Hey Y'all,

I have an '82 300 SD with over 300K on it that I was on the verge of selling since it wasn't the highway car I needed it to be. It runs perfectly but simply put, it was loud! The loudness was from two things; one was that the differential gearing was too short for 80 mph cruise and the other was that one of the pinion bearings was whining and needed to be replaced. Hmmm...do I smell a project?

The project idea really arose from the fact that I used to own a VW TDI that I put a taller fifth gear in and I reduced the noise and gained 1-2mpg's at the same time while retaining the TDI's stunning 80mph passing power. I figured that the 617 could also benefit from lowering the rpms back down toward the 2400 rpm torque peak for lower noise and higher efficiency and I hoped that the torque peak would keep the car from being doggy on the highway. I could accept some compromise but not too much as I don't want to die trying to accelerate into traffic.

To predict what might happen with the diff change, I have an Excel spreadsheet that allows you to plug in tire size, diff gearing and transmission gear ratios to get engine and road speed in relation to each other. All calculations showed that by switching the 300 SD differential to a 2.47 from a Gasser 126 V8 I would lower the rpms on the highway by around 600 rpms! The big question was whether the donor rear end would have a bad pinion bearing as well and whether the lower engine speeds would turn the car into a dog. I was also a little shaky about the required yoke swap on the diffs and whether I'd mess up the drive axle boots or joints doing the swap. I did some reading and determined that swapping the whole rear axle assembly was the most time-efficient way to get the job done though I paid a little more for the axles.

Removal of the 1985 380SE's rear end was a little nerve wracking as the flex plate's dowels weren't ready to let go of the differential yoke but a properly placed screwdriver did the trick in separating them. We were careful to keep the drive axles in as straight a line as possible to keep the CV joints and boots intact. It took about 30 minutes to extract the 380 rear end including carefully removing the diff speed sensor wiring from under the rear seat in case I want to switch to ABS later. (The donor has ABS and I've done VW ABS swaps in the past so I'm not afraid of the job.)

After the donor rear axle assembly was out, I put my car on the lift and promptly removed the rear exhaust section which is so damn easy! I wish it was that easy to remove the exhaust on VW's. Anyways, I unbolted the drive shaft flex plate and found that it was cracked all the way around and didn't have long to live. I removed the center heat shield and tied some baling wire to the two rear heat shield studs to hold up the drive shaft as you don't want it to hang! Removing the heat shield was necessary to get the proper angle on the drive shaft to get the rear end out.

The outer CV joints are held in by two long 8mm bolts with tubular spacers and big washers. They are accessed by removing the center caps on the rear wheels. Four 19mm headed nuts hold the diff to the rear subframe and the differential mount is held to the body with four 8mm bolts with 13mm heads. With a tranny jack under the differential I removed all of the attaching hardware and pulled one outer cv joint out of the rear hub, angled it down and out while being careful to move the diff to keep the axles in as straight a line as was possible to avoid damaging the axles. (I didn't know at that time whether or not they would have to go back in and I don't like destroying any parts, ever.) With the aid of a helper, we lowered the assembly to a cart with the donor differential and with a deep socket I removed both 12 point 30mm nuts holding the differntial yokes to the two diffs so that I could swap them. (The 380 flex plate and yoke has a bigger bolt pattern than the 300SD's.) The 380's front seal looked o.k. so I cleaned up the SD's yoke, checked it for seal surface damage and reattached it to the 380 diff with a brand new 30mm nut from the dealer. I tightened it to around 145 pound feet and chcked for smooth turning with some resistance. Being a geek I weighed both rear axle assemblies on a commercial scale and the much larger 380 diff weighed 126.3 which was exactly 23 pounds heavier than the 300's diff. The difference in weight is due to the fact that the 380 differential has a mechanical friction plate-type limited slip mechanism inside! I was (and am) excited to think of being able to put power to the wheel with the most traction. LSD should be standard issue with a 40,000 car. (Please, no drug jokes!)

The rest of the install went flawlessly with a new flex plate and a new diff mount. I was really struck by how simple thes cars are underneath and was really ready to see if my experiment would work as planned. If the experiment worked, I would have new enthusiasm for a car that I really liked in many ways.

I set the car back on the ground, washed my hands and fired her up. I engaged drive and quietly (at least for a diesel running on 20% biodiesel could be) and motored out of the shop. Even at 15mph I could tell that the bad pinion bearing whine was totally gone. I accelerated and it was holding gears longer but accelerating quite well. The speedo moved slower than what was visually happening to the scenery but I knew that I would hafta source a speedo from a 380 or 500/560 to jibe with the 2.47 rear end. But the most incredible thing about the first drive was that the car was so much quieter! It was like it was a totally different car. My Dad used to have a 380SEL and the feeling of composure from that car was now present in my car. Even at speed, the car had plenty of power as the rpms never got above 3,000 rpms unless my foot demanded it. The torque was doing exactly what I wanted; accelerating the car without drama or excessive noise. Downshifts were easier to provoke and the acceleration was great. Highway cruising at 80mph had the engine spinning at 2900 rpms and 70mph was around 2400 rpms as verified by a hand held GPS unit.

The swap was only a few days ago and now I am much more interested in fixing all of the little things wrong with the car like the cruise, windshield, door check weld, worn out seats, and questionable tires. I have all of the plans in my head for using a radiator sized intercooler and that project is back on the list of things to do to make a reasonably priced, relatively efficient, large comfortable cruiser with style and teutonic elegance.

Side note: I have yet to source a speedo head with a working odometer but my baseline mpg's was 26-27 mpg for about 7,000 miles while using the stock 3.07 rear end. I will report back to this thread as to my new MPG's with the 2.47 diff swap. I am hopeful that by lowering the rpm's down toward the torque peak my mpgs will go up a little though I don't expect a lot of change. All of the reading I have done about efficiency says that by running an engine closer to its torque peak, you decrease fuel consumption. We'll see.

Just for pleasantness alone, this swap is Absolutely worth the effort. I paid $300 cash for the whole rear end including axles plus $40 for use of the lift and help when I needed it. The diff mount and flex plate was not expensive and the new 30mm nut was $5.00. I spent about 3 hours total on the job and a coupla hours doing service manual research. The car is completely different and I really think that this diff should have come standard from the factory. I have already noticed the difference with the limited slip on loose gravel as I got the car to rotate it's rear end with the throttle. The engine is almost silent at speeds under 50mph and my hopes are high that there will be an efficiency gain. Perhaps the biggest efficiency gain is the one that made me decide to keep a car that I had put a lot of time into as opposed to selling it to buy a quieter Benz...

Jim Royston