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  #1  
Old 09-13-2008, 08:29 PM
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Running 87 Octane when 93 Octane is specified

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?

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Old 09-13-2008, 10:50 PM
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Quote:
Originally Posted by pwjeep View Post
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?
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  #3  
Old 09-13-2008, 10:54 PM
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Cheap out on the fuel and expect to pay for it in the long run.

save $3 or spend thousands on a new engine
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  #4  
Old 09-13-2008, 11:32 PM
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The owners manual will address this question. Generally it is OK if you don't accellerate hard and you keep your RPMS under 3 or 4K. Everyone likes to believe all kinds of great things about premium gas. The only difference is it resists knock more. But the truth is these threads are worse than oil threads. Read the manual. All cars are not the same. If you like to rabbit stomp all around then run premium.
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Old 09-14-2008, 02:43 AM
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You can always retard the spark to run on regular (87 octane) if the computer doesn't, but you sacrifice performance. As long as there is absolutely no pinging/knocking, you won't damage the engine; knocking is extremely damaging. If you later have a valve job, chances are the head(s) will be resurfaced, which will increase the compression slightly (unless each cylinder chamber-top is honed to compensate) and thus requiring further spark retarding. In the end, it's a trade-off between higher($) gas or lower performance.
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Old 09-14-2008, 06:42 PM
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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.
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Last edited by lkchris; 09-14-2008 at 07:01 PM.
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Old 09-14-2008, 06:57 PM
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Originally Posted by lkchris View Post
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?
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  #8  
Old 09-14-2008, 07:03 PM
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The computer will sense the knock and retard your spark, so you won't hurt anything. You will lose some power, and mileage.


But its false economy, and really if you are looking to buy a Mercedes and the extra $3 a tank matters...
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Old 09-14-2008, 07:05 PM
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Quote:
Originally Posted by lkchris View Post
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.
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  #10  
Old 09-14-2008, 07:06 PM
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Originally Posted by mpolli View Post
????

What do you mean?
How do YOU think you can do it?
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Old 09-14-2008, 07:09 PM
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Originally Posted by Hatterasguy View Post
Thats what knock sensors are for.
No it's not. They are for minor problems perhaps caused by carbon in cylinder, not for gross changes. This is "legend" that has been built up over the years, mostly again by wishful thinkers. The Mercedes manual would approve regular fuels if your fantasy were correct.

https://www.mbwholesaleparts.com/StarTuned/pdfs/LiquidJune06Star.pdf
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  #12  
Old 09-14-2008, 07:16 PM
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Um well thats how ignition systems work. Thats why modern cars don't ping anymore...
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  #13  
Old 09-14-2008, 10:24 PM
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PW: This was an interesting question with a load of OPENIONS. Here is some facts, help yourself, OK. I have three vehicles, all say use 93. A Volvo S-90, a 96 S-320, a 97 SL-320. I've been runing REGULAR sense NEW. VOLVO 80K, S-320 103K, and SL 78K.I use Mobile 1, 10w30 all year round. Volvo=23mpg, S=26 @ 75mph, SL=24 @ 65mph. PW, these are facts, not my openion. Have fun, the older S's are god ole sherman tanks, and the 320's get great milage for such a large car. Mine is the long wheel base model, love it. Ron
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  #14  
Old 09-15-2008, 12:19 AM
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Quote:
Originally Posted by RConrad View Post
PW: This was an interesting question with a load of OPENIONS. Here is some facts, help yourself, OK. I have three vehicles, all say use 93. A Volvo S-90, a 96 S-320, a 97 SL-320. I've been runing REGULAR sense NEW. VOLVO 80K, S-320 103K, and SL 78K.I use Mobile 1, 10w30 all year round. Volvo=23mpg, S=26 @ 75mph, SL=24 @ 65mph. PW, these are facts, not my openion. Have fun, the older S's are god ole sherman tanks, and the 320's get great milage for such a large car. Mine is the long wheel base model, love it. Ron
Thanks for those facts, but those are some pretty loose facts. The two MBs require 91 octane, not 93, and the Volvo claim seems suspect as well. Here's my personal experience - which I would characterize as anecdotal rather than factual - in a car that requires premium fuel the additional cost of premium fuel is justifiable in better mileage. I have always found this to be true. Try running a 6.3 on regular. You may be proud of your 24-26 highway numbers but those are not good numbers for a 104 motor on the highway. I get 29+ in an S320 on premium which easily justifies the approximately 5 percent premium price premium.
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  #15  
Old 09-15-2008, 08:03 AM
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Originally Posted by deanyel View Post
Thanks for those facts, but those are some pretty loose facts. The two MBs require 91 octane, not 93, and the Volvo claim seems suspect as well. Here's my personal experience - which I would characterize as anecdotal rather than factual - in a car that requires premium fuel the additional cost of premium fuel is justifiable in better mileage. I have always found this to be true. Try running a 6.3 on regular. You may be proud of your 24-26 highway numbers but those are not good numbers for a 104 motor on the highway. I get 29+ in an S320 on premium which easily justifies the approximately 5 percent premium price premium.
Don't forget that a premium first tier gasoline has additives that will keep the fuel delivery system clean.........

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