young

can someone explain this thread to me in simple english? or just summarize it...

the fuel settings change to 93 RON actually make it worse?

https://mbworld.org/forums/showthrea...&threadid=6756

SoCal240/6

I am trying to be good in this life, because I know if I am not, I will come back in the next life as a car in the hands of an "enthusiast" . . .

trench

Ouch, that thread gave me a headache.

young, I think that you have your car set correctly for the type of premium fuel available in your area. Here in northern CA we only have 91 octane so "base" is fine for us. I imagine the base setting retards the timing slightly from the 93 setting you have. I have no idea how much HP gain you actually get from this - although, wasn't there another thread regarding this complete with dyno pulls (I'm too lazy to look it up)?

Cheers, BT

WyattEarp

Well I don't remember enough about the otto cycle and automotive engineering to recall correctly but I believe this is the crux of your problem (problom) .

<i>Assumption 0:</i> Spark timing has to do with the point in time (measured in degrees of crankshaft rotation wrt to Top Dead Center) at which the spark ignites the fuel during the compression cycle.

<i>Assumption 1:</i> Higher RON fuels are less volatile (i.e. have better anti-detonating qualities.)

<i>Assumption 2:</i> To the use characteristics of explosives (which seems reasonable) Higher RON fuels have a higher RE (Relative Effectiveness) factor. (i.e. the amount of potential energy stored in a liter of 95 RON fuel is greater than the amount stored in a liter of 91 RON fuel)

<i>Assumption 3:</i> Advancing the timing means allowing the piston to come closer to TDC without allowing detonation (fuel self ignites due to the pressure and heat existing in the cylinder)

<i>Assumption 4:</i> The ideal moment of ignition is exactly at Top Dead Center, assuming you could carry out the spark, ignition and resulting explosion instantaneously. This is of course the approach to 100% efficiency and almost but not quite perpetual motion. Of course the second law of thermodynamics disproves perpetual motion, but 100% efficiency would be nice.

<i>Assumption 5:</i> Moving the setting from 91 RON to 93 RON advances the timing.

If <i>Assumption 1:</i> holds true then the goal of any efficient engine is to expend all energy spinning the engine in one direction. The way to achieve this is to strive for <i>Assumption 4:</i>. If you have a hard time visualizing this just think that the other four cylinders are on some other part of the cycle (power, exhaust, intake) since ignition takes place before you reach TDC (at or close to the pinnacle of compression) you are actually expending some of the energy fighting the natural direction in which the piston (and the rest of the engine) currently wants to move (braking the piston) to send it back on its way to the power stroke.

The problem seems to be that the base setting acording to stephens post is 95 RON so changing it to 93 RON would retard the timing therefore getting you further away from a<i>Assumption 4:</i>. After studying the screen carefully it does say: <b>"This correction is used to retard the ignition timing................"</b>. This would agree with what he implies. The timing is already set as far advanced as the computer will let you and I guess only ECU reprogramming can help there.

Now about the fuel richness issue there is some ambiguity I'd have to clear up, but if you follow the motto of the timing settings. The correction screen reads: <b>"A change is made to the fuel quantity with this correction ............."</B>. All other things being equal, more fuel will only get you more performance if you were running lean to begin with. Otherwise you will just run rich and have unignited fuel in your exhaust gases. If of course you had straight pipes and made the car run very rich to make sure you always saturated the possible ignition on the compression cycle you could have a nice display of fireworks coming out of your tailpipe.

My suggestion therefore is to leave the timing setting at base if all above holds true and to change to the rich fuel setting only if you plan on getting a pulley kit. After careful study that is what I am going to do.

Bos

I was beginning to wonder about the fuel setting / ignition change myself. When I first changed them the performance felt better, but now I experience hesitation on occasion. Now I'm beginning to think my initial impression was all in my head.

Wyatt... Great post!

young

wyatt, i asked for simple english (inglush )

i'm sort of w/ bos. the perceived changes may be just mental - either way, good or bad.

so, you're basically saying that the changes to Stage 2 and RON of 93 are actually moving away from optimal performance.

and you're also saying, correct me if i'm wrong, go get the kleemann pulley kit and i can leave it at the settings that i have set it to, right? i'm ok w/ that

WyattEarp

Stage II -- Yes with pulley.

RON 93 -- no go back to base (RON 95) especially if you get the pulley.

You also have to figure that the differences in timing between the settings is probably minimal since I doubt mercedes would take the chance of having one of their cars spitting and sputtering up and down the road after someone decided to change the setting.

As far as simple english goes I apologize. At least I didn't include any equations.

young

ok, by utilizing my remaining brain cells, i think i can make the following conclusion <strike>(which basically confirms trench's post).

stephen's post makes a reference that 91 US octane rating is equal to 95 Euro Octane. so when he says base of 95, he's referring to the euro rating.

when i refer to 93, i mean, of course, the US octane rating. which is what, 97-98 Euro Octane? so by correcting that setting to 93 Octane, i am advancing...</strike>

try again.

the RON settings in the computer is actually using EURO octane ratings. so a 93 is actually equivalent to a US Octane of 89. (which means i should be using 89 fuel right now! hey, cheaper gas ) which begs the question, where is the adjustment for US93/Euro97or98 fuel?

now, if it is using US octane ratings (which i would have assumed in the first place), then i'm ok and my initial thoughts regarding this setting is correct and i'm good. this is what trench said in his post.

WyattEarp

I agree if the computer settings are in US octane then 93 RON would be the one to use. However the computers instructions legibly read "This correction is used to retard the ignition timing................" it doesn't say retard or advance. Second lets say that they are using US Octane settings then what is base 89? The only real way to know is to ask MB however most indicators lead me to believe that these are Euro octane numbers.

Spyke

A perhaps trivial bit of information... because of the way efficiency is calculated for IC engines, most anything with a blower will be rated as over 100% efficient. (of course were talking two differnt types of efficiency here, i realize that, but still)

Regarding your comments on firing the plug at TDC to avoid the engine having to work against combustion presuure, keep in mind the piston doesn't move much at all when it's near top dead center.

what you're trying to do is get the highest Mean Effective Pressure, and lighting the mixture at TDC isnt going to do that for you.

speedfrk

OK, this is so screwed up I don't even know where to start. What you see on the pump in the US is an average of the RON and the MON (research and motor octane #). So a fuel with a RON of 93 will really have a pump octane # of about 91. 93 pump octane will have a RON of about 95. RON is always higher than pump octane due to the averaging with the motor octane #. Premium gas does NOT have any higher energy output than regular gas-it only allows the engine to be run at a higher state of tune. That includes a higher compression ratio, advanced timing, etc. Since supercharging in effect runs the engine at a higher compression ratio, premium fuel is needed. The interaction between ignition timing, fuel flow and power output is so complicated, that you can only make some very general assumptions. In general, advancing the timing will increase power and economy to a point. This is all dependent on the way the engine was designed. Advancing the timing means you are firing the plug (more) before top dead center. The timing changes constantly with engine rpm, so the number you always see quoted is at idle. But advance curves can be changed infinately in a computer controlled engine. Ignition timing, fuel flow and rpm are usually mapped 3D to determine the best settings. It is very complicated, and most of what you read in these threads is either wrong or overly simplified. If Mercedes provides a different map for use with high octane fuel, it is fine to use it if you have that fuel. Remember, if it calls for 95 RON, that is 93 pump-which is just regular premium in the US.

Lynn

The spread between MON and RON is usually about 8 points. 91 in the pump is usually 95 RON. Please note that I said usually, not always. The spread can be less, and rarely it is more.

mdp c230k

Wrong, a gallon of gas has the same P.E. at any octane rating. Octane does not add power to the gasoline. It serves to stabilize to gas under higher pressures and temps without self-combustion allowing an engine to have higher compression without detonation. The engine will make more power due to the compression, not due to the fact that the high octane gas has more power than a lower octane.

WyattEarp

Thank you sir I am duly humbled. :o I am sure you took great delight in pointing this out.

vadim

Wait a minute... He merely corrected your assumption, why instant personal attack?

mdp c230k

Vadim,
If you read almost any post by WyattEarp you will see a trend of personal attacks. Usually, this is due to insecurity. I have not met Earp, so I can't make a propper diagnosis, but it seems very likely. I have learned to take all of his comments in this light. Sad, ain't it?

young

alright. break it up. break it up... (pushing people apart) go home, and sleep it off...

back to the original topic: i'm convinced that the setting is actually correcting for less than 91 US octane. i've setup an appt to have it reverted back to base (the service consultant was very curious about the whole thing but i couldn't explain it to him very well)

WyattEarp

I'm not sure how my response was construed as a personal attack. I even put the little faces on each comment to convey meaning. First face is embarrasment second face is a frown.

As for your proper diagnosis Dr Freud I am not insecure at all. However I am very direct. Finally there is nothing sad about it except perhaps.... oh well enough said.

MB-BOB

mdp, I think that WyattEarp has tried to be less biting and more friendly lately... I have actually enjoyed his informative thoughts earlier in this thread. I believe he is trying to turn over a new leaf and deserves a second chance. We all make a$$es of ourselves, occasionally, and I for one, appreciate the courtesy of a second chance when I mess up. We all need to be less sensitive here if we expect others to be less sensitive...

mdp c230k

point taken...

WyattEarp

Oh gosh i've made an a$$ of myself. Seriously I think it's mostly context and personality. I have a direct, no nonsense demeanor. Some people find this insulting and some find it funny.

One final thought to mdp. When accussing someone be careful you are not committing the crime yourself.

trench

Back to the original topic...

young, as far as my understanding of the settings goes, I don't think you need to change the setting on your car. I think the base US setting is the middle setting for the computer which allows for US pump octane rating of 91; which is the minimum for premium in the US and all that happens to be available around my location. However, in some parts of the US they offer 93 pump octane as premium. You should get a little bit of a performance boost by being able to advance the timing a bit without incurring detonation (maybe 1-2 hp max). Which is what your car is set for (93) the other lowest setting is for people who do not have access to (91) in their location and have to use a lower octane gas (89). I think the lower setting is necessary because at high altitude locations only lower octane gas is usually all that is available (because with less air at higher altitudes the excess octane isn't necessary to prevent detonation); however with a forced induction engine the motor pumps air into the manifold regardless of local air density at altitude and detonation could occur without going to the lower octane setting on the CPU(which will incur a power loss).

BT

avlis

trench, I think the question at hand is whether or not the number we see on the fuel setting screen are in US octane or RON. Until recently we thought (assumed?) it was US octane. Now we have reason to believe it's RON.

young

i think the main issue is that i suspect that the setting for 93 is actually using the Euro RON which is equivalent to our 89 Octane. does that make sense? so i've really just set the setting to 89 octane. i think that wyatt's interpretation of the screen message is correct.

trench

I realize this, although I swear that whomever originally posted that one could change the octane settings said that the cars came from the factory with the CPU set to the middle setting (sort of like the Bose AudioPilot) and that there were two additonal settings one above and one below. I could be mistaken though.

BT