So this morning i bring in my 08' c300 sport for some minor repairs.
I asked them to hook it up on DAS to check for any updates since i been noticing some random rough idling. The SA suggests that i use mid grade gas in winters since it's easier for the engine to burn. Is that correct??

 

First - I'd tell him to put it in writing on the dealership letterhead and sign it for when a warranty claim is needed for ANYTHING on the car.

Second - I would think the lower octane would be more difficult for the car to burn. I'm not an engineer or mechanic, but the manual calls for premium - the car get premium at every fill up!

 

Actually your thinking is incorrect. Premium requires higher compression than mid or normal. In this case, the lower the octane rating the easier is the combustion. There is a relationship. Now, that is not to say that premium, when recommended by the OEM, isn't warranted.

 

It is perfectly safe to run mid-grade gas in your vehicle today. Our engines have knock sensors and according to the detonation the ECU detects, it will switch to different ECU maps (regular, mid-grade, premium; essentially, but there are many more maps).

Now depending on the quality of the gas if the mid-grade gasoline yields detonation that the ECU has to retard ignition for, it may also make the fuel mix more rich, thus yielding similar power per stroke as premium, but using more fuel (worse MPG). On the other hand, the engine may just simply retard or delay ignition to prevent knocking, but keep the fuel/air mix the same (same MPG, but worse HP).

So no, it will not harm your car. The only thing that may happen is there may be less detergents or additives or no "secret blend" in a gasoline station's mid grade that the premium fuels have. So it could leave more carbon deposits and/or hurt HP/MPG even more. But, it is in fact easier to ignite lower octane fuels. And in the U.S. every gasoline car by law must run on 87 octane (unless diesel or electric or alternative fuel for obvious reasons). You will see in many owners manuals that it almost always says 91 octane RECOMMENDED.

In my experience when I was forced to put in a 1/2 tank of mid-grade because I was on a road trip and they were all out of premium, I found the car had much less power and made it feel very sluggish. No real difference in MPG, besides me maybe pushing it more to make up for the slower acceleration...

Bottom line -- I would just stick with premium, but why not try out a tank or two of mid-grade, you never know, you might be impressed...

 

Your service advisor has been smelling exhaust fumes too long...Here is an old post where we beat this topic to death

https://mbworld.org/forums/c-class-w204/347240-what-grade-gas-do-you-guys-use-c300.html

My post (on page 2) echoes mostly what was said by jctevere above....here is the repost

"
First of all, think of higher octane gas as being more refined (kind of like filtered). It has fewer impurities in it vs. a lower octane fuel. (this is all tested on a special engine made by Dresser Waukausha Engine in WI) Gas with a higher octane is more consistant (in terms of temperature) when it burns (fires).

Most (if not all) new cars come with a "knock sensor". This sensor on the engine block, is basically a vibration accelerometer that detects "out of sequence" ignition vibrations. This is were a cylinder fires when it is not supposed to. When this sensor picks up this out of sequence vibration, it sends a signal to the engine ECU to retard timing until the vibration goes away.

The higher compression of your engine, the more likely an out of sequence may occur. This is due to very complicated Thermodynamics, but let's just say the increased pressure during combution can cause pre-mature detonation before spark is fired. This can cause structural falure to all sorts of physical compontents in your engine. This is why timing is retarded to prevent this from happening.

So, to sum up, a higher octane gas is less likely to cause these misfires vs. lower octane and the higher compression your engine, the more you should be careful with octane you should be. This is why race gas has such a high octane rating as most have high compression engines (tuned this way) or they are running super-chargers or turbos (or NO2) which increases compression.
"


VPDFAN

 

The octane rating is simply how the refineries denote how easily a given fuel will combust. The higher the octane, the more temp and/or pressure it will take to ignite the fuel. Higher compression engines often require premium fuel to prevent ignition before the proper time, when ECU calls for ignition (spark).
As far as other differences in the fuel between regular, mid and premium; I don't think any of the major refineries add additional detergents, use additional refining or filtering or otherwise make any changes other than the amount of additives to adjust the combustion characteristics.
Basically, as long as you are not flogging your car like a rented mule, your ECU shouldn't have any trouble preventing premature ignition. Under "normal" driving, it is likely the timing will not need to be retarded much if any at all.

 

Correct about the refineries not adding any detergents or additives between regular through premium, but what I was talking about is the individual gas station themselves. You will see many off-brand name trucks filling up mobil, shell, bp, etc. Because the gas is all the same to every station. However, it is the station that will add its own additives and detergents to each class of gas (regular, super, premium) and those amounts will differ. This step is what distinguishes shell, bp and mobil gas from off-brand chains.

 

I am not sure if I am following you correctly.
I have never, ever heard of an individual station adding "ingredients" by dumping them down the tubes at the station. I am pretty sure that government mandates only allow the finished and approved fuel mix to be pumped into each fuel grades seperate tank.
As far as each name brand such as Shell, BP, Chevron, etc, if they are not fully refined and mixed with their additive packages from the refinery, they add their own additive packages at the fuel depots and test them to insure the proper mix before trucking out to the stations.
If you buy fuel from a "no name" station, you are likely getting a mix of most any surplus fuel they picked up though they all have to meet minimum federal requirements per grade of fuel.

 

I should have been more specific. By me saying the individual stations I meant to say the local service or distribution station. Not each shell, mobil, bp, etc on the corner of every intersection. Because, to my knowledge, the refinery pipes the same oil to each distributor. Then at each local distribution plant for the respective company is when levels for additives, dyes and other government standards are met.

After that, the trucks bring the oil from the distribution plants to a each individual gas station. I could be wrong, but this is what I have read and understood the process as. Haha.

 

Thought i'd add my 2c: A lot of newer stations only have 2 tanks in the ground: One for 87, and one for Premium (93 or 94 usually). The pump mixes them appropriately when you pump mid-grade gas into the tank.

Another comment is that I, also, haven't heard of stations adding anything special to their premium gas that isn't in economy.