Can anyone clarrify why we shouldnt put gasoline in lower than the 91 recommendation?(other than that its in the book)

I'm not saying i put anything lower than 91 its just that i have this professor who owns the same car i do and constantly debates on which octane to put in and saying that im wasting money etc etc.(during cigarette breaks)

you think i should take his word? Does anyone knows the truth behind the octane?!?!

This topic has been beaten to death so many times, lol.

Ok, yes, MB recommends minimum of 91 octane when filling up. Why, because MB designed their engines to achieve maximum performance on 91 octane. a USA Today article quoted someone from Mercedes when doing a write up on waste of money on octane fillups. MB rep said, "if you use anything less than 91 octane, you will most likely not achieve the performance numbers (hp really) that is listed with your vehicle". But today's computers in cars adjust to whatever octane you put in the car. So if you put 87 in, it won't kill your car, your computer retards the timing to compensate for the lower octane. Also said, you can't feel the difference if you put 89 vs 91, unless you put it on a DYNO. So there is really no difference, other than 91 required stickers on the car and listed in the manual.

The ECU is smart enough to retard timing for the lower octane, so no worries about your engine blowing up or anything like that. But there has been tons of debates on 91 vs 89.

Sometimes I put 89 in the ML cuz the difference between 89 and 91 can be around $5 on a full tank. I myself wouldn't go all the way down to 87 octane, minimum 89 or 91 when it gets cheap at times.

-G-

I believe it can cause a Check Engine light condition.

You mean 87 can cause a check engine light or 89? With 89, I haven't seen it yet....even in the manual, it says you can put regular (87) if there is nothing else available.

-G-

Yes, the ECU is supposed to adjust for lower octane gas but I don't think the range is that wide. There is a specific setting in the software for octane....my understanding is this is for countries/areas that cannot get 91 octane. Why would there be a setting if the ECU already compensates for it? I have never seen this setting but only read about it .

Possible. But I remember from posts long ago that some ML owner(s) put in 87 or 89 all the time, I don't think anything has happened to the such yet.

-G-

Your professor probably drives like an old lady and will never notice the differennce between grades of fuel. You can put in anything you like and the ECU will adjust timing and mixture to compensate but like GDawg said,you won't get top performance out of your engine. Under extreme cases (hot weather, heavy load and 87 octane) the ECU might not be able to compensate enough and destructive detonation can occur. I know that in my CLK430 I can put almost anything in it and it never makes a sound. It's down on power but never pings. Howewver, my E55 won't run over 5300 RPM on anything less than 91 without rattling my fillings! It's even worse now that I put the PowerChip in.

Yup, as long as you don't full thottle every time, you can hardly notice the difference. And for octane, I wouldn't go as low as 87, atleast special or middle grade (usually 89 oct).

For most drivers, 89 or 91 will be good enough. My .02.

-G-