Can I use 100LL in my homebuilt’s auto engine?

Today’s question comes from a Mr. R. Burger of South Africa. He has a V-8 auto engine in what, I assume, is a homebuilt aircraft. He was wondering if he can use 100LL in his plane without any negative effects on tanks and other fuel system components, spark plug life, engine timing and oil selection.


Today’s question comes from a Mr. R. Burger of South Africa. He has a V-8 auto engine in what, I assume, is a homebuilt aircraft. He was wondering if he can use 100LL in his plane without any negative effects on tanks and other fuel system components, spark plug life, engine timing and oil selection.

Let’s start out with engine oil selection. The most critical point here is to stay away from any product that has a 100% synthetic base oil. The Rotax 912 engine was originally approved for use with only synthetic-based engine oil. I have heard of numerous lead sludging problems when these engines were operated on 100LL and this synthetic product. Most of these pilots changed to a mineral- based heavy duty oil and the lead sludging problem went away. I recommend that you use mineral-based heavy duty oil like the Chevron Delo, Mobil Delvac, Shell Rotella oils or other brands that meet the same specs.

Engine timing is very dependent on the make and model of engine being used. On many new engines timing is electronically controlled and can only be changed by reprogramming the electronic controls. Many of these engines have knock sensors and will automatically advance the timing to take advantage of a higher octane fuel. On some of the older engines, the timing was retarded from best power so that the engine could operate without knocking at full load. On these engines, you would be able to improve your power and fuel economy by advancing the timing when the engine is operated on the high octane 100LL fuel. If you use both 100LL and auto fuel, you may need to retard the timing whenever you operate on the lower octane auto fuel. I would recommend that you “”play”” with your engine and see if you get a significant increase in power from advancing the timing. If you do with no negative side effects, you may wish to use 100LL or a blend of 100LL and auto fuel.

One of the advantages of blending a leaded fuel with an unleaded fuel is you get more octane than the average would suggest. For example, I mixed a 100LL with a 90 motor octane auto fuel. A 50/50 blend gave me a fuel with about 97 octane.

Once you get your ignition timing set for best power, you can play with the fuel to determine if some 100LL is needed and, if so, how much. Experimenting with knock in an air-cooled engine is dangerous because once it starts knocking, engine damage is not far away. In a water-cooled auto engine, you have more margin of safety. However you can still damage your engine if you subject it to severe knocking, so be very careful.

Use of a leaded fuel can have a negative effect on spark plug life. Most modern automotive engines have project core nose plugs that operate well above the burn-off temperature for lead. If you have a more retracted nose or colder plug, you could experience some lead fouling, especially if you operate at lower power settings. If you are getting some misfiring and your plug electrodes have a yellowish/green tint deposit, you are getting lead fouling. The addition of TCP should help eliminate the problem. TCP also may reduce the tendency for exhaust valve sticking, should your engine experience this problem.

Leaded avgas should not harm your epoxy-lined tank or any of the other fuel system components. The biggest change you will see is in the exhaust stacks. They are normally covered with a black substance when auto fuel is used. If you use 100LL, the stacks will have an off-white, almost amber, coating. In addition, you should take extra care to minimize skin contact with the used oil due to the lead content.

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 Visser@GeneralAviationNews.com.

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