Confusion surrounds Motor octane rating

Earlier this year I gave several state IA renewal seminars. I always enjoy these because I get to greet many old friends and find out what is going on in the industry. I also get a lot of excellent information from people who are actually doing the work, along with some great questions.

One of the questions was a version of one I receive at almost every session: “Why can a Rotax with 9:1 compression ratio run knock free on 91 R+M/2 auto gas and a 8:1 compression ratio Lycoming need 100LL with an R+M/2 of around 104+?”

I have addressed this before in this column, but I thought I would try to shed some more light on the subject — or maybe just confuse people a little more, because we all know that there is no shortage of confusion on this subject in our industry.

The major problem is that the octane of a fuel is more of a characteristic than a property. If you measure the freeze point of a fuel and it is -60°C, you know that above that temperature the fuel will be a liquid.

But if you measure the Motor Octane of a fuel to be 100, all you really know is that in a CFR knock test engine, the test fuel knocked at the same level as a 100% Iso-Octane fuel.

When the knock test measurement procedure was first developed, all we had was the Research Method. But engineers have a bad habit of not leaving things well enough alone, and several engineers went out and compared full boiling range fuels with different Research octanes in real world autos. They found that there was a very poor correlation between the Research Octane number and how the fuels performed in the field, so they ran a large program to improve the relevance of the octane test to the real world.

They found that if they increased the CFR engine test from 600 rpm to 900 rpm and increased the intake air temperature, it greatly increased the correlation to the real world. They called this the Motor Octane rating.

But since the Research number was eight to 10 numbers higher, and higher is better, the marketers kept it.

Then with some minor changes, this method became the lean rating and was used in the aviation industry.

But as aircraft power increased with turbo and supercharging, the aviation industry, especially the military, felt the need for a better test. That’s why the supercharged or rich rating was developed.

As we look at unleaded aviation gasolines, everybody claims that all we need is a 100 motor octane fuel minimum.

A few humble observations if I might: First, when they changed from the Research to the Motor octane, the results dropped on average from eight to 12 numbers. Think about this: An rpm change of 300 rpm and an increase in intake air temperature changed the rating from eight to 12 numbers in the test engine.

The second observation is that the Motor or lean rating did not relate well to super charged aircraft engines and the rich rating does not work on unleaded fuels.

Third, the octane requirement of an engine is dependent on many factors, including engine bore, compression ratio, rpm, load, combustion chamber shape, head temperature, cam profile, ambient air temperature/relative humidity/barometric pressure, plus a lot of other little things, like combustion chamber and valve deposits.

At the present time, all of the 100LL produced is made from aviation alkylate, plus some additives. This means that the fuels are almost identical chemically, so the octane rating system works well.

But what is going to happen when companies produce unleaded fuels with very different compositions?

Life is going to get a lot more interesting — notice I said interesting and not necessarily better.


  1. says

    Confusion surrounds Motor octane rating

    • Why do they call this a ‘Motor’ octane rating – I would think a more accurate title to be ‘Engine’ octane rating.

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