The difference between 100/130 and 100LL

Some readers, including Noel Dennis, were confused when I made the statement in a recent column that 100LL is actually 100/130 (The definition of insanity: Finding a solution for 100LL requires looking at the facts, Sept. 7 issue).

Dennis writes that he’s been been buying 100LL for many years and thought that 100/130 was different from 100LL. He remembers buying 100/130 one time and it was green, not blue like 100LL. He also noted that the chart for adding TCP to the fuel calls for more additive when 100/130 is used than when 100LL is used.

I understand his confusion. A brief review of history may be in order. After World War II, there were generally three grades of leaded avgas available. The first was 80/87, which contained a maximum of 0.5 grams of lead per gallon and was dyed red. This was used mainly in the general aviation market. The second was 100/130, which had no lead limit, but usually contained about 3 grams of lead per gallon. It was dyed green. This was the highest volume product because it was used by almost all of the commercial aircraft at that time. The third product was 115/145, which had no lead content limit and generally contained from 4 to 5 grams per gallon with some batches containing more than 6 grams per gallon. The fuel was dyed purple. Its primary user was the military.

In the 1950s, the military began replacing its piston aircraft with jet aircraft. As the demand for 115/145 declined, the market for it went away. Of all of the piston aircraft engines produced, only one or two were ever certified on only 115/145. All of the other engines were certified on 100/130, but allowed higher takeoff power settings when 115/145 was used. All of the major oil companies stopped producing this fuel with almost no negative effect on the the aviation community. A few specialty fuel companies still produce 115/145 at a slightly higher price for use by air racers and other unusual applications.

By the early 1970s the avgas market had become so small that the major oil companies were finding it uneconomical to produce and market two separate avgas products. Their logic was that they had to produce a product that met the 100/130 spec, so why not just eliminate the lower volume 80/87 product? The only negative to using 100/130 in an 80/87 engine was the higher lead content. The solution was to offer a 100/130 grade of fuel with a limit on the amount of lead in the fuel. The oil companies found that they could still meet the 100/130 octane specification with only 2 grams of lead per gallon if they increased the severity of the refinery process and topped off the blend with toluene concentrate.

A new product and a revised ASTM D-910 spec were born. To differentiate the new 2 gram limit fuel from the old unlimited lead level fuel, they chose a new color, blue, for the product. It seemed too confusing to have a 100/130 and a 100/130LL, so they started calling the new product 100LL but the new product still met the same 100 lean rating and 130 rich rating octane requirements as the old 100/130 fuels.

It is interesting to note that with the old 100/130 fuels, most refiners would add lead until the fuel passed the 100 spec. Then the rich rating was almost always well above the 130 level. This meant that most fuels were around 100/137 when they were shipped. With the new 100LL fuels, most refiners add 2 grams of lead and then add toluene concentrate until they meet the rich rating specification of 130, so most new fuels are more like 104/130 fuels when shipped.

Also interesting to note: When the market switched from 100/130 to the new 100LL fuel, the number of knocking complaints went up significantly.