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Old 10-12-2006, 11:54 AM   #1 (permalink)
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87 octane fuel

Question: Why use 93 octane when the car runs fine on 87 octane?

Information: I use Chevron gasoline almost exclusively in my 05 E500 and lately have been using 87 octane with no apparent (to me) decrease in performance. I experience none of the typical octane related issues - pre-detonation under load, rough idle, etc. Chevron puts Techron in all of their grades so it keeps the engine sparkling clean. So other than a performance decrease, (which is not noticeable to me) what it is the argument for using 93 octane gas- other than the manufacturer's stated requirement?
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Old 10-12-2006, 02:41 PM   #2 (permalink)
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Quote:
Originally Posted by jpuzas
Question: Why use 93 octane when the car runs fine on 87 octane?

Information: I use Chevron gasoline almost exclusively in my 05 E500 and lately have been using 87 octane with no apparent (to me) decrease in performance. I experience none of the typical octane related issues - pre-detonation under load, rough idle, etc. Chevron puts Techron in all of their grades so it keeps the engine sparkling clean. So other than a performance decrease, (which is not noticeable to me) what it is the argument for using 93 octane gas- other than the manufacturer's stated requirement?
Jeff,

Seen this very conversation on many other MB forums, and the conclusions are the same as your question -- somewhat of a performance hit under certain conditions.


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Old 10-12-2006, 03:02 PM   #3 (permalink)
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The problem is your knock sensor is continually detecting knock and retarding timing causing higher exhaust gas temperatures not to mention higher emissions and creating a hellish environment inside the catalytic converter.

Yes you can run 87 octane, for that matter you could probably run 80 octane BUT IT WILL NOT allow your car to run properly.

MB spent MILLIONS of dollars (Euro's) putting your engine on a test stand to determine how to make it work at optimum levels. Cutting corners by using the wrong grade of gasoline puts severe stresses on your motor not just in terms of detonation but thermal stress that over the long haul can cause major damage to internal components.

Bottom line is this if you can't afford to use the correct grade of gasoline in your Mercedes maybe you should consider selling it and drive a Ford?

DB
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Old 10-12-2006, 06:58 PM   #4 (permalink)
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Hmmm... This is exactly the kind of information for which I was looking. Could you explain one step further? How does the retarding of timing cause increased exhaust gas temperatures? Does that mean that my 87 944 which requires 87 octane produces higher exhaust gas temperatures than my 87 928 which requires 91 or above octane? I have also read that higher octane gas needs to be ignited sooner because the flame front takes longer to initiate and then burns longer than a lower octane fuel. So the lower octane gas burns hotter for a shorter period of time and creates higher peak brake mean effective pressures in the combustion chamber?

In my experience an engine usually only knocks when under high loads. If I rarely use wide open or mostly open throttle positions then I should be OK right?
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Old 10-12-2006, 07:12 PM   #5 (permalink)
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Retarding the injection timing cause combustion to occur later in the stroke. This decreases combustion efficiency by virtue of a reduction in cylinder pressures. This reduction in cylinder pressure causes more gas energy to exit the cylinder(s) without being converted into mechanical energy. The increase in waste energy out the exhaust port causes an increase in heat that the engines cylinders and exhaust components must withstand. The higher temperatures of the combustion gasses can and will melt the catalyst resulting in higher back pressure and eventually an expensive repair. Obviously the software of the ECU is programmed to limit this heat but the ECU's first priority is to prevent detonation and pre-ignition so if the fuel is low enough in octane the ECU could command enough of a timing reduction that you will melt something down stream.

Don't confuse this with the temperatures that occurs during ideal combustion condtions. Yes if you run optimum octane fuels the ECU will advance the timing so far as to maximize the release of energy at the optimum crank angle within a given window of time. The cylinder pressure will be very high as well as the peak combustion temperature. HOWEVER this release of energy is occuring at a point that the engines crank will be able to convert that energy in to mechanical motion. This allows sufficient time for the peak combustion temperature to fall below that which can harm the downstream components.

This is where MB had to do much research. They had to determine all the conditions and ECU control limits to effect optimum combustion in addition to meeting a specific set of emissions regulations.

Obviously building these motors with higher compression ratios allows for more power with a given amount of dispacement while maximizing the efficiency of the motor. This again places higher demads on fuels to run within the design criteria that the engineers determined to be optimum for the engines performance capabilities.

DB

Last edited by drivbiwire; 10-12-2006 at 07:15 PM.
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Old 10-12-2006, 08:28 PM   #6 (permalink)
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For my first post I will say you do not need high octane gas if you do not use the engine to the fullest [ie full throttle]. Although MB may spend a lot of money on engineering what gas to use maybe they need to spend a little more time with different fuels and lower reid vapor points. The engines do start much better when it gets cold out and we recommend that when the engine starts to run poorly on super then they should try midgrade or even regular. Why would we recommend that? Because that is what our factory rep. says.
Since most of us rarely use full throttle you can get by with less octane since you are only compressing a vacuum. Beware though, I have seen many times that if you use lower octane fuels the check engine light may come on with a code that indicates that the ME ECU has retarded the maximum amount of timing. All the money saved on the gas will be used to get a diag. and to clear the code.
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Old 10-13-2006, 05:23 AM   #7 (permalink)
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I'm staying out of this thread -- you guys are way over my head! My mechanical abilities are still in diapers!!

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Old 10-13-2006, 12:48 PM   #8 (permalink)
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Quote:
Originally Posted by paulv
I'm staying out of this thread -- you guys are way over my head! My mechanical abilities are still in diapers!!

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I Second that.

20 cents more for 91 octane x 15 gallons = $3.00. Not a major saving ..... just use the higher octane and drink less coke\soda.
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Old 10-13-2006, 01:15 PM   #9 (permalink)
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Performance aside, the higher octane also has more beneficial "additives" that will keep yopur engine running cleaner. ( yes, even the Chevron)
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Old 10-13-2006, 01:22 PM   #10 (permalink)
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Quote:
Originally Posted by paulv
I'm staying out of this thread -- you guys are way over my head! My mechanical abilities are still in diapers!!

Regards,
paul...
Paul! You surprise me. I figured you for a blasting debate on this one!

In reality DBW is correct, assuming the engine is used to its full potential (60%+). Most drivers use less than 25% of their engines' rated output, so ... those that use lower power settings can get away with lower octane.

Mercedes, unlike Ford, Nissan and the rest are selling Horsepower to the uninformed consumer. They, like I did for 15 years, are pulling every possible pony out of every cubic inch. Face it, it's much more attractive to buy a sedan with 350hp than one with 120hp, bragging rights.

In reallity, so few people extract 100% of their engines rated power. How many of you have taken the tach to 5500 rpm and held it there at speeds of 150+ ? Speed limiters aside, I would guess none of you reading this have and if I am wrong, you felt uneasy about the possibility of a connecting rod flying through the hood.

I live in two mechanical worlds, auto and aircraft. Aircraft engines are designed to put out their rated power for hours on end, in some cases over 1000 hours. However, strict use of high octane high quality fuel is mandated ( Don't get your feathers ruffled DBW, I'm not refering to jets). Racing engines do the same but for much fewer hours. In some cases their outut can be measured in minutes. Once again, they depend on exotic, high octane fuels or they will blow apart at start up. Common characteristic, they cost thousands more and put out 100% of power BY DESIGN.

My 45 cents being said, if you drive hard you should use the higher octane fuel. If you drive at road speeds of 70mph+ for lengths of time, use higher octane fuels. If you drive in commuter traffic, park at the office and repeat the same at 5, you can "get away" with lower octane.

And no .. the E55 gets premium.
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Last edited by lear31a driver; 10-13-2006 at 01:36 PM.
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