Gas octane ratings - Mercedes-Benz Forum

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post #1 of 33 (permalink) Old 08-15-2005, 04:40 PM Thread Starter
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Gas octane ratings

I would like to see what you are running in your new ML's. Please let us know what ML you have, and gas octane ratings you run.

I have a ML350 and run high octane, super. Most of the time I run Shell V-power.

Has anyone seen a difference using middle grade vs. super?

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post #2 of 33 (permalink) Old 08-15-2005, 06:31 PM
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RE: Gas octane ratings

http://www.benzworld.org/forums/foru...osts=14&fid=18 Read that.

Basically you can run any octane if you want because the ECU will throttle down the engine so you won't knock and ping. This of course drops your performance quite a bit also.

I recommend, for best engine performance and longevity to always run the highest octane you can.
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post #3 of 33 (permalink) Old 08-15-2005, 08:13 PM
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RE: Gas octane ratings

And I second that... The engine computer just retards the timing to compensate for crappy fuel. (Which is why Montana can run a Benz on 94 Octane, and that same car runs in California on 91 Octane.)
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post #4 of 33 (permalink) Old 08-16-2005, 05:36 AM
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RE: Gas octane ratings

Mercedes engines are built to very close tolerances and are designed to run on high octane fuel. It makes it clear in most all owner manuals that running low octane fuel can damage your engine.

While opinions on this issue may vary, I had a conversation with my mechanic about this one time. He said that engines designed to run high octane should never be fueled with anthing less than 91 octane unless you live in higher elevations. He further explained that poor fuel can damage the catylitic converter(s) and cause excessive carbon build up on the tops of the pistons. When this happens, he said that the carbon will glow red hot at optimum engine temperature and can cause a variety of problems such as burning a hole in the top of your piston or pieces of it breaking off and causing internal engine damage. He has seen a lot of damage caused by those who wanted to cheap out and buy low grade fuel. You'll pay now or most likely, pay later!

His opinion was: If you're going to drive an expensive vehicle that you paid a lot of money for, you should be safe and spend the money to buy the premium fuel. I totally agree with him.

OTOH, if you run high octane fuel in an engine designed to run regular, you're most likely wasting your money.
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post #5 of 33 (permalink) Old 08-16-2005, 09:11 PM
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RE: Gas octane ratings

I would like to know how it would damage the catalytic convertor... I did not know that.
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post #6 of 33 (permalink) Old 08-17-2005, 05:56 PM
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RE: Gas octane ratings

From what I understand about this, high octane fuel (91 and up) burns slower and cleaner while lower octane fuel burns faster and dirtier. The dirtier burning fuel lends to the carbon build up on pistons. The resulting effect seems like an oxymoron, purer burning cleaner but slower and cheaper burning faster but not clean (the faster the explosion, the less efficient the combustion of fuel). This results in minute amounts of carbon and raw gas being forced into the catalytic converter which clogs and damages it (a bad converter many times will smell like sulfur burning). This is also why engines will knock with cheaper fuel. There are actually 2 explosions taking place in the chamber which results in the effect of a hammer hitting the top of the piston giving it the metallic sound.

While the engine timing is adjusted to compensate for knock, hence allowing the fuel to burn cleaner, buying cheap fuel isn't in no way good for a car designed to run high octane. While many folks run cheap fuel in their vehicles, when 91 or higher is required, without noticeable damage, there are the many who are just as unlucky and damage their engine.

If running cheap fuel, I would definitely be more concerned with what it might do to my engine more than the converter, since the computer does compensate somewhat for low octane.

There are many other things that can damage your catalytic converter as well such as:
1. Driving with a bad O2 sensor
2. Plugged injectors (causes unburned gas to ignite in the converter, thereby damaging it)
3. Piston Blow by
4. Leaking head gasket
5. Fouled or bad plugs
6. Bad intake gasket

I am by no means an expert, but I hope this helps! [:)]
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post #7 of 33 (permalink) Old 08-17-2005, 06:33 PM
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RE: Gas octane ratings

Knock is actually the fuel exploding before the spark plug ignites it, if my understanding is correct.
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post #8 of 33 (permalink) Old 08-17-2005, 07:25 PM
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RE: Gas octane ratings

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BenzofBend - 8/17/2005 8:33 PM

Knock is actually the fuel exploding before the spark plug ignites it, if my understanding is correct.
You are absolutely correct! The 2 explosions I was referring to is the one that takes place prematurely causing the knock and the correct one a nanosecond later as the result of the spark.
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post #9 of 33 (permalink) Old 08-18-2005, 05:56 PM
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RE: Gas octane ratings

And the loss of power resulting is not from the premature explosion, but from the car's ECU pushing back the timing so far to compensate for the premature detonation, that the car actually never has ignition close to TDC in the compression stroke, but rather, on the downstroke of the piston.

(Sorry if this is the car tech nerd thread all of the sudden, I just want to make sure that my knowledge of the tech is correct.)
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post #10 of 33 (permalink) Old 08-18-2005, 06:21 PM
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RE: Gas octane ratings

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BenzofBend - 8/18/2005 7:56 PM

And the loss of power resulting is not from the premature explosion, but from the car's ECU pushing back the timing so far to compensate for the premature detonation, that the car actually never has ignition close to TDC in the compression stroke, but rather, on the downstroke of the piston.

(Sorry if this is the car tech nerd thread all of the sudden, I just want to make sure that my knowledge of the tech is correct.)
Thus why the knocking goes away! My Dodge Ram truck has this problem and the dealer cannot seem to fix it. While it does okay on regular driving, if you have to get on it, it just doesn't have the power it should for a 5.2 liter. My truck is rated for 87 but no matter what octane I use, it runs crappy. The last time I had it in, they adjusted the settings in my computer and I lost 2 mpg and I'm now getting the sulpfur smell we previously discussed (rich fuel). I know it has got to be destroying my cat! OTOH, I run 91 in my ML430 and it runs great!

BTW, I love Car Tech Nerd (CTN) Threads![:D]
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