I was just reading all the documentation that can with my newly purchased 91 300E and came across a caution regaurding fuel requirments. MB says "To maintain engine durability and preformance, only premium unleaded gasoline (91 octane or higher) is to be used" it goes on to give a list of 4 precautions to do when using regular gas... no abrupt acceleration, mix with premium, don't exceed 2000rpm with a light load, don't push the gas pedal more than 2/3 if fully loaded.
Should I heed these warnings? How will regular gas effect engine duribility/longevity. Will reg. fowl injectors/filters/fuel pump?
I've been using regular gas because it's about 8-10 cents cheaper than premium.

Your car has an M103 motor - 6 cyl. There have been posts here and on another MB lists that indicate mid-grade gas(89 octane) being adequate for these motors. I too have an M103 motor and have always used 93 octane premium. Personally, I wouldn't use 87 octane in one of these motors. My 2 cents.

Mike Murrell
'91 300-SEL

Hi,
I just posted a similar statement on this topic. I cut and pasted it below:

I don't know about the new MB's but my 1986 MB really likes high octaine. Like 92. I tried to save a
couple of bucks by using the lower priced stuff, but I really noticed a difference in the mileage I
was getting. 86 octaine gets me roughly 18.75 MPG, while the 92 gets me around 23.5 MPG.
Sometimes more. Especially if I slightly overinflate the tires a pound or two. The difference in price I
think is well made up for with an extra 75 miles per tank! (Depending of course on how I drive and
how late I'm running) Since I spend an hour and a half per day commuting almost exclusively on the
freeway, I have time to notice these things. I'm going to try for 25 MPG with the tank I have now.
I've heard people argue that the brand of Gasoline makes a difference. I haven't found that to be
true. The only thing effected by different gasoline brands is my pocket book!

10 cents cheaper still isn't cheap enough when you take into account the difference in MPG. The math works out to be cheaper with premium.

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Kyle De Priest
1986 300e
1972 BMW R75/5 (Best there is)

Hello EADG,

The sky high fuel prices lately have been giving all of us headaches. Instead of opting for lower octane fuel as a way of saving $$$, may I give you another suggestion? If you do not know already, you can check out www.gastips.com and see a comparative listing of the price of gas offered by the gas stations in your area. The service is for Canada only, and I think it does cover Ontario. I usually check it out, especially durin' the summer when the prices were VERY high, and end up finding high octane fuel at a price comparable to regular at most stations. Hope this helps a bit with your problem.

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http://www.people.cornell.edu/pages/...ures/duck4.gif <- WaVe & DaNCe the DiScO DuCk if you spot a '95 black pearl/black Mercedes-Benz E420!!!

OK, looks like everybody is using the premium gasoline for the gas milage, which, I must admit, never considered. I'll spend the extra couple of dollars at the pump for the higher octane.
DEPRIEST, thanks for pasting your previous post... I totally forgot about the archives, anyways, the higher MPG off setting the premium price is definatly food for though.
DuckMuck. unfortunatly Hamilton is not listed on the Gastips.com site :( I'll be e-mailing 'em asking to add Hamilton & T.O.

Cheers for now.
EADG
91 300E (pics coming soon)

Nevermind MPG or fuel costs, consider engine longevity and the replacement cost of said engine.

------------------
Cheers!

Yen-Hsen Liem
'93 500E black pearl/black leather; 89,000km; 245/45-ZR17 Michelin Pilot SX; 17x8.25 factory EvoII
'93 500E bornit(blackberry)/black leather; 69,000km; european delivery; 245/45-ZR17 Michelin Pilot SX; 17x8.25 factory EvoII
'88 560SL desert taupe/dark brown leather; 89,000km; Euro headlights
'87 190E 2.3-16 black pearl/black leather; 55,000mi; Euro headlights
'70 280SL white/red; 135,000mi (original); factory alloys; Euro headlights

Mike M:
You have a M103 engine not a M103 motor.
P E H

the issue is NOT fuel economy.

because the M103 has a high compression ratio, it requires a high octane fuel. If you use fuel with an octane rating lower than that recommended, and the engine is placed under a high load, your engine will "knock".

Knocking occurs when the fuel pre-ignites due to the effects of compression (like a diesel), and this is bad for the engine. Imagine this happening as the air-fuel mixture is being compressed, before the piston hits top-dead-center on the compression stroke, and you can imagine the stresses on that piston and those wristpins.
This can be heard as a "pinging" noise when the engine is placed under load. Hence the precautions about gentle use of the throttle if your fuel has an octane rating below that recommended.

i give my car what it wants, but i also sometimes drive it hard ...

Hey boys.. I was happy with the fuel economy argument.; but the real questions I was asking (read up) were conserning the subsitution and detrimental effects of reg gas, if any. Bah

Cheers for now
EADG
79 280E (fire-balled)
91 300E

I believe "bobbyv" hit the nail on the head. "Knocking" is a no-no, over a prolonged amount of time of; city driving, up hills, high speed, and basically anything which stresses the engine from medium to hard, can be very bad.

The result, possibly over-heating of the combustion chamber (cooling system not designed for the added heat) and a warped head. Or possibly a cracked/broken piston under extremes (acceleration, heavy stress). Anyway you cut it, if you abuse the engine, it will fail.

Hope this answers your questions.

Thanks for the answer roas. I filled up with premium this afternoon.

End of discussion.

CHeers.
EADG
91 300E

I have always had bad experience using lower grade gasoline in performance engines. My dad borrowed my car once and he filled it up for me when he was done. He put regular gas in it. The difference was immediately noticeable. Mileage and performance were down. After a quarter of a tank I syphoned the rest of teh gas out and we put it in my dad's Dodge Ram. Seems to work OK there, and he has a big v-8 in it. Filled my car back up with high-test and everything was back to normal.

I also think it will keep your engine better over time.



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Jason Priest
1986 420SEL

How about a 85 500sel euro the mannual says 97 octane but the highest i can find is 93. what do you think/

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Dan Younes
---------------
1985 euro 500sel 223k
1986 420sel 86k soon 2 be
1981 300sd 285k sold
1979 240d 298k sold
1983 300sd 272k sold
1989 chrysler 2.2l Turbo
Leabaron 140k
1989 Dodge Carvan Se 295k
20001 Mitsubishi Mirage
1994 S500
(hopefully someday)

Gasoline is one of the major 'operating' costs of driving for many people since it costs $0.08 -$0.10/mile

Owning a MBZ for say 15 years and 150,000 miles costs at least $30,000 in depreciation and repair for a per mile cost of $0.20--roughly twice the fuel cost. Insurance costs another $15K for another $0.10 per mile.

Trying to save 8% of $0.08/mi, at best, gets you $0.007 per mile, or maybe $100 a year tops. If that amount of money makes a significant difference to you--sell the car now, you cannot afford it. You will collapse when the cost of 'hard parts' damage comes due.

Benzman500:

You need to see which method your Euro Octane rating method is based on. There are/were two methods that were used in the US; Reasearch and Motor methods and each used a different scale to arrive at the octane ratings. I no longer have that information in my brain; however.

If you look at US gas pumps and the Octane rating, you will see the number as derived from the formula R+M/2 which is the average of the two methods. I do remember that one of the methods was based on 100 being the highest octane number while the other could go higher (i.e., 110 Avgas, etc).

Perhaps someone else on the list with "recallable knowledge" on this might help. I do know that I run 93 octane in my 71 250C and it does very well - no ping or knock and the plugs are not damaged. Works well even when the ambient temp is in the 105+ range.

My .02

Dan


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Dan Taylor/ Tulsa, OK MBCA '71 250C/'81 300TD-T

Just to follow-up on this old thread, and input 2017 conditions.
Today in Texas premium gas is 60-70 cents more than regular, or about 30% higher!!!
This is a whole new ballgame for the regular vs premium discussion.

Not factoring in increased fuel MPG, that's an average savings for me of about $400 a year!

I drive a '91 300SE with the M103 engine.
I have run strictly 87 octane for the last 20 years and 200,000+ miles, with zero observed fuel-related problems. I have always gotten about the same avg MPG, whether using premium the first 7 years, or regular the last 20 years.

The M103 is faaar from a high performance engine, making only 59 HP per liter and having a low compression ratio of only 9 to 1. Having an aluminum head makes that low number even less susceptible to any octane knock.
My old car is heavy, and it gets hot in Texas; but I have never heard any hint of octane knock, and she's running great pushing towards 275k miles.

That's just my experience of course - yours may differ.
But I've saved at least $6-7k over the years buying regular - that's serious money.

So, my experience prompts me to recommend regular on these old engines,
and and save about what the car's worth every couple of years.

Cheers,
DG

I'm pretty sure the original car ( 91 300E M103 ) has knock sensors so it would pull timing back as needed.

More data points:

I'm pretty sure that it is a USA requirement that a car has the capability to operate on regular and not damage its self.

Most / all modern engines ( mid 80's and up ) have computer controls and knock sensors that pull ignition timing back as needed.

Some engines loose lots of timing so performance and fuel economy could suffer more that is saved by using low grade fuel.

My 97 SL320 ( 3.2 L M104 inline 6 ) needs premium fuel otherwise performance suffers too much. 4,000 lb / 3.2 L = 1,250 LB / L

My 97 C280 ( 2.8 L M104 inline 6 ) happily runs on mid grade, due to it being a 1,000 lb lighter car and more motor per Lb. 3,000 lb / 2.8 L = 1,071 LB / L

Naw, absolutely no knock sensors on the '91. No way for the engine to know if it's knocking and pinging. No manipulation of ignition timing other than the hard mapping in the computer for speed, load, temp, etc. The only feedback system is the O2 sensor which manipulates the EHA fuel valve to maintain proper exhaust gas rich/lean mixture. Pretty crude old CIS system.

DG

Knock sensors first appeared with the 3.0 liter 104.980 engine.

Weird, I've never seen more than ~20-25c difference between 87 and 91 octane.

Dallas seems to have the greatest delta in prices: yesterday at the Shell station:
87 octane: $2.20
93 octane: $2.90

Over $60 to fill the 2015 ML350, which I am still afraid to use regular in, what with direct injection and 12/1 compression. Of course the GM/Ford/japanese cars are getting more horsepower on similar engines using 87 octane.
Ya, you vill use der premium, and you vill like it!

DG

It has been an average of 25-30¢ between grades here for the past couple of years or so. Means 50-60¢/gallon between 87 and 93 octane. I run premium in the Honda since it returns an easy 5-6mpg better fuel economy (~100 miles per tank), despite the car being designed for "regular" 87 octane fuel. The cost is a wash between the two, but the car runs better and I can go longer between fillups with the high-test.

The SL gets 93 octane on principle. It will run on 87, but once it gets hot, it starts pinging which doesn't do it any good.

DI engines are far more tolerant of low octane than port injection.

I recently switched to regular for my 95 E320. Here's why:

https://www.cstatic-images.com/stock...4451470707.jpg

The old arguments why we need to stick with premium don't apply anymore for cars with knock sensors.

I poked around further to find out why premium is so much more expensive. There's no refining or other technical reason; the gasoline manufacturers simply believe the premium buying crowd are driving upscale vehicles, and they don't mind paying extra for their premium fuel. They're gouging the supposedly well-to-do folk.

There must be one gazzillion threads discussing the basis for this subject.
M.B. set the timing of the M103's depending on the market and quality of the Gasoline.
You need to look in the owners manual for your particular vehicle and it will state the Octane to be used.

The famous R16 resistor sets the timing and hence the Octane rating to be used. Just do a search for R16 resistor.
here's an image of the resistor:
https://farm5.staticflickr.com/4073/...be27e863_b.jpg

Some markets had variable fuel so MB included a resistor wheel - you could set the timing yourself depending on the available fuel octane rating.
here's an image of the wheel.
https://farm1.staticflickr.com/199/4....jpg?zz=1

my m104 will use regular,but if I'm doing mountain driving in summer,I use premium,my plugs last longer.But when winter hits regular is fine.