Well,

this just keeps going around. I can only say this: anybody who understands how a 4-stroke car engine works knows it does not harm the engine regardless of the gasoline octane rating. Too low (or perhaps too high also) could make the engine not to run but to harm it, no.

Low octane fuel is cheaper supposibly because they needs to add less anti-knocking chemicals in the fuel (used to be lead). This chemical is expensive additive (so they say) and this is why higher octane gasoline costs more (personally think it is just another way to make more money from the gas).

Higher octane fuel means more air with fuel can be drawn in the engine without premature detonation. This means air and fuel mixture can be compressed to a higher pressure and still make a timed detonation with the spark plug. This gives the best power output from the engine with the least amount of gas.

Using lower octane fuel limits the amount of air that can be compressed simply because the heat from the engine and compression and smoldering deposits inside the cylinder would ignite the air fuel mixture before the correct timing with the spark. To compensate for this knock sensors in the engine play the major role as they sense the knock (premature detonation) and start controlling the detonation timing and I believe also the throttle opening to limit air intake so knocking will not happen.

All of the control systems in the engine that have to work with the lower octane fuel compromises the best performance the engine was designed for. This means that the fuel mileage goes down when low octane fuel is used. The savings at the pump are lost with lower fuel mileage. The net result is your cost per driven mileage is pretty much the same but with high octane fuel the drive is far better as the full engine power is present.

I trialed for this with my Audi that I had and the difference in engine power was very noticeable between high and low octane fuels. No difference in how the engine ran, just that power was far down when trying to sprint with low octane fuel.

Also, for concideration: Japanese cars have for decades already been built with high compression ratio engines. This type of engine is built to run high octane fuel but all of them are marketed to run with 87 octane fuel. Reason for this is marketing. How would you sell a Corolla to a house wife who would never consider spending money for high octane fuel?

This is also the reason for the myth that high octane fuel increases the engine power. It is true with people who have been running with low octane fuel in a car that was designed for high octane fuel.

For so long time people have been driving cars with fuel that is lower octane than what the engine was designed for giving them "mediocre" performace. Then for one time when they fill up with the correct high octane fuel the power kincks in and they are amazed how high octane fuel gives more power when the truth is that low octane gives less when used in an engine that was designed for high octane fuel. There is no difference in the energy content between high and low octane fuels.

Using higher than design octane fuel in an engine that was designed for a low octane is total waste of money as you will not gain any more performance from the engine as it was not designed to give any with higher octane fuel.

What I am trying to say is that if the engine was designed to run with high octane fuel please do so. This gives you the best fuel mileage and engine performance. There is NO SAVINGS / mile in fuel cost when using low octane fuel. Low octane fuel could actually end up costing more / mile in an engine designed for high octane.

But again, low octane fuel DOES NOT DAMAGE the engine.

 

Okay....this will keep going unless we point out what I stated. Again.

Use what gas is recommended for your car. Forget about the damage debate, it's a skew off the main point: Just use the correct gas. My E Class is a 98 with 206K miles on it, I use premium gas just as I do in my CLS and R. It is what is recommended by MB, obviously for a reason(s).

I digress, people will do what they wish.

 

I'll trust the automotive engineers at M-B-and all the other manufacturers. The E class Operator's Manual says lower than 91 Octane WILL damage the engine.
BTW, it's about time to end this discussion.

 

I'm all for ending the discussion

What the manual says is this (from the W212 MY2011 manual, page 388)

"To insure the longevity and full performance of the engine, only premium unleaded must be used. Use only premium grade unleaded gasoline. The octane number should be at least 91 octane." It does tell you to drive carefully and not under load or at max speed if you find that you have to use a lower octane.

So I guess if one has a lease then fill it with whatever

 

MB knows best when it comes to these things, believe me they want their repair/warranty costs kept low...

 

what happens when you put low octane fuel in the new DI engines

will the damage be greater than in the old engine

 

High compression engine needs higher octane fuel to perform properly and to prevent pre ignition.
preignition will harm the engine(pistons, rings, ring lands and cylinder walls).
Computer controlled engines can minimize preignition by the use of knock sensors, when it hears a knock, preiginition happened, that's when it retards the timing. Just because it can retard the timing doesnt mean that it eliminates preignition completely.

Preignition means that the fuel is being ignited and is burning too fast(piston is still comming up on it's compression stroke and preigniton is forcing the piston downward causing the piston to **** in it's bore. That's why you hear the knocking sound.

 

Actually, once the ignition timing is retarded, there is no preignition. That is what the Knock sensors and adustable timing do. None of the current gas engines are "high compression" engines. They are "higher compression" engines when compared to the engines prior to ecu's, variable valve timing, fully computerized ignition systes, etc. Your question is based on the inaccurate premise that dmage would occur currently.

I encourage you to search and find any owner that has had an engine failure related to octane level. I have never come across a posting anywhere of such a problem. The only fuel related "disasters" I have heard about for the past 20 years have to do with someone putting diesel ina gas engine car or gas into a deisel engine.

Just read the manual and follow the instructions, rinse, and repeat.

 

I never stated that immediate damage will occur nor did I state anything in a form of a question.
if you dont mind a piston that rattles in the cylinder bore then ignore anything that I said.
If the ecm retarded the timing due to preignition that means preignition happened, there is no sensor in the fuel tank that will tell the ecm what kind of fuel you just put in it, ecm only responds to conditions.

Damage to the piston rings or lands can be caused by few things and it usually appears in a form of smoke out the tail pipe or loss of power, many times it is simply written off as that the engine is worn out/ tired.

 

The problem with this kind of discussion is that nobody dare to use lower octane, less than 91. My last poll from 45 samples, 42 people ALWAYS use premium and 2 people may mix it.

https://mbworld.org/forums/e-class-w...-gas-poll.html

So my verdict:
So regardless whether we agree or disagree, bad or good economy..your brain will "order" you to use premium the second you stop at the gas station...

I know you all love premium gas...

 

I, Sir, owe you an apology...I forgot to quote peeta with regards to the question i refer to. That said, unless your rings are damaged, the change in Octane (currently available US octane ratings) you will not have a piston rattling in a cylinder. Perhaps during the 60's or 70's, but not since the late 80's. The real problem with this thread is that everyone is right. Really. It is an academic discussion. The ocnfusion comes from not all of the "correct" things folks are saying apply to modern advanced engines taken as a whole (including the electronig nannies).

 

I am sorry but you are WRONG!!!

 

In the 60's and early 70's we had the luxory of using lead as an additive to boost octane and engines were high(higher) compression compared to the late 70's and 80's. When the electronic control entered the automotive industry once again they were able to raise the compression and power.
Even thou the piston to cylinder wall clearances are minute in modern engines they are still there and it's enough for that piston to rattle if the preignition happens.

It is difficult to blame just the low octane fuel as the reason that the engine smokes or lost compression, usually other things were the actual cause like running low on oil, not servicing the engine properly, overheating and abuse.

When person is suffering from clogged arteries, it was not caused just because he ate twinkies everyday ( it helped)

 

I've never felt too comfortable with super high compression motors (seem to be trouble prone after time, or will lose compression at a point, etc.), so using 91 is without question to me on one. I consider the current V6 to be a high compression motor, for an everyday/non "race" spec motor. The new 3.5L will have even higher compression, which definitely will make proper care and Gas octane more important.

 

You are mistaken. Lead did nothing to increase octane. It was an anti-knock lubricant. It provided a barrier between the rings and the cylinder walls. That somehow lead raised the octane level is a common misconception. In fact, lead does not burn or impact the ability of fuel to burn as does octane. it was designed to leave a residual coating on the cylinder walls and act as a lubricant. Todays "far tighter" tolerances, and super-hard friction reducing cylinder wall coatings, and improved Motor oils have replaced the need for residual lubrication from additives such as lead. Smoking is most commonly caused by oil seeping around worn rings (the result of poor lubrication) and entering the combustion chamber.

 

If it was really ok to use regular grade fuel wouldn't MB advertise/promote it? It would be a great selling feature especially in these years of high fuel prices...since they don't then I'd say it's not ok.

 

MB optimized the performance with 91 octane. If it was mine, I would run 91 just to have all of the performance I paid for. That's just me. For an individual that does not tend to exploit the performance of their cars at or near the limits, and choose to run a lower octane fuel, my point simply was that no harm will happen with 87 octane, and, this is true with virtually every car made today. Diesel has a cetane rating. Most pump diesel is 40 cetane and my diesel is rated by MB at 40 cetane. When I fill up with diesel rated at 45 cetane, I can tell you that I get 3+ mpg more than 40 cetane diesel. It is harder to find but some BP stations in my area have Premium Diesel 50 cetane. I have gotten as high as 44 mpg using that fuel. I had a Volvo S80 T6 that was rated with 91. Keep in mind that lower octane fuels ignite easier and tend to burn more completely. So, optimally, one would want to run the lowest octane fuel that provides the maximum power. In the case of MB E's that would be 91 octane. With my S80 I got my best mpg and "seat of the pants" performance with 89 octane....I put 110K miles on it in 2.5 years. Never a problem, never a ping, never a knock, and it was an impressively quick car (a very nice straight six cylinder). I would certainly consider another in the future. There is no harm in experimenting between 87 and 94. You will not rip the fabric of the universe or damage your engine. I would be curious if anyone has notice a difference between 91 and 93 octane. Some ECU's will adjust for higher octane too, yielding a slight bump. Hyndai Genesis actually publishes two different HP and Tq. specs for 87 and 91 and it is about 12 hp difference if I remember correctly.

 

This is exactly what I'm saying. Right now a lot of people will not buy an M-B simply because it requires 91. I'm sure they more than anyone would like to advertise that 87 is okay, if it actually was.

 

Since when has MB made cars for people who need to worry about 20-30 cent/gallon higher price of the premium fuel?!?

 

Right on!

+1

 

Page 363 of 2010 Operator's Manual: "Only use premium unleaded gasoline." "The octane number must be 91 min." Also: "To maintain the engines durability and performance, premium unleaded gasoline must be used."
The cost of fuel is a concern for many, even some M-B owners. Don't the A and B class and Smart use regular fuel? M-B is pushing the Smart and getting ready to import the B class.
That said, I am still running 93 octane. Could mix 89 and 93, but won't even try that.

 

Robert G is correct and you are mistaken. Understand that lead does indeed have antiknock properties, meaning the gas is less likely to autoignite in high compression engines. Since we know that the octane rating or number is an index of the anti-knocking properties of gasoline, please explain then how lead has nothing to do with this rating? If it doesn't then it has no anti-knock properties at all. I've never heard the term "anti-knock lubricant". How you put those two words together? It can't have anti-knock anything without increasing the octane number!!

Just know we are not actually talking about literal octane hydrocarbons (molecules), we are talking about a rating scale called on octane rating based on the antiknock properties of gasoline. You know, that little yellow sticker on front of the gas pump. Anybody that knows gas, or specifically race gas for high compression race engines knows you can't buy an unleaded gasoline with an octane rating near as high as you can with a leaded gasoline. Hmm... wonder why that is. Lead does not raise the octane level, it raises the "octane rating or index" level. Again that little sticker on front of the gas pump. That's what we are all referring to.

 

Have you seen the state of the economy lately? Some people that drive S Classes can't even pay their mortgages and every dollar saved counts to them, even the wealthiest people hate paying OPEC the extra $'s for gas; its also a good feeling to know your car can still be a real performer and save $ on gas either through lower octaine fuel and/ or new DFI technology. Also MB makes C classes selling in the low $30's and YES these people care to save the extra $$$ during each fill up. If MB thought it was ok to use regular grade gas they'd say so and be promoting it; period.

 

Oh..I was talking about actual, factual, stuff like what octane is. I am sure you are probably correct in a non literal or factual way. My bad.

 

Just because you dont hear a knock or a ping that doesnt mean that it's not happening.
It's your car and your engine , use any fuel that you want . If you want your engine to last as long as possible then use what the OEM recommends and perform all the maintenance that is needed by the OEM recommendation.

Iminpa, you can correct me all you want and then some, believe me you will not hurt my feelings in any way, at the end of the day, what I said is all true and research has been done by people much smarter then you and me.
You brought up the 60 and 70's vehicles into this conversation and I responded with added info to bing it into the perspective.
Happy motoring to you and everyone