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2001 ML320
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Discussion Starter #1
I know it says to use 90 in the book for gas, I have had other cars where it said that and didn't matter. Will it make a big difference using 87?
 

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ML550, W212 E350 4 matic, 1966 Corvette C2 convt.
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Use what it says with all vehicles. You are most likely not an engineer, have not tested the engines. You are most likely just cheap.
 

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2008 GL 320 CDI, 1999 ML 430, 2001 Porsche C4
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My SA here in West Virginia told me that they spoke with Mercedes regarding what type of octane rating is acceptable. 89 octane is acceptable. I am still worried about what it will do to the engine so I usually mix it, 1/2 a tank of 89 and 1/2 a tank of 93. I do not know if it does anything but I have been doing this for the past 3 months when the gas prices have gone up. i have not noticed any change in performance. I have not heard any knocking sound or hesitation associated if you put 87 octane.
 

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CURRENT: 2011 SL550, 2011 C300 FORMER: ML350, CLK550 Cabriolet, C240, ML320, 300TD
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Your ML has a high compression engine that requires 91-octane fuel. Using fuel with a lower octane rating can cause problems ranging from a barely noticeable reduction in power to increased engine noise to permanent engine damage with long-term use. Octane ratings are linear, so you could mix 93 and 89 to get an overall 91 rating. I fill my nearly empty tank with 93-octane at Costco, then at half a tank, I fill with 89-octane from a reputable brand (Chevron, Exxon, Shell) to get the additives.

- RODNEY
 

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Ml 500SE
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Penny wise and pound foolish!

Come on you guys think about it! I pay $2.54 a gal here in Cal for 91 octane from shell. 89 octane is 8 cents less per gallon. 1/2 a tank is 11 gallons. You're saving 88 cents a tank vrs. possible burned pistons and upper compression ring, or valve damage from detonation! I saved and invested money all my life so I could own the last 2 Mercedes outright and have the gas money to run them! The German engineers know what they're talking about. Do what they say and these engines are bullet proof! It's your money, but just my 2 cents...thank you!! [8D][:D][:)]
 

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RE: Penny wise and pound foolish!

Dakat, I am in total agreement with you about not being foolish over trying to save a small amount of money at the risk of high-cost damage. However, my half-tank scenario actually gives me 91 octane. In my neck of the woods, I can get either 87, 89, or 93 octane. I have not seen any stations selling 91 octane in a long time. Using 93 octane is wasteful, since the engine only requires 91. Using a higher octane than is necessary could have as much potential for damaging the engine as lower octane. I buy 93 octane at Costco for a few cents less than brand-name 89 octane. I then “cut� it with brand-name 89 octane. The result is I get the brand-name additives and I always have at least 91 octane in the tank. I figured I save around $100 per year, which I can spend on something more useful, like a good bottle of Scotch [:D].

- RODNEY
 

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2000 ML430
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101 Posts
I just buy the highest octane because you figure if it damages the car then it won't be out there. If the higher octane is meant for special cars, it won't be out there. Just don't want the engine gods to come knocking down my door asking me to replace my engine.
 

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'00 ML320 Elegance
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That's interesting...

rudeney - 3/16/2005 2:54 PM
I have not seen any stations selling 91 octane in a long time.
That's interesting. Over here, we see only 91 octane, with only a few stations carrying anything higher. Esso has a 92 octane instead of 91, and PetroCan and Chevron both sell 91 in addition to 94 octane.

I myself fill up with 91 octane only; the station that I go to almost solely charges 6 cents more for premium fuel vs regular. Most other places charge 12 cents more [:)]

I figured I save around $100 per year, which I can spend on something more useful, like a good bottle of Scotch [:D].
LOL, touche. Not so much of a Scotch drinker myself though, but good point [;)]
 

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Ml 500SE
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RE: Penny wise and pound foolish

This forum is so much fun. I agree with SOSH I think there are some cheap people out there! Now, RUDENEY pay attention! My owners manual says 91 octane MINIMUM!!! You can't over octane an engine. If someone told you it will burn the valves, that is a wives tale! What qualifies me to say that. At 66 years of age I've had plenty of experience with the internal combustion engine. It all started with being an aircraft engine mechanic in the Air Force in the 50's. The big 14, 18, and 28 cylinder engines!
I've seen the destruction that detonation can cause. Let me explain it this way. The higher the octane rating the less likely the air/fuel mixture will detonate under pressure and heat caused by the ignition process, or self ignite like a diesel if you will. So, your engine may not detonate at 90 deg. C, but it might at 100 deg. C. if you use less than 91 octane. If you use more it doesn't care. If I could get 103 octane I would use it out here where the desert can get up to 125 deg F at times for the added protection! I don't know if gas octane is linear. It would be interesing to take a sample of your "mixture" and send it to a lab and have it tested! Having been a "gearhead" all my life and built many high RPM engines that lived in boats and racecars I'll stand behind what I've said here! What you do with your money and your scotch is your business. Just my 2 cents!!!!! [8D][:D][:)]
 

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RE: Penny wise and pound foolish

Drew: Only having 91 octane available may have to do with elevation. I know that the higher the elevation, the lower the octane rating that is required. For example, in Denver, they actually sell 85 octane. The reason being is that the thinner air results in a lower propensity for premature combustion, so a few lower octane points can be used.

David: Trust me, octane ratings are linear so 10 gallons of 89 octane and 10 gallons of 93 octane = 20 gallons of 91 octane. As for using a higher octane, 93 vs. 91 likely would not cause any problems, but it would still be wasteful. I’ve always gone by the rule that you use the lowest octane fuel your car requires, but make it a good name-brand fuel with appropriate additives. With all due respect (as you have about 20 years of experience over me!), using fuel with too high an octane rating can result in incomplete combustion, which can foul O2 sensors and cause premature deterioration of catalytic converters (things that you didn’t worry about on aircraft or racecar engines).

- RODNEY
 

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Rudeney: I'll give you the "octane is linear" thing as I would never do it, so I don't care. I remember a friend of mine that knocked the top out of a piston in his then new 55 Buick Century doing that back then. Higher octane than you need will produce incomplete combustion I believe to be another wives tale! Octane requirements change with temperature, barometric pressure, and altitude, so the higher the octane the closer you are to one size fits all. The Germans have figured that 91 octane should protect you all around. Coming from an engineering background I always need proof, so I'll give you the incomplete combustion scenerio for now. I'll give it some research! Thanks for the input!! [8D]
 

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It is linear. I used to design gas pumps and the way we got the mid grade was to

mix the Lo and Hi graded in linear proportions. The interesting this is that the mixing is performed right at the nozzle level.
 

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ML55, E320
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All the talk about low octane causing pinging, knocking, burned pistons, etc. is true. It's especially problematic when the engine is under load, like accelerating or going uphill. The saving grace with our MLs is that the engine has vibration (knock) sensors. When the engine management computer gets the message that the engine is vibrating, indicating detonation (pinging/knocking), it retards the spark timing (firing the sparkplug in each cylinder earlier in the compression cycle) so that the compression inside the cylinder is lower when the sparkplug fires and lower octane is required to avoid detonation. This causes some loss of power, but it may not be very noticeable because the engines in the ML have good low end torque.
 

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OK guys I've done my homework and I'm back. AdamNelson and rudeney especially pay attention. Go to www.streetrodstuff.com/Articles/Engine/Detonation/ Adam Nelson you are correct about the sensors in our ML engines. However when you retard the spark it fires LATER in the compression cycle. Everyone make sure you read all 8 pages of the report, but especially page 4. All this should explain why we must use no less than 91 octane in our engines, especially in hilly hot country. Thanks for the input from all parties!! [:D]
 

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Thanks for the info, David. It was a very interesting read. I’ve always understood everything the writer stated, but never in the “correct� technical terms. Regardless, I still stand behind my methods for “mixing� my own 91 octane fuel and saving a few $$ in the process. If I knew that Costco’s gasoline had decent additives in it, I wouldn’t do it, I’d just use nothing but their 93 octane. As it is, their price for 93 is generally a penny or so less than name-brand 89, hence my reason for mixing the two for 91 (and getting half a tank of fuel with additives).

Once thing I find interesting is the fact that some manufacturers (I believe Honda is one) are building high-compression engines, yet advertising that they do not need regular fuel because of the knock sensor. Of course they qualify that with the fact that it will result in reduced engine performance, but it seems to me that they might also want to qualify it with the fact that it might result in reduced engine life!

- RODNEY
 

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Rudeney: Maybe this can be the last word on this subject. I never know just exactly how much gas my ML will take. So, how do I know how much half a tank is? Over time you may be under or over your 91 octane goal. That's why in an earlier message I said it would be interesting to test your "mix". 91 is as high as I can get here on the west coast. I have another home in Montana where I go in the summer, and 90 is as high as I can get there. I'll do what Mercedes says to do for my 500, and feel safe with that. I don't drink or smoke so maybe I have more money for gas. It's been fun.....thanks!! [8D]
 

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Maybe this can be the last word on this subject. I never know just exactly how much gas my ML will take. So, how do I know how much half a tank is?
Not trying to get the last word, but to answer your question, I have found that the fuel gauge on my ML is extremely accurate. In fact, I have found the fuel gauges on all the European vehicles I have owned to be this way in contrast to American and Japanese vehicles. Anyhow, I generally do my 89-octane top-off when the gauge is just above half a tank and I can generally add just over 8 gallons in.

As a side note, we drive to Atlanta about once a month to see the kids. I usually end up burning close to two tanks, and they will all be filled with 93 octane simply because I’m stopping at whatever brand-name station I find convenient and don’t worry too much about trying to save money. Of course even the most expensive stations in Georgia are still cheap compared to here at home since they have a lot less tax on their fuel.

- RODNEY
 

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Has anybody tested the fuel consumption with low and high octane fuels?

The knock sensor retards the spark for low octane fuels making the engine less efficient.

You may find that the extra cost of higher octane fuel is offset by lower fuel consumption and better performance, although going above the recommended octane level is unlikely to benefit much.
 

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Hey Rudeney, maybe I've been married for 44 years because neither one of us wanted the other to get the last word! I just filled up the 500 for $2.69 a gal for 91 octane. Talk about taxes!!! [8D]
 
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