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Old 08-04-2002, 08:40 PM
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Join Date: Apr 2002
Posts: 21
Octane question (MB126)

I am looking at my owners manual from my 1983 380 SEL and deciding what is the correct fuel (octane). The manual reads:

"Unleaded gasoline: Average Octane of Research and Motor 87 (RON of 91).

What does this mean? should I use 87 or 91 octane? and what does "RON" stand for? Why the two different numbers?


Scotty P.
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Old 08-05-2002, 09:26 AM
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Join Date: Apr 2002
Posts: 3,469
that means 87 octane in the US. Once you pick a rating that you want to use, stick with it. IMO it seems like mercedes like to be run on the same stuff everytime.
No Benzes in the stable at the moment, but itching for one again.

Former Mercedes in the Stable:
1983 300CD Turbo diesel 515k mi sold
1984 300CD Turbo Diesel 150 k mi sold
1982 300D Turbo Diesel 225 sold
1987 300D Turbo Diesel 255k mi sold
1988 300 CE AMG Hammer 15k mi sold
1986 300E Amg Hammer 88k mi sold
1992 500E 156k mi sold
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Old 08-05-2002, 09:43 AM
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Join Date: Dec 2001
Posts: 565
My 126 owners manual calls for 91 octane and that's the pump rating. I personally would not put 87 octane in an MB V-8 engine.

My 2 cents.
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Old 08-05-2002, 06:12 PM
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There are two widely used methods for computing octane, the one with the lower numbers is used in the USA, so yes, 87 octane in the USA is what the manual says to use.

Being far from a professional driver, I use either what the manual says, or the lowest octane the engine will not knock going up a hill on. Hence 89 octane in the Cadillac, even though the manual says 87 (and "slight engine knock when driving uphill is acceptable"--not in my car!).

Jaguar states the use of lower octane fuels is acceptable if the timing is slowed, so I imagine the same would be true for Mercedes. Now, who would but a performance car then slow the timing....
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Old 08-05-2002, 08:07 PM
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Generally, gasoline octane is given in relation with two methods of testing. RON and MON
RON: means Research Octane number
MON: Means Motor Octane number.
MON is always higher than RON.
All petroleum companies (at least in Canada) has to post octane rating based on
RON + MON /2 (average of RON and MON)
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