What kind of gasoline?? - PeachParts Mercedes-Benz Forum

ManikNikki · #1 09-29-2004, 04:11 AM

I own a '85 380SE, and was wondering if someone could tell me what the minimum grade gas I could use would be? The manual is not clear, so I have been running the highest all along...just to be safe. But, that is getting really expensive with the current gas prices!!

I know the higher grade the better gas mileage I get, but I would like the option to run lower if possible. I have 87, 89, or 91 to choose from where I live. So, would 87 be safe?? Thanks so much for your input anyone!!

nglitz · #2 09-29-2004, 09:55 AM

The optimum fuel for any given engine is the lowest octane that will not knock or ping under the most severe conditions encountered.

That's all the higher octane rated fuel does for you, resist ping/knock, i.e. detonation. If you want to be cautious, try this approach: let your fuel level get fairly low and put half a tank of the next lower octane rating in the tank. If it knocks or pings, fill it with the grade you were using as soon as possible. If it doesn't, continue on with another half tank until you're confident that the grade is safe for your engine, i.e. it won't knock.

Do it all over again with the next lower octane rating, until you find your engine's requirements. Personally, I run my '87 260E on regular even though the MB recommendation is premium. Zero knock.

Cars with turbo or superchargers and/or knock sensors are a bit different. They are essentially "adjustable" engines and will react differently. You also may find different requirements for summer & winter, or carying heavy loads, or mountain driving. And so on.

Advantages to using lower octane ratings are fewer deposite on plugs & valves from less anti-knock additives in the gas. The only advantage of higher octane rating is more knock resistance. More than your engine needs is useless and a waste of $, as you've discovered.

Just MNSHO,

donbryce · #3 09-29-2004, 10:17 AM

Quote:

I know the higher grade the better gas mileage I get...

My '85 380SE runs just fine on regular gas. And I can't see the logic in the higher octane giving better mileage. More power, perhaps, but that is the gain for the cost of either using more fuel of whatever octane, or the same amount of fuel at a higher octane.

mpolli · #4 09-29-2004, 12:37 PM

Nglitz is 100% correct. Octane is a measure of knock resistance and that is it. It is my understanding that 10 or 15 years ago some gas companies were making claims about mileage and power and were fined by the government for false advertising. You will notice they do not make these claims any more. They tend to make a lot of "toothpaste" claims such as "no gas cleans better" or "helps to remove deposits that can rob power". But any gas with additives (required by law) can help to remove deposits. Yes some have more additives than others. So we have fancy names for these such as Techroline, Pro Power, and V power etc. And I have been told that using a higher than necessary octan can INCREASE emmissions. Here is a funny story: My friend is very picky about her 2003 531i. She always used Shell premium. Then one day they started advertising that it contained "V-Power" Her manual didn't say anything about V-Power so she switched to Chevron!!

Another thing, both me (last week) and one of co workers (last year) were advised by our local dealer to use regular (87) in out Mercedes (me C230, him E320) in both case to REDUCE knocking. Go figure. (Any comments on this are welcome since it makes little sense to me)

Mike

earossi · #5 09-29-2004, 01:11 PM

There are myths and truths to everything. To begin with, you are correct that the higher octane prevents knock and increases power(by virtue of allowing you to runn more spark advance than on a lower octane fuel). You are incorrect in believing that the higher octane improves mileage UNLESS your engine was pinging severly on the lower grade fuel.

There is little truth that higher octane fuels produce more deposits. Actually, there are probably less deposits since most of the majors over dose their premium grade gasoline with detergent based additives.

It is correct to say that the optimum sustainable performance/power from an engine occurs right on the verge of pinging.

And, it is correct to say that you can safely use lower octane fuel to run your car as long as you do not hear predetonation (ping) and are satisfied with any perceived degradation in power.

The problem with pinging is that you may be experiencing predetonation, but not be able to hear the audible pinging sound suggested. Past evidence suggests that engine life is markedly shortened if the engine continues to be operated in a pinging mode.

Mercedes has safesided the issue with its older cars by calling for premium fuel. In reality, you can probably get away with running midgrade (89 octane) with no perceived loss of power or reliability. Dropping to regular grade is much more risky.........particularly in an older car that does not have knock sensors.

Engines with knock sensors will retard engine timing when they "hear" engine knock, so there is little chance that you will cause engine damage by running regular fuel. I doubt that your 1985 vintage car has knock sensors, so you can't be certain if damage is occurring or not.

Assuming a price spread between gasoline grades of about 5 to 10 cents per gallon, your incremenal cost of running a car about 15,000 miles per year (assuming an average fuel economy of 15 mpg) is between $50 and $100......not enough reward to risk a loss in performance or reliability for your engine. Since the price spread between gasoline grades has remained in the 5 to 10 cent range, independent of the overall fuel cost..........one should think twice before switching to a lower grade of fuel.

nglitz · #6 09-29-2004, 01:40 PM

For starters, the phenomena of concern are preignition and detonation. No such animal as "predetonation".

There will be no loss of power going to a lower octane unless detonation occurs. **Unless** the engine in question has a knock sensor and retards ignition/lowers boost. Not a problem in an '85 380SE; no turbo, no knock sensor. If anyone read my first post, I advocated sneaking up on the lower octane fuel. I'd say that qualifies as "thinking twice".

mpolli, I would also think twice before taking any more advice from that dealer who told you to reduce octane to reduce knocking. He hasn't a clue. I've heard people tell me that "Premium is a hotter fuel so it makes more power". Wrong on both ends.

Lastly, there is no "insurance" factor in running higher octane gasoline than your engine needs. It won't give more power or better economy, either. Nor, like one commercial implied, will the pretty girls smile at you because you spend too much on gas.

Regular is fine,

mpolli · #7 09-29-2004, 03:04 PM

Unfortunately I received this "advice" a day after I topped off with premium so I have yet to run the test. I am skeptical, especially since the noises I hear are somewhat different than any other pinging I have heard before. But as I get older I find that I must just give things a try. I find the more I learn, the less I seem to "know". I will report my findings as soon as I can. My friend who took the advice said it did work! He suspected it had to do with the engine control software... Honestly I wouldn't be surprised. Another factor could be that here in Seattle at least, our gas has 10% Ethanol. And the premium is 92 not 91 so I don't know if any of that could be an issue. The dealer tech said this info came from the regional service manager.

Mike