2.47 / 3.07 diff swap on '83 300CD - PeachParts Mercedes-Benz Forum

Alberta Luthier · #1 08-23-2007, 01:48 AM

I finally got some other problems in my life sorted out and finished my MB diff swap project. I just got in from a 500 mile road trip and really like it. Now it's got really long legs! It may be a bit flat in forth gear from 60 to 70 or so but pulls well from 70 up, and at 60 to pass I punch down to 3rd run it up to near 4000 rpm let it back into 4rth and it pulls great. I was driving like a fiend for hours on a roller coaster road with grades to over 9% and lots of tight corners, and pulled about 36 MPG. (needless to say the hairier driving was not in a very highly populated area)
I don't know it it would work as well on an SD 'cause I think they are heavier than my CD. Also the 2.47 diff didn't just click into place as the 2.47 is 5/8 inch or so lower at the input shaft. So I used washers to space it tipping it upwards a smidge to point the input shaft at the front engine mounts and spaced all other mounts and center support bearing to line things up. Is it a bit slower 0 to 60? It probably is but not so I notice it. Should others run out and do this? Maybe not if you drive slower or want more power at lower speeds. For me,I found the cars power more than enough stock with a nearly useless first gear and didn't like the sound of the high revs at 75 to 80 mph. Now at 75 it is so quiet it is almost spooky. For me it has made the car nearly perfect!
Alberta Luthier

t walgamuth · #2 08-23-2007, 05:42 AM

the 126 bodied cars dont really weigh much if any more and they are more aerodynamic so the gear should work fine for them too.

Tom W

dieselnut14 · #3 08-23-2007, 07:28 AM

Nice read, I am swapping my out in the near future, I like the idea that the power is there and it really is not a dog, what impressed me the most was the 36mpg, I'll report back how mine went, what did you do for speedo, odo correction? I would like to hear what is the best way to do that.

Peter

Hit Man X · #4 08-23-2007, 08:35 AM

Quote:

Originally Posted by Alberta Luthier

For me,I found the cars power more than enough stock with a nearly useless first gear and didn't like the sound of the high revs at 75 to 80 mph. Now at 75 it is so quiet it is almost spooky. For me it has made the car nearly perfect!
Alberta Luthier

Drive a 300SEL, that first gear is REALLY useless.

But I'm glad to see more people doing this swap, I will be when I decide to go through my rear suspension.

R Leo · #5 08-23-2007, 09:43 AM

Quote:

Originally Posted by Alberta Luthier

Now at 75 it is so quiet it is almost spooky. For me it has made the car nearly perfect!

That's exactly the way I feel about my wagon after installing the 5-speed. If/when I ever sort out the mid-range vibration issues and get the a/c going it will be one helluva roadtripper.

I'd wondered if a 2.47 would produce similar results.

Alberta Luthier · #6 08-23-2007, 12:12 PM

Quote:

Originally Posted by R Leo

That's exactly the way I feel about my wagon after installing the 5-speed. If/when I ever sort out the mid-range vibration issues and get the a/c going it will be one helluva roadtripper.

I'd wondered if a 2.47 would produce similar results.

I would be very suspicious of the drive shaft support bearing. I changed mine, the rubber was crappy and I am thinking that this may be what goes first, then the 'flex disks'.
My flex disks looked perfect but since I had it all apart any way, and I really didn't want to do any of this twice, I did everything.... support bearing, flex disks, axle boots, diff seals, trany support, diff rear mount, the works!
Good luck with yours,
Alberta Luthier

Alberta Luthier · #7 08-23-2007, 12:33 PM

Quote:

Originally Posted by dieselnut14

Nice read, I am swapping my out in the near future, I like the idea that the power is there and it really is not a dog, what impressed me the most was the 36mpg, I'll report back how mine went, what did you do for speedo, odo correction? I would like to hear what is the best way to do that.

Peter

I did the math about 4 times and could still be wrong on the 36 mpg , but I do think I have it right. I haven’t corrected the speedo/odo and have been converting the differences in distance measurements as well as the fuel volumes. (This is a bit of a pain as I am buying fuel in litres).

I believe if you have a donor car for the diff, you want the speedo from the same car. That would be the best way. Unfortunately (if I remember right) the member I bought the diff from (mbtechman) had already sold the speedo to someone else.
In the mean time, I am going to borrow a GPS to confirm my speed numbers, then I am going to take a pic of my speedo face and run it through photoshop, moving the numbers around, print a new face and slip it over the speedo indicator needle. It wont help the odo but at least I will have a firm idea about how much risk I am running with the RCMP. Generally around here they don’t bother you if you stay less than 20 kilometres (12.4 miles) over the limit.

Good luck,
Alberta Luthier

bgkast · #8 08-23-2007, 01:15 PM

Quote:

Originally Posted by R Leo

I'd wondered if a 2.47 would produce similar results.

Your clutch would not like it. It would be like starting in 3rd with the standard manual rear.

SW · #9 08-23-2007, 01:19 PM

36 mpg sounds reasonable for a highway trip. My car has a 2.88 rear end and I consistantly get 29-30 mpg on my daily commute in Houston traffic. The highest I've gotten is 32 pmg. I verified mileage with a gps unit (Garmin extrex). It has a trip odometer and displays speed.

Alberta Luthier · #10 08-23-2007, 02:07 PM

Quote:

Originally Posted by bgkast

Your clutch would not like it. It would be like starting in 3rd with the standard manual rear.

I doubt it would be like starting in third. Maybe a bit like starting in second, and probably not quite that bad. I doubt I would have problems with it, but I am not in city traffic much at all, (also I did drive truck at one time and that taught me how to use a clutch gently), and I would not expect it to work out well if my wife drove it much. Not that she is really hard on a clutch, but lacks the skill to baby a clutch a bit.
I am unsure of the weight differances and aerodynamic differances of these differant models. If the TD is heavier, or used for much city driving I would not reccomend the 2.47 diff.

bgkast · #11 08-23-2007, 03:01 PM

You are right, it would not quite be as bad as starting in 3rd, but it would not be a easy or fun car to drive.

Here are the effective ratios with the manual 4 speed and various rear ends:

Rear: 3.69 3.46 2.47
1st: 14.76 13.84 9.88
2nd: 12.7305 11.93 8.5215
3rd: 8.3025 7.785 5.5575
4th: 3.69 3.46 2.47

Manual transmission W123s only came with the 3.69 and 3.46 from the factory. Starting in 2nd with my 3.69 rear is not hard, but I would not want to do it every day, and 1st with a 2.47 is a significanly lower ratio than my 2nd.

A transmission calculator I made:
http://schumanautomotive.com/forums/showthread.php?t=3999

LUVMBDiesels · #12 08-23-2007, 04:56 PM

I bought a 2.47 diff last winter but have not time to do the work yet. I am looking for a speedometer from a 420SEL with that rear end and will swap the speedometer at the same time. I am looking to reduce the revs on my commute as well as I can't drive under 80 on the Thruway without being rear ended by a semi

Did the revolutions go down by a significant amount? I am looking for around a 900 RPM drop

Tell me more about the issues you had lining up the new differential.
It seems strange that it would not just line up properly.

winmutt · #13 08-23-2007, 05:05 PM

You ppl must not drive up hills. 2.88 and I struggle (although this is mostly due to manual stick)

winmutt · #14 08-23-2007, 05:06 PM

Quote:

Originally Posted by bgkast

Here are the effective ratios with the manual 4 speed and various rear ends:

Rear: 3.69 3.46 2.47
1st: 14.76 13.84 9.88
2nd: 12.7305 11.93 8.5215
3rd: 8.3025 7.785 5.5575
4th: 3.69 3.46 2.47

*cough* This is not entirely true, not all manual transmissions had the same gearing. But as a generality.....

Alberta Luthier · #15 08-23-2007, 05:13 PM

Quote:

Originally Posted by bgkast

You are right, it would not quite be as bad as starting in 3rd, but it would not be a easy or fun car to drive.

Here are the effective ratios with the manual 4 speed and various rear ends:

Rear: 3.69 3.46 2.47
1st: 14.76 13.84 9.88
2nd: 12.7305 11.93 8.5215
3rd: 8.3025 7.785 5.5575
4th: 3.69 3.46 2.47

Manual transmission W123s only came with the 3.69 and 3.46 from the factory. Starting in 2nd with my 3.69 rear is not hard, but I would not want to do it every day, and 1st with a 2.47 is a significanly lower ratio than my 2nd.

A transmission calculator I made:
http://schumanautomotive.com/forums/showthread.php?t=3999

That's really handy. Do you happen to have that info for a 3.07 rear end?

I do think that a 2.88 or even the stock 3.07 would be better for most people who have the good fortune to have a manual trany and who live in larger centers. There are few traffic lights here though and almost all of my driving is highway and in light traffic. I can still burn rubber from a stop with the 2.47 diff if I really get on it and if there is even a trace of dust on the road.