87 or 89 Octane in CLS 550?

Are you really that cheap? You won’t notice much difference under light loads and normal driving because newer cars are equipped with anti-knock sensors and will adjust timing to prevent pre-detonation. Under heavy load and WOT you should/might notice a big difference in power.

In my area it only costs $5-10 more for 20 gallons of premium than regular.

It’s not about being cheap. It’s about wasting money on something you don’t need.

I know it won’t do any damage because the antiknock sensors will retard the timing if necessary. If there is no noticeable difference in power, then what difference does it make?

It’s not about being cheap. It’s about wasting money on something you don’t need.

I know it won’t do any damage because the antiknock sensors will retard the timing if necessary. If there is no noticeable difference in power, then what difference does it make?

Interesting. Not to quibble, but there indeed is a distinct «cheapness» about this, particularly since you mention «when gas prices went up». This topic recurs whenever prices jump, but it actually costs more when it’s cheaper. On average premium is about 20-25 cents more per gallon than regular, when gas was $4.50 a gallon it was less than a 5 percent premium. Now it’s more than ten. So you’re really paying LOTS more when the fuel is less expensive.

Second, I like that you «know» it won’t do any damage. So why even bother posting?

The reality is that moving around a spark (dynamic timing), enriching mixtures and other compensating mechanisms can only do so much. When a car is designed for regular, it will often make changes that will often result in better performance. However, when an engine is DESIGNED for premium, there are certain aspects like compression ratio and static timing that no amount of electrical finagling can change. The manual states that regular fuel can be used when there is no alternative, but the owner should drive gently and return to premium as soon as possible.

While it is true that the electronic feedback systems will do all they can to prevent immediate harm to the engine, simply avoiding knock is not all there is. How about carbon buildup over time? Additional combustion by-products from incomplete ignition fouling the O2 sensors and killing the cats early? And if it’s retarding timing and enriching mixture guess what? That’s less power output per rev, which means you need more revs to maintain power or speed, and in turn, less fuel economy. Experiment all you want, but that’s basic physics. (If you do wish to experiment, then keep good track of things, don’t just depend on the dash readout.) And where does it end? If you could find 83 octane and save another quarter, would you?

The bottom line here is expense. I don’t want to spend money I don’t have to, but I also intend to keep my car for several years. Facing costly and needless repairs later to save a few bucks when I fill up strikes me as the dumbest way to spend money I don’t need to. I chose to leave behind my Ford/GM mentality when I bought my MB, and if I want to apply that attitude to a benz, well, shame on me.

Take care and enjoy the ride,



Ya! Und very gut!

Would you rather have popov vodka in the plastic bottleor imperial vodka? I guess if you are really all about «saving a buck or two» then by all means, use the lower quality. My 99 ML320 went 158K miles on the same fuel filter using an up and down rating, averaging mostly in the 87 octane fuel. When I finally changed the fuel filter, I was seriously in a state of amazement as to how the vehicle managed to keep running. Yes, I would recommend making it a point never to use anything less than 89 Octane.



Thanks to Marsden and all the other rational posters. I won’t respond to the insults.

I just got my dealership’s newletter and they happend to have an article on the subject.

What Grade Do You Give Your Engine?
By the way, this is no “knock-knock” joke!

You pull up to the pump and usually have three choices: Regular, Mid-grade and Premium. Are these grades the equivalent of “good, better and best?” Have you ever been tempted to give your car a “treat” and upgrade? Do you really have to feed your high performance car that outrageously priced Premium grade?

These questions are not “fuelish,” and the answers are based on something called the octane rating of gasoline. The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. This is meaningful when you recall how a four-stroke, gasoline-fueled engine works. One of the strokes is the compression stroke where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a sparkplug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. High performance engines generally have a higher compression ratio. Higher octane fuel can better tolerate the greater pressures in high compression engines.

When gas ignites by compression rather than because of the spark from the sparkplug, it causes a condition known variously as knocking, pinging or pre-ignition. Knocking generally occurs when climbing steep grades, rapidly accelerating or driving at unusually high altitudes.

Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like Regular-grade 87-octane gasoline) can handle the least amount of compression before igniting. So, the compression ratio of your engine determines the octane rating of the gas you must use in the car.

The gasoline grades have corresponding octane ratings, typically 87 for Regular, 89 for Mid-grade and 91 for Premium with slight variations around the country. Most filling stations sell three grades, but some offer a few more choices between Regular and Premium. Even so, the typical gas station has just two underground storage tanks. Midgrade and other octane ratings are created by blending these two at the pump.

The basic question we want to answer is, “Should you consider spending the extra money for Premium gas?” The answer is a qualified, “No.” The fact is, for most car and truck engines, Regular grade 87 is fine.

That’s good news, but what if your owner’s manual recommends Premium grade fuel? The key word is ‘recommends,’ which really means it’s okay to use Regular under most driving conditions. Virtually every expert – even those from oil companies and auto manufacturers – say that using Regular will not harm engines, but you’ll probably not get the ‘advertised’ performance. In other words, you may add a second to the zero-to-60 mph time a reviewer said you should expect from your car. Again, using Regular gas will not damage the modern engines of most of today’s cars.

Modern engines with advanced computerized engine management systems rapidly adjust their ignition timing at the first indication of knocking. For this reason, using Regular will not void your manufacturer’s warranty unless, in the rare case, your car’s owner’s manual states that Premium grade gasoline is REQUIRED. The owners of high performance engines that require the highest octane are well aware of this fact because high performance was an important factor in choosing their particular cars.

However, if you own a “classic” car or a vehicle that is 10 years old or older and lacks such stuff as electronic fuel injection, computerized engine management and knock sensors and the owner’s manual specifies Premium grade fuel, DON’T USE ANYTHING OTHER THAN PREMIUM! If your vehicle has a supercharger and you drive aggressively, you might also want to use Premium. In this latter scenario, knock sensors cannot sense the condition fast enough because the supercharger boosts pressure too quickly. Of course, if your engine does experience knocking, no matter what vintage or sophistication, feed it the next higher grade of fuel or bring it in for a diagnosis and possibly a tune-up.

Using Premium grade fuel in a vehicle designed to operate on Regular is a complete waste of money. It won’t add to performance since the engine is not designed to make use of the higher octane. Don’t believe that bunk about Premium gas doing a better job of keeping an engine clean. All grades of gasoline contain detergents and additives intended to promote clean combustion.

The bottom line is that, it’s not a big difference in the cost of a tank of gas and if it contributes to better gas mileage, which I wasn’t aware of, then I’ll just use premium. The reality is that I don’t worry about how much I spend on gas for my cars. I have a boat that burns 40 gallons per hour and marinas charge about a dollar more per gallon. So when I burn over $150 worth of gas in my boat in one hour, it makes $75 to fill up my car once a week, not such a big deal.

MTI: Your comment «This is what happens when «just anyone» can buy a Mercedes.» really demonstrates your ignorance. Owning a Mercededs or any other car does not make you special. This is my first Mercedes. I’ve owned 7 BMW’s up until now. Your 10 year old C Class really doesn’t make you a «Somebody»