I don't know where this idea came from about Avgas being for low compression, low rpm, but its common. It may come from some confusion on the different rating methods and labelling. In the US, octane for road gas is the average of the motor octane and the research octane tests. Whereas octane for Avgas is given as lean mixture and rich mixture tests.
But comparison is relatively straight forward. The first or only number on Avgas is the lean mixture octane. Shell states "
Avgas is measured on Lean Mixture (similar to MON)" and the second number represents octane tested under rich mixture conditions.
So 100/130 as well as 100LL are roughly comparible to a road gas with MON of 100. In other words, 100LL has a MON roughly comparible to a race fuel with 100 MON.
Interestingly, there's a new grade of 94UL Avgas being tried out. It's supposed to be based on the 100LL but with no TEL and has a Lean mixture or Motor Octane of 94. This is intended for lower compression aircraft that used to run lower octane Avgas that now is difficult or impossible to obtain.
In fact 100LL is required for high compression and supercharged aviation engines, whereas a lower octane fuel is all that the majority of prop planes need. But a lot of companies like Shell aren't making the other grades anymore.
As far as what has happened to avgas with the demise of the large airline a majority of the large aviation freight haulers switching to either jet engines or turbine powered aircraft is the old low octane avgas went bye-bye many years ago.
I owned and flew a 1960 Comanche 250 that had the engine switch to a newer 1965 260 HP 540 C.I. opposed flat six cylinder air cooled Lycoming engine after the original engine blew a oil line in flight damaging that motor before I bought that plane in 1991. Both of those motors had 8.5 to 1 compression ratio
The 250 HP motor was limited to 2550 RPM and the newer higher HP 260 motor was limited to 2700 RPM max.
I flew that airplane around 900 hours and it required 100 LL, I learned on the second flight to lean it out some before flying it, not take of at full rich
, leaving Bullhead, AZ airport to return home to Hesperia, CA
It foul out the both spark plugs on one cylinder shortly after take off scaring me a lot
I had bought and used aviation fuel in several of my race cars back when I could still buy 130/145 and later to 115/130 Av gas at the local airports. I stop using it when I found pits on all the exhaust valves in my 415 M.W. stocker motor in the mid 1970s
Their are two major manufactures of piston aircraft motors being used in todays small aircraft, Textron Lycoming and Continental.
Both of the Piper airplanes I owned and flew required 100 LL as the fuel in them per the operator hand book, most Continental powered aircraft require 85 octane which hasn't been available for sale at most airports in the U.S since around 1995 or earlier
A lot of those Continental powered airplane owners have apply for and received permission from the FAA to use automotive gas in them after doing some modification to meet the FAA specs to get permission from them to use it.
Some of those owners and pilots still buy the 100 LL to use in their airplanes and they had to learn to lean the motors out properly to safely operate and fly them with that fuel
100 LL has a lot longer shelf life than any of todays pump gas or race gas, but it has it limitations.
That is my main message, be careful of what fuel you use and watch the plugs and tune the motor to the fuel your using just like I had to do when I switched to E85 in my current bracket motor with 14.8 to compression ratio
