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post #11 of 47 (permalink) Old 04-16-2010, 09:30 AM
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You will destroy your catalytic converters burning 87, which will void your emissions warranty. You also lower your MPG, thus erasing any pump savings you think you may have achieved.
Please explain how lower octane will "destroy" or affect the catalytic converter. Higher octane fuel simply allows higher compression levels before it explodes in the cylinder. As I understand it, that has no impact on the catalytic converter. There are other users here with extensive professional experience who have said that they never have seen a catalytic converter damaged by lower octane.

Further, sometimes higher MPG can be obtained at lower octane. I have been tracking MPG and octane with a spreadsheet since purchasing my 1991 300e in June of last year. The manual indicates I should use premium, but a trusted indy suggested that I could run mid-grade. With more than 60 data points, well distributed between 89 and 93 octane, I find that I consistently get 1-2 miles per gallon better with the 89. All other factors such as temperature, oil consumption and yes performance remain constant.

Now the particulars of my car and a 2006 ML350 vary. Specifically my 300E has a compression ratio of 9.2:1 and is not really high compression. The ML350 is 10.7:1 and might be; I am unsure where the dividing line is.

That said, I don't know enough about the ML350 to weigh in on what octane is best. However, it is wrong to say all Mercedes need premium fuel. Sometimes they run better on lower octane fuels.

Canards such as lower octane destroying the catalytic converter need to be called out.
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post #12 of 47 (permalink) Old 04-16-2010, 09:49 AM
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Not all fuel is burnt up in the engine. You see it on the track sometimes-flames from the exhaust pipe. The owners manual says something to the effect that those fumes can ignite in the cat and cause damage. Not sure as the exact working being that I do not have an owners manual on me right this second. Considering that low octane is slower burning that higher octane, I could see how more fumes could end up in the cat and then be ignited by the high temp.

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post #13 of 47 (permalink) Old 04-16-2010, 09:59 AM
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On the contrary, lower octane fuel is faster burning by definition. It also has a higher BTU value than higher octane fuel--delivering more energy per gallon.
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post #14 of 47 (permalink) Old 04-16-2010, 11:28 AM
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Further, sometimes higher MPG can be obtained at lower octane. .
You still seem not to understand the issue. Running 6 cylinders on regular gives it 4 cylinder performance.
4 cylinders use less fuel.
Very likely if you fill it up with Premium and drive as slow as you do on Regular -the mpg will go even higher since the engine will work the way it was designed to work.
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post #15 of 47 (permalink) Old 04-16-2010, 04:21 PM
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Oh how I love octane threads!

The key issue is predetonation (ping) from using lower octane fuel when the engine is under load. So, when you're stomping on the pedal on an onramp, or going up a fairly steep grade, the increased load can cause predetonation . . . which the engine's "knock sensor" will detect and cause the ignition setting to be retarded so that you don't damage a valve or piston, which will in effect lower efficiency during that period and likely send unburned fuel vapor through the exhaust system and throught the catalytic honeycombs . . .

So, if you and your passengers are less than average weight, don't carry much cargo, live in an area with no hills or highways, at fairly high altitude and your weather is mostly cool . . . you MIGHT be able to consistently use non-premium octane rated gas with no detriment to either your powertrain or fuel economy.
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post #16 of 47 (permalink) Old 04-16-2010, 04:39 PM Thread Starter
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Ok, so I will be trying out 91 Octane fuel and seeing whether it makes a difference or not. I'll let you guys know. I'll drive how I usually do and see if it goes up. The reason why I didn't read the manual is because...well, when you buy a car from a dealer, the grade of one of the most important things that goes into it, like gas, should be told to you...or at least I would expect it to be. Apparently we've been using Premium in our other cars until we were told that they didn't need them, so we didn't think this one needed it when weren't told it did.

But thank you for all your responses, and I will let you know how it goes, performance, gas mileage and whatever else.

Thank you!
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post #17 of 47 (permalink) Old 04-16-2010, 04:51 PM
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While you're at it, keeping track of mpg, performance and the like . . . also keep track of how much you are actually saving, per tankful, of hydrocarbon based fuel. All you need to do is write down the price that's listed on the pump for "regular." Then, and only then, evaluate the "value" of a lower octane fuel.

When did Mercedes-Benz owners and enthusiasts become such gasoline tightwads?
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post #18 of 47 (permalink) Old 04-16-2010, 05:08 PM Thread Starter
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Ok...so what exactly should I keep track of?
MPG.
How many miles I'm getting per tank.
Engine performance.
Value of lower octane fuel.

Is that it?
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post #19 of 47 (permalink) Old 04-16-2010, 05:31 PM
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"Performance" is a tricky thing to measure, especially if all one has is a "seat o' the pants dynometer." Do keep track of your MPG and cost per gallon, but also take note of how much it would have cost by putting in regular, so that you can see the actual "savings."

Your truck's fuel capacity is 25.1 US gallons. Assuming it's bone dry, and for discussion sake, assume you're filling it up today in St. Louis, MO. Today, regular is running about $2.80/gal; midgrade is about $3, and premium is about $3.15

So your fillup costs would be $70.28, $73.30 (+$3.02) or $79.06 (+$8.78 or +$5.76)
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post #20 of 47 (permalink) Old 04-16-2010, 05:41 PM
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MTI and SLR2098:

There is a warning in MB owner's manuals issued in countries where gas quality and availability is errant. The warning is to use an octane booster additive if no gas of the rated octane for the engine is available. It even goes further to make a point to stop using that as soon as the right gas is available.

You are the other side of the coin of those who wish to use a gas of a higher octane than specified. The difference is that in this case there is no damage to the car, while the lower octane creates combustion at the wrong temperature and has other connotations, mainly through the GRS mechanism, affecting the plugs, engine and exhaust, the injection, and the electronic feed and response action.

Excellent example of penny wise, pound foolish.

A car would show appreciation for preventive maintenance by never leaving his owner stranded.

Defensive Driving is never having to say you're sorry: a true love story between man and machine.
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