I copied this off a question and answer board from my work. Thought it might be of interest to some.
Comment: Maybe you can settle a long-standing debate for us. It revolves
around hi compression engines [ie. factory 11.3:1]. One side of the debate
says that octane ratings are computed differently now than they were back in
the musclecar era but the 92 rating that we currently have is essentailly
the same as the old 100+ octanes from way back when. The other side says
that the older fuels actually had more octane and were more resistant to
detonation in those engines. We both realise that CARB fuels that have MTBE
and TAME in them burn differently due to the ethers but the main debate is
the octane requirements of old versus new engines.
Comment: Some cars and trucks built in the late '60s and early '70s (ie:
high compression engines), require gasoline with octane ratings as high as
101, is there a different formula for figuring octane now or did oil
companies market higher octane gas during that time period?

Response: The octane quality of motor gasoline is measured using two ASTM
test methods, which have changed little over the years. The two methods are
the Research octane number (RON) method and the Motor octane number (MON)
method. RON provides a higher number than MON and the difference between the
two is called sensitivity. Prior to about 1973 when federal regulations on
octane posting began, both the oil industry and the auto industry used the
RON to refer to octane quality and vehicle requirements. Some premium
gasolines were promoted as 100+, but the U.S. average peaked at 99.9 for
leaded premium. The peak for the average MON was 92.2. Then the federal
regulations required the posting of the average of RON and MON, or (R+M)/2.
Thus, the high-octane leaded premium gasolines would have been posted at
their peak as 96. In 1971, the auto industry began making engines with
reduced compression ratios in preparation of their need to install exhaust
catalytic devices to reduce emissions. The catalysts cars would need
unleaded gasoline and the autos recognized the oil industry couldn't take
the lead out and still maintain the same octane level. Leaded premium
gasoline disappeared in 1981with an average RON of 97.3, an average MON of
89.2, and an average (R+M)/2 of 93.3. The simple answer is that today's
unleaded premium does not provide as high of an antiknock performance as the
leaded premium sold for muscle cars in the 1960s.

The response is pretty good. I would quibble with the statement that the industry reduced compression in anticipation of the catalyst. The reduced compression ratio would yield higher exhaust gas temperatures that would cause continued oxidation of the unburned hydrocarbons, and would reduce peak flame temperature for a reduction in oxides of nitrogen. As for a RON of 91 in 1981 , I think that is pretty optimistic. Good fuels were gone by then. The original government position was to have 91 octane (r+m/2) as the minimum. That did not come true.