I want the diff to have a higher ratio than stock 2.47. for example e63 amg has a 2.82 ratio and SL63 has 3.06/3.07 ratio.

I'm wondering has anyone gotten the electronics working with new diff ratios?

My current idea is to swap in a diff and TCM from e280, which has 3.27 diff and also uses identical gearbox.

 

So you want the car to have lower top speed?
Why?
Messing up with digital cars can open whole can of worms. How do you synchronize transmission speed sensor with different speed from wheel sensors?

 

Well, with current gearing and grear ratios in 7G auto box my theoretical top speed is 274mph (438km/h) and if i would be using 3.23 gearing it will drop to 209mph (335km/h). Only place i can imagine doing such speeds at is German Autobahn and I dont get there often (ever actually ).

Also RPM growth on highway speed of 75mph will be neglible (from 1900rpm to 2300rpm) hence i believe my real life fuel mileage drop will be insignificant. 98% of my driving will be done on roads that have a 60mile speed limit and i dont do more than 65 there.

I firmly believe Mercedes used this tall gear in e550 to make the performance gap between e63 and e550 big enough to justify the 60k USD or so price difference they charged when these cars were new! M273 engine produces flat 530nm torque from 2800-5000rpm while M156 in e63 does a bit above 600nm in the range of 3800-5800rpm. Any ECU tuning company can get you ~30nm more from the e550 with flat curve ie. raw performance difference in these engines will be less than 10%.

Coming back to my original topic, I did some studying and correct me if I'm wrong but 7speed auto was common on all W211 facelift cars. I also believe all gears have identical ratios which means that the TCM units (i believe and correct me if I'm wrong) which are EGS53, are identical and interchangeable. So assuming i get a TCM module form say E280 which has a 3.27 rear end, i should be easily able to run Richmond gear 3.23 ratio in my rear end. Difference in ratios is so small (it essentially equals going up in one tyre size height index) that software should have enough tolerance in it.

Now, the question is if the TCM module data is somehow linked to other electrical modules in the car (ECU etc) in the car and can we somehow hack those links?

Here is a forum post that discusses it for EGS52 TCM unit.
http://*******.com/Thread-Change-rea...-doesn-t-works

 

What you are thinking about is common issue with pickups, where owners want better pulling power for those 20,000 lb trailers.
Don't think it applies to your situation
The 400 rpm difference on top gear will drop mpg quite noticeable, so IMHO you want to shoot yourself in the leg.
The truth is that computer interlocking and software programs are whole maze on those cars and MB is not helping with those issues at all.
I heard MBUSA will not let USA mechanics to open failed transmission on guarantee repairs and after fitting new one, the old one goes to Germany for inspection.

 

kajtek1, You ask, how do I synchronize transmission speed sensor with different speed from wheel sensors? If i install a e280 TCM unit which has similar rear end ratio, will this not understand the rear axle ratio and make it work? Is the TCM car specific (linked to VIN for example)? Cannot this be tweaked With Vediamo? If i understand correctly, there is no electronics monitoring the rear axle, only ABS sensors.

I completely agree that what i try to achieve has very little practical sense, but hey, if i wanted to have a practical car, it would have been a 220cdi. This car is a bit of a hobby for me and i really do no more than 7k miles per year (small town, i walk to work), so fuel bill in absolute terms is not an issue.

 

What I am trying to say, that by swapping the computers without knowledge how the system was build can leave you with nice lawn ornament.
I read about it few times when somebody started engine swap and the car ended on junk yard, or parted out.
Than I don't think you will achieve anything. With the HP you are limited by tire traction, so lower gearing will become useless.
In my diesel I can play C/S switch and can't tell the difference between starting on 1st or 2nd gear. Plenty of torque to the wheels regardless.

 

Gotta go with kajtek on this 1. If you really need to be quicker off the line, stick on a set of shorter tires. It'll be easy to reverse, and your car won't care even a little bit. The difference in engine noise, rpm, mileage, etc. between 1900 and 2300 rpm at 75mph would drive me nuts but hey, maybe you're already there or it's just a short drive...
At any rate, let us know what you do and how it works out! Life is too short to be one of the common sheep.