Octane Requirements for 74 Merc

We have a 74 450 SEL. The manual says to use minimum 91 octane to avoid pinging or predetonation, but that was in 73/74 using the Research method of determining octane. I know that they changed to the average of research and
motor octane around that time, so what we see as 91 at the pump today is not the same as 91 Research octane. I have gone down to using 89 octane modern gas with no problems. My question is, will I be ok using 87 octane pump gas, since I've read that modern octane gas is rated about 6 points lower than the research method? Lastly, anyone have a good estimate of the motor octane rating of 87 pump gas? (We live in the Southwestern US). Thanks!
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A gasoline's octane rating basically indicates the temperature at which it will NOT preignite. Preignition is a fuel's tendency to self-ignite before the spark occurs; such preignition is caused by the engine's compression ratio, glowing carbon deposits and any hot spots in the combustion chamber. Anyway, one doesn't want preignition or "pinging" to occur because the fuel explodes before the piston reaches the very top of its stroke so the engine works against itself so to speak.
"Pinging" is more likely to occur in hot weather than in cooler weather so if your 450 doesn't "ping" now the fuel is OK to use. You've already experimented by using mid grade fuel, you could try a few gallons of regular in a relatively empty tank so if it pings there's tank capacity to add some high octane fuel to eliminate the pinging - end of experiment!
These '70s gas engines were quite detuned to achieve emissions goals. Detuning included lower compression ratios than used in Europe, USA versions' power suffered but that was the technology available then. So, the engine may be well able to run on regular fuel, but "pinging" is the warning of its limits.
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"T.G. Lambach" wrote:

In New Zealand we have 91 and 96 unleaded; my 1969 220 runs fine on 96 but today had to be refuelled in a country town with only 91 available. Only put 4 gallons in, and it pinked badly at low revs. Back in the city, went to an outlet that sells racing fuel [said to be 100 octane] and put a gallon of that and two of 96 to dilute the 91. Fine again. Cheers
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Remember that quoted US octane numbers are about 5 below the European/international ones, i.e. 98 octane international is 93 US (the definitions are differenent).
DAS --
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NB: To reply directly replace "nospam" with "schmetterling"
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