DFWdude

Perhaps I should be embarrassed to admit this...

My 2001 C320 had the 3.2L engine (91 octane required)... Sometime after 2005, there was a large, seasonal gas price increase, which prompted me to try a tank of 89 octane. I noticed a 10 minute period of peculiar performance as the onboard computer adjusted. Thereafter, the engine resumed its performance level as before. Curious, after a couple tanks of 89, I tried Regular 87. The car did not like 87 at all, but refueling at half tank level with 89 the performance was back to normal. I used 89 octane thereafter, until the car was sold last month. No problems at all (for 18 years).

The 3.5L in the E350 is a further development of the C320's 3.2L. The only reason I have not tried 89 in my E-Class is that I shop at Albertsons/Tom Thumb grocery stores, where I earn $1.00 off per gallon with my grocery purchases. So the price for Premium (93) minus the dollar per gallon is always less than the non-discounted pump price for midgrade (and regular, too). So, I've stuck with Premium since I acquired the car in 2018.

Each time we get a huge spike in gas prices, I'm tempted to try mid-grade 89, just to see what happens, although I seriously doubt there will be any ill effects. But so far, I've resisted.

So there, I've admitted it. Shoot me.

MBNUT1

I suppose one could say that and sure they both have knock sensors but, the M272 has 4 vs 3 valves per cylinder and 10.5 vs 10 to one compression ratio and 1 spark plug vs 2 per cylinder which I suspect would strongly influence it's tolerance to octane rating.

Rickman30

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pierrejoliat

He's pulling your leg, refineries make regular and premium, 89 octane is mixed at the pump.
Also, these cars can adjust to run 89 octane, but not regular, unless its a flex-fuel car, in which case it will run on regular or E85, the gas mileage and performance will suffer, especially with E85, which is why the flex-fuel cars come with bigger gas tanks, different spark plugs and ignition coils.

pierrejoliat

That is made by Sunoco, it will make little difference over 93 octane gas as your car cannot adjust ignition and spark that far out of range to make any difference, you would have to program the car to run that fuel, turbo boost as well to see any gains. Lead is not used anymore, you would have to add it, like for old cars.

Rickman30

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DFWdude

pierrejoliat is a career petroleum engineer. I believe he owns a related company, too.

BenzV12

I run premium grade fuel only

dmatre

We have run mid-grade (89) in our 2014 E350 since new. Now has 138k miles, and never has a problem.

Siegmann

105

PatPend

This makes me smile.

Rickman30

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Arrie

One thing here to realize is the type of the fuel supply in the engine, port injection vs. direct injection.

With the “old fashioned” port injection fuel octane rating is very important as the fuel/air mixture sees the high compression before the ignition spark. We don’t want the fuel/air mixture to ignite before the spark so we need high octane rating of the fuel.

DI engines are a bit different for this. Fuel is not mixed in air outside the cylinder so it is only air that sees the high compression before the spark.

Fuel is injected in the high compression cylinder just at the time of the spark or just a microsecond before that. Danger for knocking is minimal regardless of the octane rating or at least way lesser problem compared to the port injection. This all makes me wonder if I should try a tank of 87 to see how it runs in my DI engine.

DubVBenz

I've run premium in every gas MB I've ever owned. On my diesels I'll actually seek out higher cetane than what is available in most places near me (40). I've found between 45-50 cetane at various gas stations in Virginia. In my older M117 powered cars, I've removed the timing retard resister that was shipped with US spec cars to accommodate lower octane fuel which gives me a bit of a bower pump if I maintain discipline about using higher octane. Well known mod on those older engines.

cetialpha5

Not sure about the better part, only an electrical engineer here. Basically octane is ability to resist detonation. If you don't have detonation at a lower octane, then a higher octane isn't better if there's no detonation to begin with. Higher octane just costs more money so I would say higher octane is actually worse because it costs more money without adding any extra benefit. It'd be different if the btu content of higher octane gas was higher and therefore there's more energy in the fuel, but it's basically the same. You can only retard, advance timing so much and if you're already at the ideal setting, advancing it just makes it worse not better.

You basically need an engine designed to take advantage of the higher octane and that's more of a purpose built engine, using higher octane in an engine not designed to take advantage of it doesn't really do anything.

pierrejoliat

Correct, and sometimes premium has less actual BTU's depending on the additives, Toulene and Ethanol have less BTU's, by volume, so flow would need to be increased if those were used to increase octane.

MBNUT1

Love when people share real knowledge.

pierrejoliat

Kind of Arrie, Mileage will go down and your O2 sensors wont be happy,

Rickman30

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S18NGL

Just out of interest, the other day I heard a radio report (yes, I still listen to the radio!) discussing the cost of living and the recent fall in inflation rates across North America and Canada. One of the contributory factors was said to be a decrease in gasoline prices, due to the gas companies changing to a (cheaper) fuel mix for winter driving. Certainly I have noticed a fall in gasoline prices for the last month or two; I'm now paying less than I was in September.

Does anyone know if this would affect the octane rating, and possibly the performance of our cars during winter months?

Arrie

Why?

pierrejoliat

Gasoline is oxygenated in summer to reduce vapor point or vapor pressure, as it is colder in the winter months, gasoline is not oxygenated to improve cold starting. no price or performance changes

pierrejoliat

An engine designed to run on premium fuel, will not burn regular as efficiently, usually some type of dished piston is used to reduce this in cars designed to run regular in direct injection. hence more unburned fuel will be pushed out the exhaust valve into the exhaust system, this can mess with O2 sensors.

Crito

Correct me if I'm wrong, but the reason DI is high pressure is because the gas doesn't get squirted in at the start of the compression stroke but rather near the end. So there's no chance of low octane fuel igniting early. If it was squiring it in at the start of the stroke it'd be low pressure just like port injection. I believe a lot of modern DI engine also use the Miller cycle, which has all the advantages of Atkinson but without the weird connecting rods.

pierrejoliat

DI is high pressure, making the fuel delivery more precise and to atomize fuel better, also because it's being introduced to the cylinder under compression, yes. So there would be LESS chance of pre-ignition. Miller cycle indicates intake valve remaining open and pushing some air-fuel mix back out the intake valve, I believe you would need a scroll-type supercharger to facilitate that, a turbo would not produce the boost necessary at lower RPM's

Also I would add the ignition and combustion cycle is different here than in Europe, I believe the flex-fuel cars here have a similar sequence to those Euro cars.