Octane requirements?

[mark300d]

Hello all,

can anyone tell me what octane is required per the owners manual by the following models:
- 190E-16V
-190E 2.6L
-300E (don't know which engine?)
-400E

I am gearing up to buy a new car. With the gas prices going the way they are here in Los Angeles I would like to learn all I can about the fuel requirements so that I know how much pain my wallet will be in.

Thanks.
Mark.

[David C Klasse]

All of them require a min 91 Octane.
I think only 84-85 190 2.3 didn't need 91, don't quote me, but it was something like that.
Everything above and after that is min of 91.

[chowpit]

If you get a 300E/E320 with the 104 engine you can run 89 octane. The same applies for the 400E with the 119 engine. Reason being is the fuel management system has a knock sensor that will retard the ignition timing when spark knock is detected.

[David C Klasse]

Scott,
Then theoretically speaking, I could run 89 in my car because it has a knock sensor, and it could retard the timing, correct?
Be more specific, hic. Otherwise I'll come back and BITE!!!!!

[chowpit]

Your theory is correct, brat.

[hill]

I have noticed quite a few posters mention using NN octane for this and NN for that. Do's any one have a fuel requirement chart based on compression ratios.

Fuel octane requirement is much more complex than simply what compression ratio is present. The timing and mixture condition have much more to do with it. Also the head material, an aluminum head can conduct more heat away from the combustion chamber, thereby allowing a higher compression ratio, all other things being equal.

With the engines utilizing a knock sensor and the associated controls, you CAN use low octane fuel with negligible engine damage, but it is usually false economy. When the knock is sensed, the timing is retarded and possibly other actions taking by the engine management system that result in poorer fuel economy. So, it might cost less per gallon, but require more of those gallons to get the same distance down the road.

Good luck,

[Mercedes Fred]

Why did MB settle on premium-only for its engines? Are all mb gas engines premium only? does premiums benefits far outweigh its fill-up costs?

[Jim Anderson]

I think thats how they measure octane. They have a varyiable compression engine and find out when it starts to knock.

[300EE320]

The difference between mid-grade and premium is $0.10 per gallon around here. That's about $1.50 per tank. I don't think that's a very good place to try saving money when you've decided to invest in a Mercedes. Just pack a lunch to work once in a while and you're even.

I'm with Larry on the knock management functions. The system is there to help avoid catastrophic failure. Even the manual says not to run anything other than premium unless you have no choice, and then to go easy on it and put the proper fuel in at the first opportunity. The manual does not say "put in any fuel you want, we've got you covered".

There are many great (non Mercedes) cars out there if super-economical driving and great gas mileage are primary considerations.

A somewhat related question I have had for a long time - is it beneficial to use premium on older cars? I have a 85 190E 2.3, whose manual states 87 as the required octane. The possible reasons that I can come up with, to use premium, are:

1. Older engines have carbon deposits, so knocking may be more, so better to use premium.
2. Premium fuels may have more cleaning ability.
3. Premium fuels may give marginally better fuel economy, so some of the extra cost evens out.
4. Premium fuels have greater fraction of ethanol, so they burn cleaner, so they will be better for those 15 year old cat convertors.

Now, are these reasons just fallacious, or, put together, they may justify spending an extra 15 cents a gallon on an old car (for me it comes to about $100 a year extra).

My wife's Land Rover also requires 90 or better. It's a 4 litre very low reaving engine. One would wonder why in hell they'd require you to run 90 Oct for this snail on wheels. I've ran 87,88,89,91, and 92 in and have never noticed one bit of difference in it's performance. But it seems that this particular vehicle has a problem with carbon build up on the exhaust valves. The company knew about this, and figured the higher octane gas would burn better, thus less carbon build up on the valves. Running it harder (not babying it like tea-tottlers do) will also have the desired effect of not having carbon build up on the exhaust valves.

So when the car makers suggest you put x octane or higher in your car, I'd just suck it up and do it. They made the car, hence they probably know what they're talking about.

Mateo

87 300E (140K - Like new)
97 Land Rover Disc. SE7 (Wife)

[deeph8]

I have the same question concerning what to burn in my 1982 280e. It has a higher compression engine than the states version. I have the owner's manual, but it is in German and I can't read it. To confuse matters further, it was fitted with all sorts of anti-smog stuff when it was imported in 1985. I have been running premium, but I'm not sure if this is right. Can I damage it by running too high octane?

The testing mechanism for octane measurement is indeed a variable compression ratio engine. All of the other parameters that I mentioned, however, remain constant. For THAT engine you could easily draw a chart, but it would be good only for THAT engine with THOSE particular mixture and timing parameters.

In the real world, all engine designs have different combustion chamber and valve configurations as well as different fuel and ignition management systems. That means multiple, interacting parameters.

Have a great day,

A major car magazine (sorry, can't think of the name) just did a test involving the use of different grades (octane ratings) of gasoline on several contemporary engines. The engines ranged from BMW's and Saabs that have sophisticated engine management systems that automatically adjust timing to accomodate fuel grades both higher and lower than the manufacturer's recommendation, to a Dodge Ram Van with a 360 equipped with a more rudimentary engine management system. Most (but not all) engines performed better, and achieved better fuel economy, with a higher than recommended grade of gasoline. But, the improvements were just in the 2-4 percent range. A Honda engine actually demonstrated less performance and economy under these circumstances.

All engines performed more poorly with a lower than recommended octane, even those (e.g., Saab) that can be run on a lower octane according to manufacturer's recommendations.

The conclusion of the article was that using a different octane (whether higher or lower) than the manufacturer's recommendations is a waste of money, except during hot weather, a circumstance in which an engine's octane demand rises.