I'm rusty on my organic chemistry any more, but API has two ratings for motor fuels - road and motor. The tests use a common standard test spark iginition engine to find the point at which the fuel will spontaneously combust (in a 4-cycle gas engine, "knock" or pre-ignite)

Per EIA.gov: "The (R+M)/2 you see on the label refers to the average of the research octane number (RON) and the motor octane number (MON) ratings. To determine the RON, the fuel is tested under engine idle conditions with a low air temperature and slow engine speed. To determine the MON the fuel is tested under the more stressful conditions of higher air temperature and engine speed.

"Historically, RON and MON were determined on separate testing machines specifically configured for each test. Current designs (see image below) allow the same engine to perform both tests. Despite this flexibility, many testers still prefer to use more than one machine with each specifically set up and calibrated to perform either RON or MON tests."

The octane rating is a relative figure - different engines will react differently to the same fuel. For example, your old 440 six pack with 11:5 static compression, primitive carburetion and chamber design will rattle like a poltergeist at a seance on 87, but a new GM LS running 11.5 static with the small, fastburn chamber and direct injection will wail without a peep.

Today's premium (92-93) is comparable to 96-97 by the older standard, similar to older standard pump premiums. Innaday, my beloved '63 Marauder with the 390 was okay with the 87-88 octane as long as I didn't ask much or the weather wasn't too warm. But it ran like it had a B12 shot and a double espresso on the 92 Amoco gold. I could bump the timing several degrees, even tho it was a 2V it would one-wheel-peel well down the block.