Many popular myths about octane and the supposed ill-effects of running less than premium in a MB routinely resurface--killing catalytic converters, fouling injectors, destroying pistons, poor mileage and performance. Not happening. People are mostly repeating hearsay. Knowledgeable experts agree that lower octane will not damage a vehicle and that it is likely to have little impact on performance.
The biggest determinant of a fuel's impact on a vehicle is quality, and it should be noted that the detergent quality of a particular brand of fuels is likely to be identical across their product line. Bad tanks and low quality suppliers can, and have made, a big difference.
Below is the text of one of my previous postings on this, found here
. It cites experts ranging from a well-regarded participant on this board to senior people at Mercedes, Porsche and the Society of Automotive Engineers. I believe it's worth reading, so I re-post.
One well regarded indy, Jonathan Hodgman of Blue Ridge Mercedes/AMG East (jhodg5ck on this forum), says: "IME, I have Never seen lower octane fuel hurt a cat." In fact, he uses 87 octane in his commuter wagon. His comments can be found here: Should I be running Premium fuel? (post #15)
Other experts seem similarly unconcerned about octane level. Below are some interesting quotes from "Why use premium gas when regular will do?
," USA Today, July 30, 2003:
I personally use regular even though my owner's manual says you'll get better performance with premium," says Lewis Gibbs, consulting engineer and 45-year veteran at Chevron oil company. He's chairman of Technical Committee 7 on Fuels, part of the Society of Automotive Engineers (SAE) Fuels & Lubricants Council.
I go back and forth, and I'm hard-pressed to notice" whether there's regular or premium in the tank, says Jeff Jetter, principal chemist at Honda Research and Development Americas. He drives an Acura designed for premium.
Our cars must be able to drive all over the world, and so we are able to run on regular," says Jakob Neusser, director of powertrain development at Porsche's research and development center in Weissach, Germany. "You don't have to feel that a mechanical problem or anything else will happen" using regular gas, even in the highest-performance, regular-production Porsches.
That same article goes on to say that the quality of the fuel is the most important factor, not octane. Carbon build-ups and other issues can contribute to the demise of a catalytic converter, but not octane.
In "The Low-Down on High Octane Gasoline
," the Federal Trade Commission notes:
As a rule, high octane gasoline does not outperform regular octane in preventing engine deposits from forming, in removing them, or in cleaning your car's engine. In fact, the U.S. Environmental Protection Agency requires that all octane grades of all brands of gasoline contain engine cleaning detergent additives to protect against the build-up of harmful levels of engine deposits during the expected life of your car.
From "Do You Really Need Premium
," Edmunds.com, August 20, 2007:
Until about 15 years ago, if a car called for premium gas and you pumped in regular, the car began to knock and ping and even vibrate. But that was before they essentially put a laptop under the hood of the automobile, said Dr. Loren Beard, senior manager of Environmental and Energy Planning, for Daimler Chrysler. Now, sensors take readings and tune the engine as you drive by adjusting the timing for whatever fuel you put in the tank.
The result is that a car that calls for the mid-grade gasoline will usually run on regular without knocking, Beard said.
Steve Mazor, principal auto engineer for the Auto Club of Southern California, said it is important to read the owner's manual carefully. The key is to figure out whether premium gasoline is "required" or "recommended." If it is recommended then a driver could opt to use a lower grade of gas, if they were willing to accept slightly reduced performance and fuel economy.
Volvo cars call for "premium fuel [91 octane or better] for optimum performance and fuel economy," said Wayne Baldwin, product/segment manager for the S60/S80. "However, there is absolutely nothing wrong with using 87 octane, as the knock sensors and engine management system 'protect' the engine from knocking."
Edmunds.com had a Volvo S40 in its fleet, so we consulted the owner's manual to see the exact phrasing in regard to fuel requirements. It said, "Volvo engines are designed for optimum performance on unleaded premium gasoline with an AKI (Anti Knock Index) of 91 or above. The minimum octane requirement is AKI 87." It appears that Volvo is making a recommendation for premium gas but is not requiring it.
The "Mercedes-Benz W124 (200, 200E, 230E, 260E, 300E, 230CE, 300CE, 260E4MATIC, 300E4MATIC) Owner's Manual
," pdf download via mediafire.com, states: "For knock-free operation the engines have to be run on fuels which have the minimum octane ratings: . . . . Unleaded premium fuels 95 RON/88 MON minimum." In the United States, Canada and some other countries, the number on the pump is the average of the RON and the MON. In this case 91.5 octane would seem to be the factory requirement, depending on how you read it.
In "13 Must-Know Tricks to Max Out Your Road-Trip MPGs
," Popularmechanics.com, author Mike Allen suggests:
Fill up with the fuel that has the recommended octane rating (let's assume it's 91) and drive for a few tankfuls. Check your fuel economy by dividing the miles driven by the amount of fuel consumed.
Now try a midgrade fuel and repeat. Make sure the driving route, traffic and weather conditions are similar. If the fuel economy is just about the same, try the next lower grade. Repeat until you see a drop-off in mpg. When you do, go back up one grade (to the one that didn't lower economy) for the best mileage.
Now, instead of dividing by the gallons consumed, divide by the cost of each tankful. This will give you the miles per dollar. If you are still saving money, reserve that better gas for times when you really need performance--like for towing.
I have followed this testing model in my own 1991 300E since buying it in June of last year, tracking the data in a spreadsheet. I logged over 40 tankfuls, well distributed between 93 and 89 octane, before even glancing at the results. Now with more than 60 tankfuls logged, still well distributed, I can say conclusively that I get 1-2 mpg better results with 89 octane than with 93 octane. Meanwhile engine temperature and oil consumption remain identical. This indicates that the engine is performing more efficiently with the lower octane fuel. And of course the fuel cost per mile is lower as well. I drive fairly hard, my mechanic friends tell me, and I have yet to hear a knock. Running 93 octane, I get about 320 miles per tankful. With 89 octane, I routinely get 340 miles a tankful or better. Recently, I got my first 400 mile tankful.
Most all of the user comments on octane I have seen on this forum have been opinions based on experience, usually second-hand and anecdotal. There are many horror stories passed along, failing catalytic converters, fuel injectors, clogged lines and more. However, in most of those stories the cause of failure is suspect or patently unrelated to octane. Few users are citing data and fewer still are acknowledged experts. As one poster to this thread says in his signature: "One anecdotal story does not constitute a statistic."
Note that all bolding and italics above is mine. Underlining indicates links to the sources cited.
Please excuse the length of this posting, but octane myths and flame wars are too common on this board. Users come here for accurate and helpful information on the operation and maintenance of their Mercedes. I feel that there is an obligation to call out postings that are inaccurate and not factually-based.
Pump what you like. Flame as you feel compelled.