Running 87 Octane when 93 Octane is specified - PeachParts Mercedes-Benz Forum

pwjeep · #1 09-13-2008, 08:29 PM

I'm thinking about buying a 1999-2005 gas Mercedes. I think I remember reading that Mercedes specifies using 91 or 93 octane premium.

Has anyone tried to run 87 (regular) octane gas in these cars? What is the performance difference? Does it "hurt" the engine or does the computer compensate for the difference?

94Wagenmeister · #2 09-13-2008, 10:50 PM

Quote:

Originally Posted by pwjeep

I'm thinking about buying a 1999-2005 gas Mercedes. I think I remember reading that Mercedes specifies using 91 or 93 octane premium.

Has anyone tried to run 87 (regular) octane gas in these cars? What is the performance difference? Does it "hurt" the engine or does the computer compensate for the difference?

The computer will compensate for it, but don't get in a habit of running cheap 87 all the time. A tank now and then is ok, 2-3 tanks in a row is not. But, at 15 cents a gallon premium it's only like $3 more filling up a bone dry tank... Is it worth it?

TheDon · #3 09-13-2008, 10:54 PM

Cheap out on the fuel and expect to pay for it in the long run.

save $3 or spend thousands on a new engine

mpolli · #4 09-13-2008, 11:32 PM

The owners manual will address this question. Generally it is OK if you don't accellerate hard and you keep your RPMS under 3 or 4K. Everyone likes to believe all kinds of great things about premium gas. The only difference is it resists knock more. But the truth is these threads are worse than oil threads. Read the manual. All cars are not the same. If you like to rabbit stomp all around then run premium.

280EZRider · #5 09-14-2008, 02:43 AM

You can always retard the spark to run on regular (87 octane) if the computer doesn't, but you sacrifice performance. As long as there is absolutely no pinging/knocking, you won't damage the engine; knocking is extremely damaging. If you later have a valve job, chances are the head(s) will be resurfaced, which will increase the compression slightly (unless each cylinder chamber-top is honed to compensate) and thus requiring further spark retarding. In the end, it's a trade-off between higher($) gas or lower performance.

lkchris · #6 09-14-2008, 06:42 PM

There is no evidence the "computer compensates" and it in fact is NOT designed to do so. This is simply wishful thinking and borders on fantasy to believe so. (I'd like, BTW, to see the "timing retarded" on one of the new distributorless cars--which describes all 1999-2005.)

The typical 6-cyl Mercedes engine is around 180 cu in and develops the same horsepower as an American iron boat anchor engine of around 350 cu in. It takes high compression--and consequently premium fuel--to accomplish this. The V8s are in a similar state of tune.

Maybe it's SOP to ignore what American car manufacturers recommend, but you do so very much at your peril with German stuff.

mpolli · #7 09-14-2008, 06:57 PM

Quote:

Originally Posted by lkchris

There is no evidence the "computer compensates" and it doesn't. I'd like to see the "timing retarded" on one of the new distributorless cars.

????

What do you mean?

Hatterasguy · #8 09-14-2008, 07:05 PM

Quote:

Originally Posted by lkchris

There is no evidence the "computer compensates" and it in fact is NOT designed to do so. This is simply wishful thinking and borders on fantasy to believe so. (I'd like, BTW, to see the "timing retarded" on one of the new distributorless cars--which describes all 1999-2005.)

Thats what knock sensors are for.

jlomon · #9 09-15-2008, 01:46 PM

You wont save any money on the lower octane fuel. The gallon may be cheaper, but you'll use more of them to develop the same power that the car would have made with the high octane fuel. It might actually cost you money, depending on how you drive.

hdbill1@comcast · #10 09-15-2008, 02:11 PM

bottom line - why chance it - iffen you can't afford an extra $3 per week - you can't afford the car. there are plenty of nice cars out there that will run just fine on reg. (hondas, toyos, buick, chevy). why did you buy this car? i bought mine cause of the way it handles and runs (126 lwb, v8). if i wanted to save money i would have purchased a much smaller car.

Lexxani · #11 09-15-2008, 02:44 PM

my M117 powered 450SL-C gets a little bi+chy if I run her on 87. She'll live with 89 just as well as 93 though. This is just my observation over the course of ownership.

Gurkha · #12 09-20-2008, 08:02 AM

Knock sensor is a high tech mic attached to the engine block and the moment it senses ping, it retards the timing. If high octance is specified then using regular fuel will lead to knock. OTOH if a engine specified for regular fuel knocks or knocks with high octane there are other issues to be looked into from timing to excessive carbon build up.

Lexxani · #13 09-20-2008, 11:37 PM

Just filled up the 1st time since installing my new heads, advancing the timing and correcting the crankshaft/camshaft alignment. . .goddamn. . .what a difference over 89 (i can not imagine how poorly she'd run on 87). . . A few more miles to break in the head and an oil change to 20/50 from my 10/30 flush Im running and its track time!

Gurkha · #14 09-20-2008, 11:42 PM

Quote:

Originally Posted by Lexxani

Just filled up the 1st time since installing my new heads, advancing the timing and correcting the crankshaft/camshaft alignment. . .goddamn. . .what a difference over 89 (i can not imagine how poorly she'd run on 87). . . A few more miles to break in the head and an oil change to 20/50 from my 10/30 flush Im running and its track time!

Instead of 20W/50 try a HDEO like Delo 400 15W-40 and you will see even better results. Engines raced with Delo 400 hardly show any wear after tear downs. There are several threads on BITOG discussing this.

Lexxani · #15 09-20-2008, 11:50 PM

hmm thanks for the tip gurkha, I'll give that a shot instead, I still need to go get oil. . .brand spankin new heads. . .a world of difference they made. . .3 way cut and all new valve guides/seals. I reused my valves and springs as they were ok, but thoes new heads gave the sluggish US spec m117 (with emissions) a real wake-up. . .