100 octane aviation fuel is not rated the same as fuel for cars. The aviation fuel uses a performance type octane rating system (remember red, green and purple aviation fuels?) while cars use the old R+M divided by 2, that is Research method of rating octane plus the Motor method of rating octane, then divide that answer by 2 to get the octane rating. Total and complete different octane ratings. Using the R+M divided by 2 method gives 100LL somewhere around (IIRC) 80-85 octane rating.
I never used 100LL in my race cars because aviation fuel is made for low compression motors running usually running less than 3000 RPM consistently, no on then off throttle movements like a race motor. My race engines had considerably more than 8.1 compression ratios and ran 8 to 10K RPM anywhere from 10 to 40 seconds, then totally off the throttle, feather.... then back to WOT, from 50 to 250 laps on 1/4 mile to 1.5 mile tracks. The race fuel I used was made for that application.
Now with that said, yes, we ran 100LL (sumped from filters and planes) in airplane tugs for years without problems, plus lawn mowers, tractors and other small engines. At the local airport the 100LL truck has run on 100LL for years, but it was made in 1973 and never goes more than 10MPH.
) use it in their oldie car (maybe no cat-converter?) because it ends up being cheaper for them, I presume.
And I'm not surprised you could tell. 
