I recently received a question on octane ratings similar to the questions I typically receive. The reader wanted to know what the rich and lean rating would be for 100, 110, 112, 114 and 118 octane racing gasolines. And what would be the R+M/2 of 80/87, 100/130, 100LL, and 115/145 avgas?
The problem here is that the octane rating of a fuel is not a physical property of a fuel, it is a performance property. It is like someone asking “What is the horsepower rating on a Lycoming 540 engine?”
The 540 is a physical property of the engine because as long as it has a standard bore and stroke, the engine will always have that cubic inch displacement.
But the hp rating of the engine depends on compression ratio, turbo charger, gear reduction, fuel injection, general engine condition, air temperature, altitude, etc.
With octane ratings, you can get a general idea, but there is not really a direct conversion. To start, the race gasoline could be a research octane number or an R+M/2 number.
If it is the R+M/2 octane, then you must guess as to the sensitivity of the fuel. The sensitivity of the fuel is the difference between the research and the motor octane number. Typically, normal fuels have a sensitivity of around 8 numbers.
For example, a 100 R+M/2 fuel would have a research octane around 104 and a motor octane around 96.
This is a generalization for typical fuels, which may not be correct for all fuels.
For example, the reference fuel iso-octane has a sensitivity of zero, while I have seen non-typical fuels with sensitivities over 12.
If the 100 octane rating fuel was fairly typical, then the motor octane could be correlated to a similar lean rating. But this correlation was developed for typical alkylate avgas blends and may not be accurate for a non-typical race gasoline.
So, a good GUESS is that the 100 octane race gas would have a lean rating of about 96, unless the 100 number was actually just the research octane of the fuel, in which case I would GUESS that the lean rating would be closer to 94.
The lean rating of the race gasoline is just a guess. The rich rating would not even be a calculated guess. It would be more just “pick a number that you like and go with it.”
The problem is that the rich rating was developed for leaded alkylate-based fuels. When the test is run on unleaded non-alkylate fuels, the answer can come out almost anywhere.
And the results are more or less non co-relatable to actual aircraft engine requirements.
For example, I have seen a 96 rich rating unleaded fuel run knock free in an aircraft engine where a 107 octane rich rating unleaded fuel resulted in significant knock. That’s why I will not even guess as to the rich rating of the different race fuels.
Now going from avgas rating to R+M/2 is a little better. Typically, an 80/87 fuel correlates to an 80 motor octane, so with a sensitivity of 8, it would have a research octane of 88 and an R+M/2 of around 84.
But 80/87 is just a minimum octane. Most refineries make 80/87 by blending aviation alkylate that is used in 100LL, but reduce the lead level to around 0.5 G/gal. So the R+M/2 could be in the mid to upper 90s.
100/130 and 100/130 low lead usually have a motor octane in the 101 to 105 range and an R+M/2 of around 106 to 110. This is based on a limited amount of test data I have seen and is not an absolute level.
The octane number of a fuel is basically its resistance to knocking in an engine. The test for research, motor and the lean ratings are run in a Cooperative Fuels Research (CFR) test engine under standardized conditions.
The CFR engine is designed so that the cylinder and head assembly can be raised or lowered relative to the crankshaft position, changing the compression ratio. When the knock level reaches a prescribed intensity, then the compression ratio at that point correlates to a given octane number.
The rich rating uses plant compressed air to supercharge the engine and the pressure and air/fuel ratio are varied and this correlated to those of known octane reference fuels.
Octane numbers are good in that they predict how a typical fuel will perform under certain conditions. But they may not be accurate for other applications or conditions.
It is like a turbo-charger. If you have the correct turbo and control system, it will work well on, say, a TIO-540. But that does not mean you can change the turbo, the control system, or even the compression ratio of the base engine and then expect it to run perfectly.