my 1985 500sel runs aobut a mile more on a gallon with 93 octan than 89 and my mannual says min. octan 97 should this matter and what kind of damage would 190k on 87 do to my engine.

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1985 500sel 220k
1981 300sd 278k sold
1989 chrysler
leabaron 130k
1990 380sl (hopefully)

As long as there is no spark knock, this fuel will not damage your engine.

Are you SURE about your difference in fuel mileage difference between fuels?

Good luck,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

yes with out a/c and the same daily drive



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1985 500sel 220k
1981 300sd 278k sold
1989 chrysler
leabaron 130k
1990 380sl (hopefully)

I believe you need to try several tanks on each fuel and average them together. Without making a timing adjustment or such to optimize for the use of a particular fuel, there should be no difference in energy availability. A higher octane fuel burns slower, preventing spark knock. There is no difference in the energy content of the fuel.

I wonder if the 89 octane is indeed preigniting and you just can't hear it. The preignition would increase fuel consumption as well as eventually destroy the engine. There are many things done in the design of the large body cars to decrease engine noise in the passenger compartment. I wonder if it would be possible to not hear spark knock.

Just my thoughts, have a great day,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

My Ford F250 with the 460ci engine used to be really sensitive to gas octane. It would definitely get better mileage with better gas, and had better power too. The reason why was because the engine managment system would advance the timing up to just before knock since it had a knock sensor. Perhaps Mercedes does the same thing?

I don't know how much extra power you'd get but I'd bet good money that you would "lose" less power to less favorable ambient conditions. That said...what is everyone's take on fuel? I know the Sunoco 94 is popular but we can't get that down here in GA. (at least that I've seen) Usual suspects around here: Amoco, BP, Exxon, Texaco, Chevron, Shell, and QT. Can't find Sunoco (or anyone else for that matter) that has 94 (or better) octane premium.

Just curious what you other rabids are using...Lee

If I remember my Chemistry correctly, the lower octane gas will NOT harm your car. My only suggestion is do not drive it hard on lower octane gas. The motor is designed for performance, thus the need for higher octane fuel.

You will have no damage to the motor by using lower octane (within reason) fuels in the tank as long as you do not drive like you're on a NASCAR track or try to race the teen-agers at the red light.

87 and 89 octane should be fine for highway driving or leisure driving... If you are stop and go in town, and runnig the A/C you should consider staying with the higher (90+) octane fuel.

In answer to your main question, fuel is fuel and the energy produced by burning it is constant, regardless of octane rating, provided it is burned in similar conditions... ie. same engine. Higher octane ratings do, however, burn more efficiently, thus reducing knocking and pre-ignition 'ping'.



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William
76 240D - 550K miles
78 300D - 200K+ miles