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  #1  
Old 02-16-2006, 08:49 PM
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126 differential interchangeability

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
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  #2  
Old 02-16-2006, 09:22 PM
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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??
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  #3  
Old 02-16-2006, 09:38 PM
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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
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  #4  
Old 02-16-2006, 10:50 PM
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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??
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  #5  
Old 02-16-2006, 10:59 PM
Hatterasguy's Avatar
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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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  #6  
Old 02-17-2006, 06:22 AM
t walgamuth's Avatar
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i predict

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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..I also have a 427 Cobra replica with an aluminum chassis.
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  #7  
Old 02-17-2006, 02:08 PM
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Wink Remember when...?

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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  #8  
Old 02-20-2006, 01:57 PM
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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
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  #9  
Old 02-23-2006, 04:06 AM
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Quote:
Originally Posted by S-Class Guru
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
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  #10  
Old 02-23-2006, 09:14 AM
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Quote:
Originally Posted by S-Class Guru
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.

"could I successfully re-calibrate the speedo".
DG
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.
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  #11  
Old 02-23-2006, 09:30 AM
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Quote:
Originally Posted by gsfent
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
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??
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  #12  
Old 02-23-2006, 09:52 AM
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Quote:
Originally Posted by Brian Carlton
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
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  #13  
Old 02-23-2006, 10:09 AM
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Quote:
Originally Posted by gsfent
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.
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