Anyone know if a '91 420SEL diffy will bolt up to a '91 300SE chassis?
I know the ratios are hugely different, 2.47 vs 3.46, but that's the
mad project I have in mind.

Tks,
DG

I can't imagine why it would not fit. Same chassis.

But, it sure will be interesting to see that 300 push the W126 with a 2.47 installed.

Better wire the hazards permanently ON.

Did you figure a way to get it to start in 1st??

Yep, you're getting the wild idea. I'm sooo tired of doing 3500 at 70.
Fix it to start in 1st gear, gain 15% torque over the current 2nd gear start, cruise at 2500 in 4th instead of 3500. No hills in Texas.
I've been chewing up gas, and all the belt-driven accessories for so long, I'm gonna try this. Maybe. Found a diffy on ebay for $99, just might take a chance and bolt it in, see what happens.

DG

I'm very curious to see how it does. I'm really wondering if you can get 25 mpg on the highway? The diesels do a bit better than this, but, the cost of diesel fuel offsets the benefit.

How are you going to get the first gear start??

They all should bolt up. A W126 is a W126. I can't think of a reason why it wouldn't be a direct swap.

After you do it please post 0-60 times and MPG figures. It would be very interesting to see the results.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

that the acceleration will be a bit leasurely but it will be fine. and i think mid twenties will be possible if you dont drive too fast. say, 75.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

You could order a Nova, a Chevelle or an Impala with a six, about three or four different V8 engines (in many different stages of tune), various transmissions and almost any final drive ratio you wanted?

There must have been a reason why that rear end was not available as an option in '91. I can only guess that performance/reliability up and down hills (much less MOUNTAINS) will suffer. If you live where it is good (?) and flat you may see no real ill effects except a certain loss of UMMMMPHHH! at takeoff?

Shouldn't hurt your gas mileage...

Cheers!

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'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435

Well guys, I chickened out on the diffy swap - again.
I would need to make it start in 1st gear everytime, and reading some old posts about that modification were not too reassuring.

But the copncept seemed sound - gear up 40%, then start off in 1st and actually get 15% lower gearing on take-off. Then get 40% overdrive in 4th.
I compared the proposed gearing, and it almost matches exactly the 1-2-3-4gearing on my little PT Cruiser. That little 2.4L pulls the 3200 lb PT really well, so I thought the 3.0 might ease the 4000lb SE along sufficiently..

I did a search on 1st-gear starting. There were some old references to Bergwerks having an electrical shift-mod method, but I can't find that website anymore.
There was a guy who modified valve bodies for $500, don't know if he is still performing that service.
He shot down the quick/easy method of changing one spring in the valve body - said it would not be satisfactory.

Then there's the question of "would the 1-2 shift be acceptable on a daily basis". It can be pretty jarring at anything more than 1/8 throttle.

Then, "would the 420 diffy fit for sure", and be in good shape out of a used car, and "could I successfully re-calibrate the speedo".
Then, "could the little engine successfully pull the car along without having to downshift to 3rd all the time". Ha, I had an old Eldo with the 4.1L that downshifted when I met a truck!

So, maybe i'll just apply the 150th coat of wax to the Benz, and keep it as the Town Car. after 14 years of good service for us, it probably needs a rest from the long trips anyway.

DG

Would have liked to have seen this. Did it with a V8 BMW and it worked like a charm.
If you reconsider, maybe 2.65 better so the car will pull in top gear on the highway. 3.46 to 2.65 still a pretty big drop. I did 3.15 to 2.65 and it was just about perfect.
Regards,
Jerry

S-Class Guru,

I say go for it. I recently changed the diff in my 1984 399SD from a 3.07:1 to 2.47:1. Although my 0-60 time increased by 2 seconds (was 11 sec, now 13 sec), the car does not "feel" any slower than it did before. Driving down the highway it feels like a totally different car - it is very quiet and relaxed.

I guess I should quantify my thoughts on acceleration. With the 3.07 diff, I always thought the car was slow. I never raced cars away from stop lights - what's the use? I thought the 2.47 diff would really bog the engine down, but it didn't. Granted, mine always starts in 1st, but you would have a hard time guessing which diff I had in.

Before Thanksgiving (and before I switched the diffs) I drove 1600 miles Chicago -> Mississippi and back. While it was physically comfortable to sit in the car for 12 hours at a time, the engine screaming at 3500rpm wore me out. With the new diff, the engine turns 2600rpm @ 70mph.

As far as the speedo, there are several "black boxes" that can take the current speedo signal and step it up or down to give you a correct reading. They cost around $100 - if you have the ability to change the diff yourself, you should be able to program the box.

I say go for it. If you don't like it, you can always change it back. Everything is easier the second time around.

Where did you find a 2.65 for an S-class? AFAIK, they were only offered with 2.47, 2.88, and 3.46 (in the 300).

I suspect that you are referring to a W124??

You are correct, I should have said 2.88 for the W126. The same thought applies, 3.46 to 2.47 is a pretty big jump, 3.46 to 2.88 may be a better match. Still close to a 20% reduction.
Regards,
Jerry

I changed the SD from 3.07 to 2.88 and I really enjoy it. It's smooth and quiet at 3000 rpm at 70 mph. Typically, as a roadcar in the NE, 70 mph is the practical limit. I certainly could not consider switching to a 2.47 at this point because the vehicle is up against all it can possibly do to climb a decent hill at 65 mph. I really am not into downshifting into 3rd to make a hill on the open highway.