Untertürkheim

The C loaners I get always sound like they are popping corn in the engine when I pick them up, I doubt that is "fine". The engine will have to constantly retard the ignition pattern and probably also enrich the fuel/air mixture. At the very least you get bad carbon buildup, and you run a high risk of causing substantial damage even in the short term.

The fact is that using 87 will void your warranty, the service dept can find out in a couple of seconds whether or not you are using the proper fuel.

I would simply try to get a 90% reimbursement agreement from your company that allows you to use premium.

narky

I second this opinion. Get it in writing, and on you go.

RLE

>>To answer your question, it is very easy for the service dept to tell what gas you are/have been running, and yes, it will void your warranty.>>

And they do this how?

EmceeSquared

Aight. I got less lazy and searched for the old posts. Read some well written and intelligent responses to previous posters on the same subject that expanded on what Akasigfreak and Unterturkheim wrote. I know that hi perf/hi compression engines are more likely to suffer from pre-ignition..I just didn't think the c350 would be in that group (still not sure this applies specifically to the c350).

Nonetheless, I've already loaded her up with the first of many rediculously expensive tanks of 93 octane fuel (or whatever it really is gas stations are putting in there).

Just because something is theoretically harmful does not mean that it does harm in the great majority of cases. ie. theoretical risk of cancer from certain levels of radiation exposure or known potential carcinogens.

Thanks to several of you who took the time crafting helpful responses - I appreciate it.

ps. Narky, that's the best avatar I've seen in a while...

naifali

I just dont understand, why would someone wanna fill up 87 in a merc. You spent so much on a car then forget about the important details like fueling up with 91. Here in singapore, i fill up 95 and thats the minimum!

akbro93704

Yeah, me being one of those people. But as another poster said, whenever I have had a loaner, they PING LIKE CRAZY. I mean really crazy. I dont care how much people say that the ECU detects the lower octane and adjusts, it doesnt adjust enough. Put premium in your car! I've even noticed slight pinging once in a while in our cars when we have ALWAYS used premium gas in them. These engines are very sensitive to octane levels.

methodz

ya, i don't have my c class yet but my old car had a knock sensor on it. if you ran lower than 91 it would retard the ignition and adjust to stop the knock. but that doesn't mean the knock isnt happening. assuming all modern day ecu's operate the same way the 87 would cause all kinds of knock that would have to be compensated for whereas it would run normally on 91. all that takes its toll if you run 87 all the time you would ruin your engine in the long run, which is probably why those rentals sound the way that they do. the dealer could probably tell this by the ecu reading and you would have a harder time than normal trying to disprove them.

if you get 87 free, like someone said just buy some octane booster to bring it to at least 91. my old car was and extremely high compression engine though and i'm not sure of the ratio in the c class but if it says 91 you should run that. there shouldn't be a need to run higher, forgot the technical reason but i could look it up, something about the engine ignoring the added octane, you would have to mod other internals to take advantage of the added octane.

besides, for those that don't get free 87 gas the difference at the end of fillup should only be a few dollas, is it really worth not using the recommended?

Untertürkheim

1. The fuel air mixture and timing can be read in real time when the car is in for service.

2. Preignition stores fault codes in the ECU, so even if you fill up with premium when you bring the car in, there is enough information stored onboard.

Untertürkheim

Compression ratio on the C350 is 10.7:1, that is high compression for a gasoline engine.

narky

So what's the verdict on these octane boosters I keep reading about? Any possible negative repercussions to using these? I've done some research on Google, and I'm getting mixed reviews. Most sources also say that not all octane boosters are created equal. So which one have you guys used and would recommend?

I would never put 87 oct + booster in my tank, but I'd consider 91 octane (the highest available in my area) + booster. If it works, I assume that performance would increase, however little.

TIA.


Thanks!

ATSOBE

In manual state ony when emgency arrive use lower octabe. You will notice your timing will slow down that why stick to what the manuel say i tihnk i read somethign about the timming in it it will slow it down becuase of not proper octane

marakbai

I agree ... how much money are you saving anyway? If the majority is free, pay the difference and get the good stuff ... a 3c difference can't stop you from driving a car, that's a tough position to be in. I normally use 94, but witht he gas prices have gone down to 91, but at least I still do the premium

JNLeChevet

The engine's compression ratio (CR) determines the octane of the fuel required. MB builds their engines with a CR in the 11.3 - 11.5 range. The spark advance curve and valve timing are designed around this CR and the 91 octane fuel required. If a lower octane fuel is used, the flame front advances too rapidly and the expanding gases begin to push on the piston before it starts down on its power stroke - result is ping/knock. The knock sensor then retards the spark timing which results in incomplete combustion. Incomplete combustion causes power loss, deposit build-up, and other expensive problems. The knock sensor is there to take care of short term knocking caused by localized heating, engine stress, etc.- not long term use of low octane fuel.

Unless you decrease the CR [dish your pistons/destroke the crank/increase the head cc's/use thick head gaskets] and reprogram your ECU, you can not use 87 octane in an 11.3 CR engine over the long term without damage. BTW, using a higher octane does not buy you anything unless you increase the CR and reprogram the ECU.

Most octane boosters on the market are junk. If the ingredients state "petroleum derivatives" or something sililar, you are wasting your money. The effective ones contain metallic alchohols (MMT, Tetra ethyl lead, etc.) and don't play nice with your emission control system and sensors. Some aromatic hydrocarbons are effective, but you need to use quite a bit to be effective and they cost far more than the difference between regular and premium gas.

You can afford a Mercedes instead of a KIA? You can afford the extra pennies for the right fuel!

maninblack