Common misconceptions about unleaded avgas

Whenever an article on unleaded 100 octane avgas is published, a few of the same questions always come up.

For instance, many readers will comment that automobiles switched from leaded fuels to unleaded fuels with no major problems, so why can’t that be done with avgas?

Well, they didn’t really solve the problem, it just sort of went away.

When I started at Shell in 1967, emission controls on automobiles were just starting in California and went nationwide in 1968.

At that time the EPA announced the lower limits for the next number of years. It was obvious that by 1975 cars would need catalytic mufflers in order to pass the limits set by the EPA. Since leaded fuels poison catalytic converters, unleaded fuels would be needed.

In 1970, GM announced that, starting in 1971, all of its models would run on 87 R+M/2 octane unleaded fuel. Except for a few rare models, such as the Chrysler Lean Burn cars, all of the automobile manufacturers followed suit.

Then for 1975 models, the EPA decreed that unleaded gasoline should have a smaller diameter pump nozzle. All catalytic converter-equipped vehicles had to have a restrictor in the fuel fill neck that had a hole small enough that the new unleaded fuel nozzle would fit into it, but the larger leaded fuel nozzles would not.

Oil companies continued to market leaded fuels until 1988 when the EPA banned leaded automotive fuel altogether.

Part of the reason behind the ban was because people were buying the higher octane cheaper leaded grade of gasoline and using it in their catalytic converter-equipped cars.

I remember going into convenience stores where they sold adaptors that featured one end that fit over the larger leaded fuel nozzle and the other end was a 4-inch tube designed to fit into the restrictor plate in the vehicle.

The bottom line was that leaded auto gas was available for 17 years after the car manufacturers started building cars with lower compression ratios and hardened exhaust valve seats, so that all post-1971 vehicles should not have exhaust valve recession problems.

In addition, they almost all had liquid-cooled heads versus higher operating temperatures found in air-cooled aircraft engines, as well as lower operating loads at cruise.

Then after 17 years, assuming 10,000 miles a year, if the car engine had a burnt valve, it was worn out and needed replacing anyway.

Also, if a car had a mechanical issue, it could just coast to the side of the road and then be towed to the shop and no one would even notice.

An air-cooled aircraft cruising at 70% load at 8,000 feet AGL is much more likely to develop engine roughness and loss of power. If that occurs over a highly populated city, it could create a very serious problem that will be reported on the evening news nationwide.

An additional factor of comparing automobiles and general aviation aircraft is the replacement rate for cars versus aircraft. If you look at cars in the parking lot of a mall, you will find very few 20-year-old cars. But if you go to an airport, you will find very few aircraft newer than 20 years old.

So the automotive world had a 17-year overlap of leaded fuel until only unleaded fuel was available and they only had minor problems with exhaust valve recession.

In the aviation world they are planning a cold turkey replacement of 100LL with an unleaded fuel to be used in all general aviation aircraft, including orphans and older aircraft, and do not expect any problems. What could go wrong?

Another comment I hear is that aircraft owners think it will be great to go to 100-hour oil changes with unleaded avgas.

But you can do that now. The University of Illinois has been running 100-hour oil changes for years as long as the school’s aircraft are run at least 30 hours a month.

One of the issues we have is that manufacturers specify oil changes either every 50 hours or every four months.

But many pilots do not remember the part about every four months, then wonder why their cam is flat when they only fly 20 hours a year and change the oil only every two or three years.

That leads to a worst-case scenario: What will happen when aircraft owners only change the oil every five years on unleaded avgas?

The third comment I hear is that 100LL is the only leaded fuel still in use in the world, followed quickly by the fact that there is only one plant in the world that produces the tetraethyllead (TEL) that is added to 100LL.

First, take a look in the VP Racing Fuels catalog, where you will find a number of leaded fuels available.

And then there is the concern about something terrible happening at the one plant in the UK that produces TEL, such as a fire or explosion.

Before I retired, I was at an ASTM meeting and I asked the representative from the company that produced TEL at that time whether this was a concern. He said his company was aware of this concern, but were in contact with plants in other countries that could be brought on line in less time than it would take to use up the company’s existing inventory of TEL.

I do not know if this is still completely true, but if there is money to be made, someone, somewhere will make it.