I read with interest, many postings recently about the pro's/con's of running lower octane fuel. My '89 300E has always idled a bit rough despite being well tuned. I have always run Mobil 91 Octane.
Another quirk which I have not seen mentioned in previous postings is the tendency for the car to almost always require two efforts to start except when very cold (winter). The dealer says this is normal???
Due to the ever increasing cost of gas, I decided to try a tank of 89 octane and am very pleased that the car runs as well as before and the idle is smoother and it now starts on the first try everytime. There is absolutely no pinging or any other indication that anything bad is going on..

I've never been able to tell the difference between premium and mid grade in my 300E.

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

Larry........
Thanks for the replies to both my posts. I think if I put some MMO in the tank, I'll have the best running 300E in the state!!!!
I did read your comments on MMO after the fact. Sure wish I had some air tools!!

robi, You might want to do a search there are a lot of topics regarding hard starting when the engine is warm.

yal
88 230E (W124)

I've used "middle grade" octane for last 100000 miles and have noticed no difference.

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87 300E
175000 miles

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