I don't think running regular would be a problem unless it starts to ping. Why don't you run a tank or two and see if it pings.

But really, I have to wonder about the low-compression motor needing this octane as well. As with anything Mercedes, especially from this era, it was engineering and performance at all costs. I don't think it's a stretch to presume in the days of the 126/124/140, if premium was the best one-size-fits-all grade to provide absolute protection of the engine in all environments, MB engineers would have recommended it, even though they may well have known 87 would be fine in most situations (of course, we are speculating). Bear in mind, those of you not getting pinging in these cars on 87 probably have engines in good condition, live somewhere below the mountain ranges, and don't drive at redline. If you're running a ragged example, or you do high-altitude or high-rpm driving, you're probably better advised to run the premium. I have a 32-valve M119 in my 400E, so I'd be afraid to run 87, but if I had an m103, I'd probably rotate 1 tank of 93 v-power for cleansing for every 3 tanks of 87.

I ran 93 in my Toyota Yaris when I went to the mountains recently, just because of the hard work the little motor has to do on the grades. It only specifies 87, but during hard shifting moves I have noticed some occasional quick pinging. So the lesson may truly be, hard driving, altitude, you probably need higher octane than sea-level granny driving, no matter WHAT your application

Just some things to ponder. First, Compression ratio does not mean the compression pressure is higher. Compression ratio is the volume inside the cylinder and head when the piston is all the way down and the volume when the piston is all the way up. The cylinder pressure is regulated by the cam shaft profile and timing. Ignition timing plays a big part in all if this. A high compression ration engine can be set up to run low octane. A low compression ration engine can be set up to use high octane. Its what ever the engineers were trying to achieve. For instance my 84 500SEL doesn't call for high octane for some reason and I have found that if everything is set to factory specs the car runs great on low octane but mileage is a little better if I use high octane and advance the timing about 10 deg.
So, use what was recommended or try both IF you can regulate the ign timing and see which works best for you. If the engine has no provisions for Ign timing changes then only run what is recommended.
My $.00 cents worth

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84 500 SEL (307,xxx miles)

I ran my '91 SE on only premium for 17 years.
But when gas hit $4 I just had to try mid-grade 89.
Absolutely no pinging or abnormal running.
So,,,, I tried 87. No pinging, no issues, even in the Dallas summer heat. The only thing I noticed is that she starts a bit easier when warm.
But, she's in good shape at 180k miles, and is now a town car, never seeing more than 4000 rpm and half throttle.

Waaaay back in '92 I noticed a bit slower starting when hot, and my MB service manager suggested I try regular gas; said it had higher volatility and wouldn't ping. I never tried it until now though.

Once, several years ago, the big thing was to disconnect the timing resistor to increase ignition timing. I tried this, and I did get hi-rpm, full throttle ping on premium fuel. So, that modification went back to stock pretty fast.

Yes, 9.2 compression could be considered high, on an old head design such as the M103. As stated, it depends on many factors of cam timing, quench area, combustion chamber design, ignition control, fuel atomization, etc. Back in the 80's, Corvettes had 9.0, and recommended premium fuel.

Oh, generally, the higher the altitude, the less octane required to avoid pinging. You always see octane ratings of 85 in the mountains.
This is due to the thinner air providing less cylinder filling, and thus less actual compression ratio. (and richer mixtures in the old CIS systems I bet).

For me, I would never do anything I thought was detrimental to "Old Pearl".
But, I just have zero evidence that says she requires premium fuel at this stage in her life. The savings is really minimal, but it's liike bending over to pick up a penny - makes you feel good. And she does start a bit easier.

Do what makes you feel good about your car.

DG

Here we are again. Same old argument. See the foto.

If your car has one of these then you can alter the timing to enable you to run with the Octane you like. These " octane adjusters" were installed in International versions of some MB M103 models. These were provided for cars that would possibly end up in a third world country which could only supply very poor octane fuel. If you have a car with one of these then the owners manual would have a section explaining the required adjustments re fuel octane.

Cars destined for most western countries with more strict emissions requirements came with a set resistor that would set the timing to a constant. In your owners manual there would be a section specifying the octane fuel to be used. there would also be a disclaimer that would say that using a lesser octane fuel than specified would risk the integrity of the engine.

Search R16 adjustments in this forum. II will also include a foto of the R16 resistor in a W124.

Been running 87 octane in my '89 for a couple years now. No problems.

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1989 300 SEL that mostly works, but needs TLC

You should use premium gas only because of the high compression of the motor. Using the wrong grade gas will cause preigntion and denotation of the air fuel mixture when its not wanted. You will be causing a lot of unnecessary heat in the piston area. Also when using the right grade gas you will greatly reduce the piston slap and prevent dieseling. Also you wont be buring all of your gas.

So bottomline you should use premium gas. Your mpg, power and operation of the motor will be better. The motor was desgined by engineers to use premium gas for a reason not because they felt like putting it there.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

I have an 88 300SEL I always use the 91 octane fuel, the car just seems to start and idle better with it.

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I'm sick of .sig files

It's more than just compression, it's the timing of the motor that makes use of lower octane fuel risky. You run the risk of detonation which will tear up your top end. Regardless of how 'fine' you think it's running, I'll bet that if you accelerate under full throttle or up a steep hill you will get detonation. The car has detonation sensors which may attempt to retard timing enough to cure the condition, but by then you will have already been detonating in your cylinders possibly damaging them. And your performance will suffer to boot.

I honestly can't see that saving $3 on a tank of gas is worth that risk.

Yes, is $3-4 per FILL UP going to make that much difference? 87 v 93 has always been around a mere 20¢/gal difference on an average 20 gal fill up... you do the math.



S-class Guru pointed out, there's FAR more that determines octane requirement than simply static compression ratio.

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Yep, and you really can't compare a modern car to a 20+ year old car because modern cars have computers that can compensate and adjust a lot.

Run whatever the manual calls for.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200