Aviation Gas

Q: Aviation Gas

I have an HO engine which requires premium gasoline. Now that gasoline has lower octane rating and no lead, I want to possabley use Aviation Gas. Doesn’t AVGas burn pistons over time? What about Cam 2 or 3? I’ve heard Sunoco has a higher octane fuel for this application.

A: You may have wanted a simple ‘yes’ or ‘no’ answer, if so: ‘no’. But since I have done a fair amount of research/experimenting with gasoline and find it a very interesting topic, here goes!

AV gas itself will not burn pistons. But this is not to say you cannot burn a piston with AV gas. AV gas is higher octane than typical pump gas but it is typically formulated to burn at higher elevations. Also, the vast majority of aviation gas is unleaded or “low lead” (nearly unleaded). As with any fuel, you need to jet your carburetor (or fuel delivery system) to correspond with the specific gravity and other characteristics of the particular fuel. Simply adding higher octane fuel to a properly tuned car will rarely increase performance, any usually degrade it. The reason you can make more horsepower or go faster with higher octane fuel is that you can tune your motor to take advantage of the particular fuel. That is, you can run higher compression and more advance with higher octane and, thereby, make more power.

If you’ve even seen Chevron’s commercial where they say something like you can make more power with their Supreme gas, you will note A LOT of disclaimers at the bottom of the ad. Basically, this only applies to cars with fairly sophisticated engine management systems. That is, ones that increase ignition advance until the sense knock and then slightly back off. With this sort of engine management system, a higher octane fuel will actually more the car go faster. While is seems counter intuitive, a typical engine will make more power using the lowest octane possible fuel (lowest so as to prevent detonation, that is). Once you exceed this level, performance will actually drop off. Many people will argue this, and may even produce time sheets showing better ET’s, but when driving on the street, there are far to many other variables. You will not see an improvement on a dyno, only a decrease.

Another concern is that AV gas is not taxed for use on the street and thus using it on the street is illegal. No FBO will pump AV gas in your car. While they will pump it in drums and you can transfer them to you car, it will still be illegal to use it on a public road. Should be o.k. on a race track but you will be far better off buying a race fuel specifically formulated for your application.

AV gas used to be a more viable alternative, but still not the best choice, when it was leaded. This is because it got most of its increased octane due to the addition of tetraethyl lead (“lead”). When mixed with pump gas or even real race gas, a significant increase in octane would occur, thereby allowing the use of higher CR’s and more spark advance.

Contrary to someone’s earlier post, there is no magical compression ratio cut off for running on pump gas. Your engine’s octane requirements are dependant on far more variables that just CR. Squish band, combustion chamber shape and layout, timing as well as even the material of your head (cast iron vs. aluminum) all make a significant difference. There is no reason you cannot set up an engine to run 10:1 on pump gas (I believe Jim Hand has done this with his wagon but an not 100% confident on this and too lazy to check right now!!). Obviously there WILL be an upper limit but it is unique to each combination. I probably would not encourage someone building an “average” street motor to go much higher than 9.25:! but this is based on the complexity, care and expense necessary to set up an maintain a motor running a higher CR on low octane fuel. Also, there is not that much power that will be gained going from a 9.25:1 to a 10:1 CR.

Sunoco does sell their GT100 fuel in some areas (check out www.racegas.com). This site also lists specifics details on their race fuels as well. VP also makes a variety of high quality fuels. If you are looking for high end race fuel (gasoline, not nitro methane) you can also try Elf or Nutec.

Fuel technology is incredibly complex and very interesting (at least to me)! If you go to a GP (car or bike) you will notice that the top team have fuel technicians that ‘brew’ fuels for not only each track but to correspond to the environmental conditions change through out the weekend.

A: As a Petroleum Engineer I can only say, “very informative and well said.”

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