So you like the idea of auto gas….

Builders,

Perhaps the second most common question I get about Corvair engines is: “Can I run one on car gas? ” The answer is yes, but like all engines that can, it comes with some limitations.

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The engine itself isn’t bothered by car gas, even gas with 10-15% ethanol in it. As long as the fuel has sufficient octane and is kept fresh, the motor will not have an issue. However, as I have pointed out countless times in the last 15 years, your whole fuel system, from the gas cap to the tip of the exhaust pipe, must be tolerant of the additives and ethanol they put in auto gas these days. This isn’t an imaginary issue, get a look at the pictures below.

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Above, a fuel petcock and screen from a 2001 motorcycle I own. you can’t see it in the picture, but the screen is so clogged that it flowed zero fuel. This is the result of leaving 93 octane car gas in it for 6 months.

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I usually try to run small engines, boats and motorcycles I own on what is referred to in Florida as “Marine gas” which is lead, additive and ethanol free, and about 90 octane.  It can be stored a long time, and it doesn’t attack parts like carbs, fuel lines and primers.  It is slightly low on octane for Corvair flight engine with compression ratios of  8.4 :1 or higher. For those engines 93 octane is the requirement.  It is a very clean burning fuel in comparison to 100LL. Contrary to popular myth, lead is not good for valves, and the byproducts of combustion of leaded fuel are highly corrosive in the presence of humidity.

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Corvairs were designed in an era of leaded auto gas, and consequently have specifically designed exhaust valve rotators available. Lycomings use the same system to protect their exhaust valves. They are very cheap because they are the same part as a small block chevy V-8. We use these on every Corvair, and because we also use very high quality exhaust valves, Corvairs can run 100LL fuel with a minimum of valve issues. No modern car engine comes from the factory with a valve train designed to run on 100LL, but our years of working with Corvairs and the fact that we overhaul and modify every Corvair head and valves allows them to be optimized for the aviation environment.

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Corvairs can effectively use both 100LL and auto fuel, but it does take builders doing their part and making sure their complete fuel system is compatible with the fuels they intend to use. When salesmen are pushing engines, or people are writing magazine articles, this is often glibly glossed over, but to be the master of your own aircraft, you have to put in a little more understanding than that. Fortunately my whole Corvair program is based on the idea of builders learning things, not just buying them.

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Above, the screen broke to bits at the slightest touch. It could have easily done this in the tank. Before using auto fuel with additives, a builder has to seriously consider every single piece of his fuel system and evaluate if it is ready for prolonged exposure to the things they put in gasoline these days. This can be done, but it means using things like fuel lines intended for exposure to harsh additives. By comparison to automotive gasolines, 100LL is a very mild substance.

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My personal preference for a Corvair flight fuel is lead free, ethanol free fuel without the additives that make it go stale in 6 weeks like most auto fuel has. For a Corvair with typical compression ratio, this fuel needs to be 93 octane for reliable full performance. I intentionally built my own 3,000cc Corvair with lower compression to allow it to use lower octane boat gas. There are many good strategies available to custom tune your Corvair build to a wide variety of fuels, and our builders have long taken advantage of this to intelligently utilize the fuel of their choice.

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Please note this is a very different from the dope who just pumps 87 octane fuel in his aircraft tank because its “cheap.” 30 years ago you couldn’t have convinced me there were such people in aviation, today when I trying to be optimistic I try to believe they are not the majority. Either way, you are in charge of your own life, and you have the option of not being one of that herd.

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wewjr.

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