A nearly forgotten fact is that, before the early '70s there were 2 different fuel octane rating methods. I did a search using 'RON+MON' and found a site with a detailed explanation of the different octane rating methods. In the US, the Research Octane Number (RON) and the Motor Octane Number (MON) were combined (RON+MON) to give the Pump Octane Number (PON). That site also has a chart showing that the 91 RON octane rating is equivalent to an 87 PON rating.

Also, for 1972, cars sold in the US were required to be able to run on unleaded gas. Compression for the US was reduced by many car manufacturers as most unleaded gas at that time was availible only in regular grade.

On an engine designed for regular fuel, there are many factors that can cause pinging, including incorrect timing, lean fuel mixture, high engine temperatures, carbon build-up in the cylinders, engine modifications (shaved heads, different pistons) or a combination of any of these factors.

I have to run premium in my 60 220S and my '72 250 (with a higher compression '70 engine) but I used to run regular in my lower compression '72 220 without problems.

Happy Motoring, Mark

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DrDKW

I've put both premium and 87 octane in my 4.5 and cannot determine a difference in performance or mileage. But I have experienced the power difference beyond the 5 degree mark and want to go there safely.

I run the ignition timing a few degrees advanced on my '84 Honda Accord for better performance and gas mileage, with the main
downside being occasional slight pinging during hot weather.
I just don't agree that all 1970s Mercedes or 4.5s require premium fuel. Given the thirsty reputation of most 1970s US market gas Mercedes, I think the abilty to run on regular gas would be an advantage, unless the improvement in performance and mileage justifies re-tuning for premium.
If you need to run premium, for whatever reason, it certainly won't hurt anything, I'm only saying in many cases, why waste money if you don't have to.

Happy Motoring, Mark

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DrDKW