and that can only be accomplished by water cooling the head. I’m sure there will be plenty of people that will argue that point but trust me – there is a LOT of dyno data to support this and in the end – everyone will find out for themselves once the lead goes out of avgas. You can expect about 500hrs before the exhaust valve seat erodes enough to take up the valve lash and then burn the valve because it can’t close properly.

    Whether anyone wants to admit it or not – we are flying 60+ year old engines – they just happen to be recently manufactured.

  2. Mauno Silpala says

    I have been following GA for about 15 years, part of it as a pilot. I cannot help but be dismayed at the amount of hand-wringing and wailing about how the end of 100LL would mean the end of GA, while not much was done about finding a substitute. Sure, there were studies about it, but they all came to the conclusion that GA simply has to continue the lead pollution ad infinitum. This despite the existence of the Mo-gas STCs that would have allowed most GA pilots to break their “lead habit” and switch to a less expensive fuel eons ago.

    There has been more hand-wringing about how E0 Mo-gas is not more widely available. I have to believe that, had the industry (AOPA/EEA & engine manufacturers) used its formidable clout to push that issue, instead of prolonging our dependence on 100LL, there would me Mo-gas at every FBO.

    Even the automotive industry initially fought Ethanol but, facing the reality, adjusted and today’s automotive engines run just fine with gasohol, with more and more power being squeezed from small engines. I understand aviation’s need for greater reliability, than what cars can live with. Again, eliminating problems with gaskets & hoses or whatever reasons are given for aircraft engines not being able to operate with E10, would seem a smaller problem to solve than looking for a new fuel once again unique to aviation, or completely new engines, which would be even more expensive.

    Speaking of new engines; as I recall, Continental developed a new engine on Uncle Sam’s (& my) Dime. It was supposed to run on Jet-A or diesel and be simpler and less expensive than the existing 100LL-burning models. The prototype was on display in Oshkosh about ten years ago; some of you may have seen it there. When is that going to start rolling out in numbers? Don’t hear much about that these days…

  3. says

    For many current users of 100LL, there is already an excellent, legal (through EAA/Petersen STCs), safe and affordable fuel available at many locations in the U.S., namely ethanol-free premium (91 octane) Mogas. While you won’t find it at more than 100 or so airports, a number have added it recently, which is a good trend. Pilots are not the only ones who need ethanol-free fuel (E0). Boaters, ATV & snowmobile owners, classic car/bikers and others all need the fuel, and gas stations are starting to take notice. See for a list of over 1200 gas stations and marinas that have E0. If you know of one not on this list, please add it. It is hard to argue against $3/gallon fuel, regardless what happens with 100LL. See also for more on this subject. Note also that nearly 100% of the new LSA fleet are powered by engines designed for premium, ethanol free unleaded autogas, not Avgas. Attend our talk on the subject on 7/27 at 11:30, Dake I Pavilion, at Oshkosh Airventure.

Leave a Reply

Your email address will not be published. Required fields are marked *