when the compression is less than 10-1. just wondering

9.2 is actually somewhat high relative to much of the rolling fleet out there. Not as high as the M104, but still...

__________________
1986 300E 5-Speed 240k mi.

The 103 engine has what can be descibed as a hemispherical (actually semi-hemispherical with a small quench area) combustion chamber. Such designs do not have good detonation resistance and have always been known as having an "octane appetite, but they are amenable to large valves and the angled valve layout yields good high rev volumetric efficiency for high specific output.

Also, there is no knock sensor as in modern cars. You can experiment around with lower grade fuels. If you hear knock, you might be able to reduce it by changing your driving habits. Since higher inlet air temperatures increase the octane requirement, you might find that lower octane fuel is satisfactory is cold weather, but not in hot weather.

I note that the newly announced C6 Corvette sports a 10.9:1 compression ratio; 93 PON fuel is recommended, but NOT REQUIRED. Knock on lower octane fuels will cause the spark to be retarded to below the knock point, so it won't make full power, particularly at lower revs where knock is usually most prevalent.

Duke

German cars are all known for high octane habits, and I guess MB hadn't gotten over the "normal" 96 or 98 otcane, high lead fuel typical of Germany up to the mid 90's, when the forests all started to croak for air pollution. Nasty.

Use the premium -- detonation at high speed is not audible, so you will get lower performance and burned valves/melted pistons. The M103 does NOT have a knock sensor, and will NOT accomodate lower octane nor adjust timing for it except for running richer to compensate for the excess O2 from poor combustion when it knocks.

The result of running lower octane fuel is poor performance, low milage, and possible engine damage. The milage gain alone will probably pay for the difference in price.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

The owners manual for my 103 calls for 91 octane.

Many posts here have indicated that mid-grade is ok.

In my area we have 87-89-93. I use 93.

The difference in price between 87-93 isn't worth the damage(long-term) that could be done.

For me, it just ain't worth it. I plan on keeping the car until someone takes my drivers license away from me.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Its a Mercedes, doesn't she deserve the best?

__________________
88 300CE 300,000kms
Black Pearl, Burgandy Leather
Euro Lights.
Its for sale. Its SOLD

In my 86 300E I always run 91 octane ( the highest generally available here in So. Calif.) but I still hear pinging. I've tried all the different brands of gas and none of them seems to make any difference. I've also tried using Techron several times over as many months with no results. It's usually not that audible, but If I'm accelerating near a wall or parked cars the noise is reflected back at me and I can hear it quite clearly. It's also noticeable while accelerating up long freeway grades. It seems to occur even under normal acceleration and I worry about the long-term effects, especially after reading about the "190E Disaster"

190E disaster!

Is there a way to retard the timing slightly, or to remove carbon buildup on the pistons and combustion chambers without pulling the head? (not sure that I have an excessive amount, but it couldn't hurt to remove any that's there)

For that matter, does anyone know a source for a reasonably priced fiber optic device that could be used to inspect the inside of the combustion chamber through the spark plug hole?

Gary

We've owned five 300Es over the years. We presently have a '90 and a '91, both with over 150,000 miles. We've owned the '90 for six years; the '91 for five. Both average about 22-23 MPG. The '91 gets a bit better mileage than the '90 for some reason. It'll do 26 on the highway.

We have never used anything but regular 87 in any of these cars.

No pinging. No damage.

IMHO, the compression ratio is not that high and then there's that resistor that retards the ignition 6 degrees (I took it out on one of the cars and still heard no pinging, but I was too chicken to leave it out)...

that was a very good explanation. from now on only premium fuel.
i also own a 88 190e 2.3 with a melted piston. so that is also a good reason. even if octane was not the cause it was a wake up call.
Rich

Hi olpos and all -

The last 3.0 engine I tore down (late '87 model) had a dropped valve that actually stuck into the piston, like a sword in stone. The carbon build-up was incredible, and the previous owner used regular fuel.

Pinging or not, carbon build-up is not good for piston rings, cylinder walls, nor the valve train (not to mention the catalytic converter).

My W124 book shows that some 2.8's and 3.0's have 10.0 compression ratio, as well as 9.2, and the 2.6 as having 9.2.

I have the 2.6, and you can bet that premium fuel will always enter my fuel tank. 91 to 92 octane is the average in my area, but 93 can be located.

I believe limeyb spoke the truth.

__________________
'87 300E | 2.6 engine
Artic White - Navy blue interior, chrome rims, very clean and sharp!
91 300E
89 300E


The rest of my collection are just cars and trucks...no more "automobiles"...

Actually I have taken to using 87 PON fuel in my 190E 2.6, but I only drive it from November to May nowadays when it is fairly cool in So Cal.

I remember when I drove it everyday I could sometimes hear it ping on hot days with the A/C on and that was on premium (91 PON) fuel.

My car has a five-speed, and I tend to shift at low revs in normal in-town driving. I've learned to feed the throttle in a little slow on upshifts to avoid the brief transient detonation than can occur if you feed the throttle in a little too fast at low revs.

Detonation damage can be cumulative, but brief transient detonation will rarely lead to a burned piston or valve, however, sustained moderate detonation can eventually cause a burned piston or valve.

Detonation causes shock waves in the combustion chamber. These shock waves "ring" the engine structure, which is what we hear as "knock" or "ping", and these shock waves increase the rate of heat transfer to the combustion chamber boundaries by up to an order of magnitude, so it is ultimately local overheating of the material that can burn a piston or valve since pistons and exhaust valves normally operate pretty close to the temperatures at which their strength properties rapidly degrade.

Carbon buildup can reduce combustion chamber volume, which can increase actual compression ratio, and carbon can also cause hot spots that can create preigntion, which can lead to detonation; 87 octane fuel will not usually lead to carbon buildup. Carbon buildup is usually the result of either high oil consumption for extended periods or an overly rich fuel mixture for extended periods.

The key is to keep you ear open for detonation. Driving the car with the radio off and a window open in the city will usually allow you to hear any detonation. If anything, as has been mentioned previously, you are more likely to hear detonation echoing off the surroundings through an open window as opposed to through the vehicle structure.

Duke

I can run regular fuel in my Honda that has 269,000 miles on it, (since it's a Honda, and can take it).....but it's premium for my Benz.

I suppose it's just a matter of preference, like keeping my Artic white 300E clean versus driving it while dirty (which won't hapen , because I have cleaning supplies in the trunk!

Remember what the little sticker at the gas cap states? "Premium unleaded fuel only".......at least that's what my sticker states on my 300E.

Good luck whatever fuel you run....but take it from me, you should consider removing the fuel tank and cleaning the rust out of it! .....(no matter what fuel you choose).

Glad to hear that you can run 87 octane Duke.....but for me, 87 means my year model...lol!

__________________
'87 300E | 2.6 engine
Artic White - Navy blue interior, chrome rims, very clean and sharp!
91 300E
89 300E


The rest of my collection are just cars and trucks...no more "automobiles"...

The times I have used premium I have never noticed any improvement in mileage. YMMV

However, to make up for the 20-cent difference in price you would have to go from 22 MPG to just over 25 MPG, assuming regular is $1.40/gal.