What is considered acceptable octane levels for my 1991 300E.. With rising gas prices i would like to run 89 or 91 as opposed to the normal 93 octane that i run.. would this be OK? would i notice a decrease in power or economy due to this change?

Oh,boy. You're opening a can of worms here.

Be aware that your car has no knock sensors and therefore no ability to protect itself from detonation.

That being said, I have run 87 in our cars for about 300,000 miles between them with no problems.

I think there is a noticeable DECREASE in power with premium. But that may be my imagination since I know premium burns more slowly.

Just my humble opinion.

YMMV!

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2012 E350
2006 Callaway SC560

Power decrease WITH premium?? hmm wow, never thought i would hear that

Power will not decrease with premium. Octane is only a measure of the gasoline's ability to resist ignition upon compression.

A gas rated 93 Octane means it will has the resistance to ignition upon compression of a mixture that is 93% octane and 7% heptane.

The reason that MPG would decrease in a modern fuel injected high compression engine with knock sensors when lower octane gas is used is that if the octane is too low, the engine will retard the timing to prevent knocking. This consequently results in an incomplete burning of the fuel and thus less of the energy contained in the fuel is efficiently utilized.

Regardless of the octane of the gas, one gallon of gas contains about 115,000 BTUs of energy.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record

Check your owner's book. I believe that 91 octane is what is called for, at least in the newer MB's. I'll dig out my book over the weekend and check.

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Paul 1987 300 SDL; 2000 ML; '69 MGB; '68 VW Fastback

My experience is that you can run 87 PON fuel in 103 engines, especially in cool weather, but you may have to change your driving habits.

Detonation is most prevalent at low revs, so you need to feed the throttle slowly. When your tank gets near empty try a few gallons of 87. TURN OFF THE RADIO/STEREO and listen for detonation. If you hear it, learn to drive around it. If it detonates significantly and you can't drive around it, fill it up with 91. You're stuck with it.

Hot summer weather with the A/C on is going to make detonation more prevalent, so even if you can get away with 87 now, you may not be able to in the heat of summer.

Duke

Wrong place to pinch your pennies ---> do what the manual says. My '02 says 93 octane, I don't second guess them.

20 gal/week x $0.20 extra per gallon = $4.00/week. Peanuts.

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2002 E320 4-Matic
2008 Subaru Outback
2009 Subaru Forester

My owner's manual (2000 320ML-215hp) states 91 as the min. Of course many gas stations in the Eastern US sell 87,89,and 93 as their octane choices. So, you may want to cut back 1-2 beers a week at your local bar and buy the 93 octane.

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Paul 1987 300 SDL; 2000 ML; '69 MGB; '68 VW Fastback

thanks for the info everyone,,,,, I have a question just out of curiousity...

There is a gas station in my town that sells gas for $2.60 a gallon i believe it is like 96 octane, i could be mistaken.. not that i would but what would happen if i were to run that in my 1991 300E?

If you put 96 octane in, you would be most likely doing nothing but wasting your money.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record

I've switched to 87 octane in my car and haven't noticed any less performance or mileage. But I do have the M119 engine in my car.

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500

There is a gas station in my town that sells gas for $2.60 a gallon
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Hell, here in California there are many stations selling 87 octane gas for that price or close to it for full serve. Self serve averaging around $2.29/$2.39/$2.49 for 87, 89, and 91 octane.

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1986 560 SEL (159K miles)