Tenknots

I suspect this is a controversial topic, but here goes. I run my 2002 CLK55 on regular gas most of the time. I have had several cars I have done so over the years, including a BMW 7 series, a 1996 Mercedes S320, and Land Rover Discovery, both of which require premium gas. In well over 40,000 miles I have had no issues. Others have their opinions, but it works for me. Sophisticated engines such as these use one or more knock sensors to detect knock and instantly retard timing. It is so fast that I have never heard knocking, though on my Rover I did once the first time I used regular, then never again. Nearly all cars are built for markets that don't even have premium available. A friend who is a Porsche mechanic says their cars are sold all over the world and can run on nearly anything without harm.

One of the arguments against running RUG (regular unleaded gas) is that there is about a 10% fuel mileage penalty. I have doubted this but until I took a trip recently, I had no hard numbers. Last month, I drove from Washington DC to Athens, Georgia. I left on a Friday and returned two days later. The trip is about 600 miles each way. There was a heat wave when I made my trip, with temps in the high 90's and low 100's - I recorded a high of 104 on the interstate.

The weather going down and back up was nearly identical, though there were about two hours of rain on the return trip. Speeds were the same and traffic was never an issue. I drove 5 mph over the speed limit, the A/C was always on, as was the cruise control. I drove the same route up and back, mostly flat highway, some mountain driving. The trip up and back was as identical as I could make it.

I had been running PUG (premium unleaded gas) the last few tanks in anticipation of this trip and filled up again with PUG just before I left. The first picture is when I arrived in Athens (I accidentally reset the trip computer about half way down, but the numbers were the same during the trip). I filled up a couple of times with PUG along the way. When I arrived in Athens, I had a nearly empty tank and filled up with RUG. I then drove to Atlanta and back (about 150 miles) plus some city driving. Just before I left on the return trip, I filled up again with RUG. The return trip was only on RUG. The results from that trip are in the second picture. I was surprised to see that the trip with RUG gave even higher mileage figures. I can only assume it was because of the two hours of rain that cooled things off (into the high 80's and low 90's for two hours).


To me, this is proof that there is no mileage penalty for highway driving using RUG in a CLK55. If there is a power penalty, I can't feel it. I can always smoke the tires at will. I suspect any penalty is in the 3-5% range, which would be about 13-15 hp. I can live with that.

The price difference in PUG over RUG was always at least 10% on my trip.

cindyclk

As you say, it's your belief that there is no mileage change. You might be right but your way of supporting your ideas are not. When you run your car with identicle conditions both ways...once with premium and again with regular it might be more accurate. Your method can't take into account all of the variables associated with driving conditions. If your computer is changing the timing, there has to be a change in power output. If you want to drive your car w/regular, by all means do so, but your way of supporting it is way off.

Tenknots

Actually my way of supporting it is quite accurate since there is virtually no elevation gain. Two nearly idendtical 600 mile trips is plenty. It's not my belief, I proved it.

What surprised me more than anything was how good the mileage actually was. 26-27 mpg is pretty good for a car of such power.

OGCLK320

Why would you risk running regular gas in an AMG, If you cant afford premium gas, don't buy an AMG, most of all you just seem cheap, and your depriving your beautiful car of what she really needs, whether or not you see a difference, why do it? Do you also run non synthetic oil, just to say your car runs fine with that too!? To each their own, but this just seems foolish,
but since you save 8-10% it must be worth it huh? Lameee!!!!

dlbehrns

I don't think the man deserves any name calling. He just posted what he did and left it up to y'all. These engines ECUs are tuned to run on a wide range of octane fuels. When you get a chip tune/remap multiple settings are narrowed to run on a specified octane like 91 on my 320. Because those settings were narrowed I bet if I ran my car on RUG it would run like crap.

MarcusF

You’re right, you engine has knock sensors. With substandard fuel (which according to the powertrain engineers who designed the engine is what 87 octane fuel is - don't kill me, I'm just the messenger), a knock or ping occurs, and the knock sensor sends a signal to the ECU. The ECU then retards the timing. You know that timing change isn’t permanent, right? As soon as the timing is retarded, the ECU starts bringing the timing back to factory setting. Since the car is still running 87 octane fuel, it pings again. The knock sensor detects the ping, sends the signal, and the ECU retards the timing again. Once again, the ECU starts bringing the timing back to factory setting until it pings again. This happens over and over and guess what that means? It means your engine is pinging.

But wait, you didn’t hear anything, right? That only proves that the small ping from the partially retarded timing was caught by the knock sensor before full timing caused a knock with a pressure spike loud enough to pass from the combustion chamber, through the engine block, past the engine compartment, into the interior of the car, where you would hear it. At 70 MPH. Over the radio.

OK, so you didn’t hear the supersized pings, but the little ones, yeah, they’re still there. Hammering everything in the combustion chamber. How often does it happen? Well lets see, if at cruise your crank is spinning at about 2800 RPM, that would mean 11,200 compression strokes per minute. Hmmm . . . . . . of those 186 times per second exactly how many of them have to be pings to do damage? Hmmm, and how long did you drive this way?

But wait, lets not forget, you saved money. 600 miles at 27 MPG, so you burned about 21.4 gallons. If you saved 20 cents per gallon (regular versus premium) you have four dollars and twenty-eight cents extra at the end of your trip. I don't know what one buys with $4.28. A cup of coffee and a Wall Street Journal? Anyway, when something expensive goes boom, take the tow bill plus the repair bill (the dealer will probably give you a “free” loaner) and divide that by twenty cents. That’s how many gallons you will have to have burned for you to simply break even.

But hey man, whatever you think is best. I’m just a guy with a different opinion. And I thought it best if I explained why I have this opinion. If you feel regular is the way to go – it’s your car, go with it. We can be friends and disagree, right?

Lastly, the building in the background of my sig photo is one of the training facilities where Porsche sends mechanics for factory training. That is my car out front. Being an ex P-car mechanic, I thought I'd stop by for a visit. I've built a number of Porsche engines. All air coolers, but I'm what some may call “old school”.

SoCalw11w123

Pinging is Bad.

The factory setting for US is 91 octane.

I once ran the 93 Octane programming, but I got tired of adding octane booster in the tank every fill up.

just pay the extra 2-3 bucks each fill-up for premium gas.

jbenz99

If Mercedes Benz say premium fuel only .... Well then I use premium fuel only !!! They know what they talking about !! I think Mercedes is one of the oldest car's company on the planet, building one of the best vehicle around since the beginning !!! Plus higher octane fuel's has more cleaning agents so hence mean engine runs cleaning less carbon deposit !!

VividRacing