Gasoline Usage

gasoline
mercedes-benz
glk-class

#1

Some luxury SUV’s have “premium gasoline required” and I have always been told that all gas is basically the same. What are, if any,the consequeces of using mid-grade gasoline instead of premium?


#2

“Basically” the same still leaves room for differences. Higher octane fuel doesn’t burn at the same rate as regular octane. High octane premium is “required” in some motors to burn properly. Using regular gas in a motor “requiring” premium can result in less performance and can damage the engine, specifically the pistons.

Cars that “recommend” premium have sensors (knock detectors) that detune the motor to run on regular without damage to the motor.

Your questions are about high end SUV’s by MB and Audi. If these mfg’s require premium then that’s what you should use in them. If you want to own a high end SUV then you should be OK with the extra cost of premium fuel. Using regular in these motors could cost more in the long run if the motor requires expensive repairs due to low octane fuel.


#3

Octane is a measure of how fast the fuel burns. It does not relate to the amount of power the fuel has.

You need to read your car’s owner’s manual to find out if YOUR SUV requires or recommends high octane.

Requires means if you don’t use it you can damage the engine. Recommends means if you don’t use it you will loose power and reduce mileage. It may be cheaper to buy the higher octane fuel for your car.

Note, buying high octane fuel for a car that neither requires it nor recommends it, can be bad for the car, and certainly will be bad for your wallet.


#4

Hi Joseph:
I agree with your post, but do question one minor detail.

Rather than how fast a fuel burns, I always understood octane to be a measure of a gasoline’s self-ignition “flash point” temperature. Faster burn rates do help to reduce detonation, but I didn’t think burn rate was a function of octane.

Thoughts?


#5

Thanks for your comments; however, I asked about mid-grade gasoline which is usually 89 octane which is 3-4 less octane than premium. Do you comments/recommendations still apply?


#6

Your owner’s manual will either say
"An octane rating of xx or above is required.“
or
"An octane rating of xx or above is recommended.”

If it’s the former, and it says 91+ octane is required, then you have to use 91, 92, 93, or whatever that is at least 91.
If it’s the latter, then you can use whatever you want but your performance and mileage will suffer. It may cost you more to use the cheaper gas.


#7

I think you’re confused on the ‘all gas is basically the same’ comment. That’s often said about different brands of gasoline, and is somewhat true, but it is not true about different grades (octane ratings) of gasoline at all.


#8

According to Wikipedia:

The octane rating is a measure of the resistance of petrol and other fuels to autoignition in spark-ignition internal combustion engines.

The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical will ignite decreases as the pressure increases or oxygen concentration increases. It is usually applied to a combustible fuel mixture.



#9

The issue is that these engines have a higher compression ratio in order to get more power. If the octane is too low, the fuel-air mixture can ignite by itself (without spark) too early in the cycle due to the high temperature, causing engine damage in the process.

If a car says premium gas is recommended but not required, then it has a knock sensor to detect this condition and retard the timing to prevent damage. However, this usually means that your gas mileage is reduced, cancelling out some or all of price difference. Also keep in mind that if the knock sensor ever fails, then engine damage can occur; I’ve heard of one car where the knock sensor wire appears to be an attractive target for rodents in cars parked out in the woods.

Your comment about all gas being the same is probably referring to the additives, which are a different issue. All gas now has at least the minimum required additives.


#10

frjohnson…

Probably your question stems from the desire/need to save a few cents at the pump every time you fill up.

Here’s a quick and dirty calculation:

If you are like the average american driver you roll approx. 15k miles/year and consume, let’s say 1000gal/year (15 mls/gal). If the price differential between Premium gas and MidGrade gas is approx. a constant $0.2, you would save $200 in one year, or $16 per month.

Questions:

  1. Is the $200 savings worth the $8000 to $10000 in engine repairs that will result from detonation of the lower grade gasoline?
  2. Did you read the Owner’s Manual? MB has a huge section on the particulars of gas needed.
  3. Can you really afford a premium car, if you indeed need to skimp on premium gas?
  4. If the cost of premium grade gas is indeed an issue, I am afraid you will have a VERY unpleasant surprise the first time you take your MB in for some repairs.

Ha…here you go all…I beat Caddyman to it :slight_smile:


#11

Same answer, plus (89 octane) isn’t “premium” which is usually 92 to 94 octane.

If you want to use mid grade gas to save some money, go ahead your car will run. If the owner’s manual says premium gas required then you risk damage to the motor. If you want to take that chance, fine its your money.

All women are basically the same. But would you rather date Roseanne or Marilyn Monroe? While basically the same, plus just isn’t premium.

In some motors the difference is important, in others is doesn’t matter. If you buy a performance luxury SUV that requires premium and you run regular or plus in it and the pistons overheat and get a hole in them your warranty might not fix the car because you went cheap on the gas.


#12

If you can afford a Benz, it is assumed you can afford a few extra bucks for “Premium” fuel…Both the oil companies and Mercedes depend on this…However, the 87-89-92 octane spread is not very much and few engines will be damaged, destroyed or otherwise harmed by shaving a few octane points off the fuel you feed them…The only risk is destructive detonation caused by insufficient octane…Detonation at the destructive level is VERY audible, it sounds like someone is playing your engine like it was a snare drum, a sharp metallic rattle sound…

Virtually ALL modern engines have a “Knock Sensor” that detects this condition and instantly adjusts the engine to eliminate it or control it by retarding the spark and increasing EGR flow…These adjustments can reduce power and fuel economy while protecting the engine from damage.

Many “Premium Fuel Only” cars will run fine on mid-grade or even regular without any problems or loss of performance. You can be sure, when your Benz trickles down to it’s third or forth owner, it will be fed a diet of regular gasoline and in all likelihood, it will run just fine on it…The only way to know is run the tank down and TRY a 1/4 tank of mid-grade and monitor the results…


#13

Most drivers do about 15k miles per year or less. The difference between premium and mid grade in price is about $0.10 in my parts.

So you are talking about saving maybe $75-$100/year using premium vs mid grade and possibly damaging your engine and getting less power for sure.

Is it really worth the “savings”.


#14

And if the knock sensor fails? A rarity, but it can and does happen. Some engines are very sensistive to octane levels, the Nissan VQ V6 is a good example. In my car (Ford 4.6L supercharged) I had to drain the fuel tank because I let someone borrow it and they returned it full of 87 octane. My car has a custom ECU tune for 93 octane, it’ll run on 91 octane but pings under even a mild load. With the 87 in it sounded like a clothes dryer filled with hammers.

Bottom line put what’s spec’d in the manual, if you’re not willing to do that then you need to look at another car.


#15

Acura and Lexus requires premium as well. Hell, even my Mazda requires it, but the turbo kinda necessitates it


#16

Well, it depends on the model and year. My ES ‘recommends’ premium, I’ve found no difference, so I use regular. I think the 2011 ES also now ‘recommends’ premium, noting a few horsepower loss if regular is used.


#17

It is totally beyond my comprehension that a person would spend mega-bucks for a fine automobile like a Benz and then want to “cheap out” on things like the quality of gas, oil, preventative maintenance and repairs. Expensive cars are expensive to own. QED.


#18

f a car says premium gas is recommended but not required, then it has a knock sensor to detect this condition and retard the timing to prevent damage.

Can you show me the car that says premium is REQUIRED that DOESN’T have a knock sensor??? Even cars that run on regular have knock sensors!!!


#19

That’s bad advice. Premium fuel only cars run at high compression, which is why they require higher octane fuel. Saying “It’ll be fine” to run them on lower grade fuel is pretty easy to do when it’s someone else’s car, but if anyone follows that advice, they risk damage.


#20

Compression ratios mean little these days. Ford’s 3.7L V6 used in the base model Mustang sports a 10.5:1 compression ratio, but only calls for 87 octane. Similar story with the 3.5L Ecoboot V6 found in the Taurus SHO. It has a 10.0:1 compression ratio and two turbo chargers but it runs on regular.

In comparision the 6.2L V8 used in the current Corvette has a 10.7:1 compression ratio and it’s premium only. And the Subrau WRX has a lowly 8.4:1 compression ration and it too requires premium.