I use 89 with my E320 with no problems (the computer makes the necessary adjustments). I am new to the SLK and wonder if I can use 89 Octane with this car (230 Kompressor)? Anyone have any experience here?

 

There's the chance that everything will be ok, but also the chance you develop a serious engine knock (faster than you can blink) and since your probably out of warranty thats going to bite the big one......this isn't like Ford where you can buy crate motors from the JC Whitney catalog

17 MB's, eh? Just spring for the extra .14 a gallon

 

The others (17) are all gone - ex wife and hers, and the kids now have their own.

 

"No problems." does not necessarily mean no difference. Even if there is no knocking and no damage, you have to realize that the car is designed to run on higher octane fuel, so lower octane will give you lower performance, and more importantly, lower fuel economy.

You might pay $0.10 less per gallon but if you get 5% lower fuel economy, you don't actually save anything.

 

During the first gas "crunch" of the 70s (maybe the 80s - I'm gettin' old) there was an article in The Star about Octane usage and how using the middle grade would not harm the engine, but would result in a little less horsepower that really would only be noticable on hard acceleration. It was supported by lots of numbers and equations. In the next quarter's issue there was a retraction. And it was obvious that the order for the retraction came from on high. And even then the retraction was somewhat weak when you read between the lines. I always used the high grade for the V12, but not for the sixes.
I know lots of people use the middle grade and was wondering what their experience has been with respect to a little engine that is supercharged.

 

I agree with the article, there shouldn't be any damage to the engine if the knock sensor is functioning properly. However, the reason there is a loss of power is because the timing is retarded when using lower octane fuel, and that also means lower fuel economy. You have to weigh the percentage decrease in fuel cost to the percentage decrease in fuel economy, but in general it's probably not even worth your time to make the calculation, just use premium.

 

Why would you want to use the mid-grade? If it is about saving money, in the long run it will most likely cost you more and in some situations make your vehicle less fun to drive.

A recurring theme seems to be that using a lower than recommended octane will save you money. Now, how much are your really saving if you use the lower octane? If you live near me, you might see 87 at about $2.11, 89 at about $2.21, and 91 at about $2.31. Most of our current MB's in the NA require 91. Let's forget about 87, it is too low. The difference in price between 89 and 91 is $.10/gallon or about 4.3%. Based on some reports on another SLK forum, using 89 degrades fuel economy by ~3%. That gives a net savings of ~1.3%. While at the pump, you will save about $1.50 for 15 gallons but you will be going back to the pump more often. Your net savings on the 15 gallons is really only about 51 cents plus you'll be at the pump more often. I put a high value on time, so for me this washes out any savings (and makes it even less desirable to do).

Now when we are talking about it being ok to use, are we talking about the short term or the long term? What about increased maintenance? Just because you don't have ping or knock, because the engine management system is able to compensate, doesn't mean that everything is as it should be. The lower octane fuel is burning less efficiently (seen in the reduction in fuel economy). If the fuel is not buring as efficiently, there will be deposits in the cylinders,and elsewhere, that will need to be addressed at some point. So, no catastrophic failures but something you'll have to pay for - or the next owner.

If it is about saving money, it would be better to get a gas card or other credit card that gives you money back when you buy gas and make sure your tires are properly inflated - or even up the pressure a little to improve mileage.

 

below 93 is no good

 

Is that 93 as in posted octane = (RON + MON)/2 or is it 93 RON?

If you live in a location where RON is specified instead of (RON + MON)/2, the minimum octane rating will have a higher value. That is because RON is tested under "gentle" conditions. Not all fuels with the same RON are created equal. Some will be better behaved under more stress (for example higher temps) than others.

In the western US, 91 is typically the highest octane rating to be found as stated above. That is 91 based on (RON + MON)/2, where RON is 96 and MON is 86 typically. In the eastern US, the highest grade is 93 and 91 is the midgrade. For some reason, in the mountain states, 89 may be the highest grade to be found - which is based on really old data that says that minimum octane decreases as altitude increases.

There is no benefit in going above the minimum octane specified. It will not do any harm but it will not improve performance either. A possible exception to this could be fuels that are rated by RON. I don't have much experience with those fuels but there could be regional/local differences (at least based on posts from other folks). For my friends here with (older) vehicles that require 93, only having 91 is disappointing.