GAMI’s search for an avgas alternative

Recently, my wife and I decided to take a road trip to Texas to visit friends and fill up on good Cajun and Tex-Mex food. On our return, I stopped to visit GAMI — General Aviation Modifications, Inc. — in Ada, Oklahoma.

GAMI builds and sells a line of parts designed to improve performance for a variety of aviation applications. It is best known for GAMI injectors. The concept of the GAMI injectors is to even out the variation in air/fuel ratios for all the cylinders of an engine. This, combined with proper instrumentation, will allow the pilot to operate on the lean side to peak. I know many people think that operating on the lean side is operating on the dark side and can harm the engine. However, if done properly, it can be done with little or no problem. There are several advantages to operating on the lean side, but the biggest is that you can decrease your fuel consumption by a very significant amount.

GAMI’s latest project is to design an unleaded fuel that will satisfy the same engines that presently require 100LL. There are presently three approaches to finding a replacement for 100LL.

The first is that of the oil companies, who want to produce a fuel just like 100LL, but without the lead. It would have an octane of about 94, but would meet all of the other requirements for an 100LL fuel as listed in the ASTM D-910 specification. This is the very safe approach because it would be almost identical to the present product so should not have any problems in production, distribution, or in an aircraft. The downside to this approach is that many aircraft would require significant — and therefore very costly — modifications to operate satisfactorily on this fuel.

The second approach is that used by Swift Fuels and others, which is to produce an unleaded fuel that meets all of the requirements of the present ASTM D-910 spec for 100LL, but with no lead. This approach should produce a fuel that can be used in almost all present 100LL engines with only minor modifications — for example, hardened exhaust valve seats would be required at your next overhaul, plus a few others. The downside to this approach is that the product is all new technology, which will require a new or modified plant to produce the fuel. Also, there is always the fear of the unknown when a new product is introduced into the distribution system and aircraft engines out in the real world.

GAMI is using a third approach: It is blending a fuel, called G100UL, using existing components that will meet the octane requirements of aircraft engines, but may not meet all of the other test requirements of the present ASTM D-910 specification. In fact, a completely new specification will have to be written for this fuel. Hopefully the engine manufacturers will then be able to add the GAMI fuel spec as approved in their engines.

GAMI’s first step in this development process is by far the most impressive. Where some have depended on the use of lean rating octane numbers to tell the whole story, GAMI has set up a test aircraft engine to measure octane requirements under more realistic conditions. I have visited and/or worked in a number of engine test facilities in my time, but the GAMI test stand is by far the most impressive aircraft piston engine test cell I have seen.

GAMI’s Tim Roehl (in back) and George Braly in the company’s test cell.

GAMI officials can control almost all of the engine run perimeters, and their data logging is first rate. They are able to measure the in-cylinder pressure for each cylinder and display it in real time.

They are definitely using good science in their approach to developing a fuel. When I was there, they ran the engine up to max rpm and load. They then leaned out the mixture to raise the cylinder head temperature up to around 450°F. Very shortly, the leaded 100LL reference fuel, which has an actual octane rating of just 100/130, started to knock. You could tell the knock by the change in the pressure vs. crank angle display. When they switched over to their candidate fuel, they were able to lean out the mixture well past that of the reference fuel with almost no knock detected.

Is GAMI’s fuel THE ANSWER to replacing 100LL? Well that is a definite, kind of, sort of, well, maybe yes type of thing.

GAMI’s product can be made from commercially available components. The cost of these components varies widely with supply and demand, so a cost estimate is going to be very dependent on a lot of factors. However, I would guess that it could be produced and sold within maybe plus or minus 20% of the current price of 100LL.

So, are there any negatives for the GAMI fuel? The big one is the high aromatic content of the fuel. GAMI has a very large test rig with three fuel bladders, a number of fuel filters, pumps, valves, hoses, carburetor components, plus other generally used fuel system parts. The parts are all connected in a series and GAMI will run its candidate fuel through the system for many hundreds of hours to measure the affect of the fuel on the hardness and leakability of the components. Hopefully, this will find any problem areas before the fuel reaches the field.

The flip side to concerns about high aromatic content fuels is the octane bonus for aromatics. We have known for a long time that aromatics, like toluene, are very good at improving the rich rating for aviation fuels. This should help ensure their candidate fuel will meet the octane needs of almost every piston engine out there that is approved for 100LL.

So, back to the question about whether the GAMI fuel is THE ANSWER? Time and testing will help determine that. There is also the question of who will produce the product and assume the liability of marketing it into the field.

As always, there are some differences between GAMI’s fuel and 100LL. Will the higher aromatic content cause any problems? Hopefully GAMI officials will be able to solve the problems and find someone to produce and market their product, since they are not in the fuels business. But this is just another example of the cost that the GA community pays for all of those lawsuits.

So will they succeed? I do not know, but the fact that they are working on the problem — and especially the fact that they are using good science — should give us all a warm fuzzy feeling that the lawyers can’t take away from us yet.

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. pilotman says

    Leguest, you are correct about the political risks, however the amount of mogas used would probably be a small fraction of 1% of the total aviation fuel used.
    Of greater concern right now is the the $50 million being diverted from jet fuel tax to the road use fund under the pretense that truckers are buying jet fuel instaed of highway diesel fuel.  That is absurd.

  2. Leguest says

    Mogas is politically dangerous. Right now we can fight user fees using argument that we pay taxes when we pay for avgas. With mogas there are no aviation taxes. 

  3. pilotman says

    Tim, diesels are great engines, but not everyone wants to spend $500K+ for a new 4-place plane or $100K plus to retrofit a used plane. There will be a suitable 100LL replacement, but when and how much doesn it cost? You are correct about the efforts that have failed, however I believe someone will succeed. in finding a suitable replacement that can be used in high-performance, turbocharged piston engines. 

  4. Tim says

    I think that these guys are just wasting time. 100LL is a dead end and replacement is not coming. Some people have been working for 20 years on that and then retired without result in sight.

    We are moving to diesels folks. Diamond with Thielert did it first. Now they have Austro diesels. Cessna’s new 182 is turbo-diesel. Continental just announced that they will be developing the line of aviation diesel engines. Drones that military are using also have diesel engines. So, pretty much what we can see the beginning of the end of 100LL and engines that run on it. Diesel engines are better as they consume less fuel, work better at altitude and do not have mixture controls. Fuel is also available all over the world, it is cheaper than 100LL and has no lead. FBOs do not have to do anything special to get it as well. Most of them already sell Jet-A which is diesel so hey, why not run current engines to their TBO and replace them with diesels or switch to autogas.

  5. pilotman says

    No matter what the replacement fuel ends up being, when the FBO’s add their margins to it , the price will be higher than today. Most places mark up the fuel $1.00 to $2.00 per gallon, since there is no competition at most airports, and fuel profits are the main means of support. That is why some self-serve places without big overheads have prices where you can save substantially. If mogas was the main fuel at an airoport the price wouldn’t be close to the price on the highways. 

  6. Mark Wiley says

    However, I would guess that it could be produced and sold within maybe plus or minus 20% of the current price of 100LL. Biggest problem with this statement is that 5.25 to 7.50 a gallon for 100LL still isn’t anywhere near 93 AKI E0 auto gas, so if the powers to be want to make a ‘universal’ fuel, this is going to be the stickler for low compression ratio engines. 

  7. Kent Misegades says

    What’s taking them so long, Ben?  Same with Swift, why is this so hard?

    In the meantime, we know that mogas can power all aircraft. 80% are already covered by an STC or TC.  The remaining 20% could use the new INPULSE water injection system from Air Plains of Wellington, KS.   Why don’t the alphabets support Air Plains in addition to GAMI/SWIFT ?

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