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+--- Thread: W116 6.9 rear diff install into W126 (/showthread.php?tid=1506)
W116 6.9 rear diff install into W126 - Biohazard - 07-18-2010
So I've been sitting on a rear diff from a W116 450 SEL 6.9 for a while now. It's a 2.65:1 Limited Slip unit. Well, my Bonneville road trip is coming up and I'd like to be able to keep up with traffic with out spinning the hell out of my engine. So, I'm swapping the sucka into the Flyin Pumpkin! My goal is 75mph cruising speed with decent fuel economy.
I have mocked it up in my W123 240d project car, it seems to fit there too. I'll give all the details for the W126 300SD install though.
Got started on the swap last night. Car on jack stands and stock rear diff removed. Here are some pics to compare the two diffs.
Back view, 6.9 diff on left. The rear opening of the housing is much bigger on the 6.9, also a cooler looking diff cover.
Top view, 6.9 diff on left.
Inside of the 6.9 diff.
Rear diff mount. In normal mounting, the 'taper' side is the diff side and the 'straight' side is the bolt side. The 6.9 rear diff is a bit longer at the back than the stock diff. At least in the 240D, I had to flip the mount around so the 'taper' side was the bolt side. The mounting surface is offset by just enough to make it fit. I don't know if those crazy Germans meant to do that, but it works. I'll let you know if I have to flip it around in the 300SD also.
So now I'm in the process of swapping the input flange from 3.07 stock diff over to the 2.65 6.9 diff. The bolt circle of the rubber damper is significantly larger on the 6.9. I've already put new output seals in the diff, just working on the input flange now. Then, it's ready to get stuffed into the Pumpkin. Wish me luck!
RE: W116 6.9 rear diff install into W126 - ForcedInduction - 07-18-2010
Thats an open diff.
RE: W116 6.9 rear diff install into W126 - DeliveryValve - 07-18-2010
(07-18-2010, 04:11 PM)ForcedInduction Thats an open diff.
Picture is kind of faint, but I see clutches.
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RE: W116 6.9 rear diff install into W126 - Biohazard - 07-18-2010
Sorry ForcedInduction, but it's a limited slip. Closer view of the clutches.
Sorry for the crappy cell phone pics, the regular camera is MIA. Here are the case stamp and special oil tag also.
The clutches are easy to miss if you don't take a magnifing glass to your screen and try to decipher the pixels.
My brother just helped me get it mounted to the subframe. More Super Duper Hi Definition Cell Phone pics to come!
RE: W116 6.9 rear diff install into W126 - ForcedInduction - 07-18-2010
Unless the clutches are in the side of the housings I don't see them.
RE: W116 6.9 rear diff install into W126 - Biohazard - 07-18-2010
Easier to see in the close up view. On the right side, just to the right of the gear, it's the verticle lines. The seperations between the clutch parts look like machining lines. They are basicly built into the right side output assembly.
RE: W116 6.9 rear diff install into W126 - Biohazard - 07-18-2010
Here is a picture of the diff mounted into the subframe.
The 2.65 rear diff mount has the same orientation as the stock 3.07 did; with the 'straight' side being the bolt side. When it was mocked up in the W123 240D, I had to flip the mount around to fit. So... the w116 rear diffs should fit the W123 as well as the W126!! I don't know if that opens up a lot of different options, but I know there is at least one diff I like that fits!
I'm back to the cave to get the rest of this put back together. It's my daily driver, need it running to get to work in the morning. I'll update more as I get to stuff or get done and test drive.
RE: W116 6.9 rear diff install into W126 - Biohazard - 07-18-2010
Just got done with a test drive, all is good! I'll be doing some highway driving tomorrow and Tuesday, will update on how it does. Need to top off the fuel tank, I'll see about milage also.
RE: W116 6.9 rear diff install into W126 - ForcedInduction - 07-18-2010
How are you tracking accurate miles without a 2.65 speedo?
RE: W116 6.9 rear diff install into W126 - Biohazard - 07-19-2010
I have the matching speedo, but it's mechanical and the 300SD is electric. I'm going to be putting the 2.65 diff back into my 240D car, so for now it being in the 300SD is temperary. For error % and calibration, I'm back to using a GPS and good old fashioned math to figure the difference.
RE: W116 6.9 rear diff install into W126 - Biohazard - 08-03-2010
I've finally made it through a tank of fuel. Filled up this morning on the way to work. Drove the car an indicated 379.9 miles, which works out to 439.9 miles actual. I timed myself over a known 3 mile interval and got an error of 15.8% with the new diff. So, 439.9mi / 15.7 gallons is 28.0 mpg! I had a best of 26.4 mpg before swapping in the diff. It still has decent acceleration, but not as quick as the 3.07 diff. I'm ok with trading that for the 75mph cruising speed.
RE: W116 6.9 rear diff install into W126 - ForcedInduction - 08-03-2010
Thats a pretty good difference. Swapping a manual transmission in place of the slushbox will return better results though, I've gotten 34mpg on pure highway and average 26 around town on winter fuel with the 3.07. Combine that with the 2.65 and you'd get some stellar economy!
RE: W116 6.9 rear diff install into W126 - Biohazard - 08-03-2010
(08-03-2010, 10:47 AM)ForcedInduction Thats a pretty good difference. Swapping a manual transmission in place of the slushbox will return better results though, I've gotten 34mpg on pure highway and average 26 around town on winter fuel with the 3.07. Combine that with the 2.65 and you'd get some stellar economy!
Those are some great numbers!! The engine in the pumpkin is going to get swapped into my 240D (4 speed car) with the same 2.65 rear diff; thats the plan at least.
As much as I like manual transmission cars, there are still a few days (read stop and go traffic for 10 miles) that I'm very very happy with my automatic. The Pumpkin is fun and all, but I like smaller cars better. So, the Pumpkin is a rolling r&d test bed for the 240D project. I can live with a slush-o-matic for that long. 
RE: W116 6.9 rear diff install into W126 - ORION_theArcher - 08-22-2010
Anybody find/fool with lower gears? 3.55 maybe even 4.10. I doubt I will push this 350 past 100 mph very often. Just looking for faster around town, 65mph being the achieved max most of the time. I'm looking for throttle response without adding too much to my 3.5. I don't want to blow any rods at this point in time.
My friend has a six speed jeep with an LS1, 40 inch tires, and 5.13 gears. I've never been thrown Into the seat so hard in my life. Not one of these "fast" kids with decent Honda and Toyota come any where near that jeep off the line and through 70. You can't expect much superpowers after that
RE: W116 6.9 rear diff install into W126 - Biohazard - 08-26-2010
My brother and I swapped the rear diff from a 300D non turbo into my dads 300D turbo (it was my brothers at the time). I believe those were the 3.69 (may have been 3.4something?) gears. The car has much better acceleration off the line! It's more fun to drive around town, but extended highway driving is not so much fun. My dad ended up swapping on the larger back tires (235/55r17) when he got the car from my brother. My dads commute is mostly highway, and he wanted to get better manners and milage. With the stock size rear tires, it can cruise at 60-65, but he doesn't like spinning the engine that fast; he is more concerned with longevity. If you're looking for in-town acceleration and limited highway use, I'd say go for it if you can find a diff that will work for it!! I believe the 350SDL uses the later style rear diff; the mounting points are different from the early style like in my car. You can probably check some of the similar model/year gas models and find a suitable ratio.
RE: W116 6.9 rear diff install into W126 - ORION_theArcher - 08-27-2010
Anyone know my gears? A neck injury is preventing my from crawling under the car.