keith0alan

Hello all,

Posted quite a while back with a question concerning swapping my 3.27 stock diff for something in the 2.xx range to lower engine RPM on the highway. After about 2 years mucking about it is in. A bit more of an adventure than I expected. After my local salvage yard getting about 3 stock ones and 2 fragged ones I finally got what I was looking for. The 2.56 diff from a diesel. After replacing the seals and cleaning it up I proceded to do the swap. As it turns out the diesel diff is about 1" wider between the output flanges. Shouldn't be a problem, the CV joints have a lot of travel. Unfortunately the stock axles with the stock diff are already as short as they can go. After a couple of weeks one of the CV joints popped apart. It was my new axle that I installed to replace the original that was worn out. Pulled it apart and figured out what the problem was. There are enough extra splines on the axle shafts that I was able to cut new snap ring grooves and move each CV joint 0.25" in and shorten the axle length by 0.5". Axles are basically spring steel and will take the end off of a carbide lathe tool. I used the tool post grinder to make the grooves. After getting it all back together the axle has a nice 0.5" of free play that should solve that little issue. There may be a stock axle that is a little shorter but I wasn't up to the research to find it. Also the diesel diff has a different input flange. Fortunately the one from the old diff fits. As far as driving with the old diff 3k RPM was 60mph. Now 3k RPM is about 75 mph. The engine noise really picks up above 3k RPM so cruising at 70-75 mph is much quieter. It is not nearly as zippy but that is what 3rd gear is for. It was getting about 20 mpg on the highway and I am going to do a run to see what the mileage has done. Some improvement I hope. Overall it makes the car nicer to drive but was a bit more work than I was expecting.

keith

QVHK

Hi, read your story about the differential swap. If you have a lot of highways during your daily ride then it makes sense to go for a differential with a lower ratio. My M103 CE had a 3.27 differential. When I converted the auto transmission to 5-speed manual, the lot came with a 2.87 differential. In my case, I was hoping to get a more responsive drive so the use of a 2.87 differential was a big mistake, the CE became lifeless, although cruising along highway was quieter. I would imagine that a 2.56 differential would lose more power at lower rev, and it takes quite a while to accelerate. I am quite happy with my 3.67 differential as car was fun to drive. However, occasionally when I am on the highway, I wish I could save some fuel and lower engine noise. Just wonder if there is any 6-speed gearbox out there that would fit the W124, so I can enjoy both power at the lower end AS WELL AS achieve fuel economy and lower cabin noise level.

keith0alan

Drove to Los Angeles this week. About 600 miles total almost all highway. The mileage was 22mpg cruising at 70-80 mph. My last long trip with the old diff was to Reno and back where I got 20 mpg. So, about 10% better mileage. At that speed the engine is about 3000 rpm. Engine noise is way down but so is power. On a couple of the long grades I did need to drop to 3rd gear to maintain speed. Around town the old diff was more fun but on the highway the new diff makes the trip more pleasant. The original diff with a serious overdrive trans would be the way to go. Another option would be a turbocharger to up the power a bit. It wouldn't take a lot of boost to make a real difference. Altogether, if I had it to do over I think I would go with the 2.87 rather than the 2.56.

pwogaman

Are you compensating for the change in odometer reading when calculating your new mileage? If you are not, then I believe your mileage calculating will have been understated by 21.71% and instead of 22 mpg, you will have gotten 26.7762.

http://www.carmodder.com/doc-ford-fa...or-d10m20.html

http://www.wallaceracing.com/Calculators.htm

keith0alan

Good question. When I changed the diff I also swapped the speedo head. This is how Mercedes does things. The speedo head is matched to the diff. Also, and more important, when I do gas mileage checks I use my GPS rather than the odometer. As it turns out the speedo reads a little low and the odometer reads a bit high. On the L.A. trip I also traveled at a bit higher speed that the Reno trip.

Cruising on the highway it is nice to have the engine in the "sweet spot" at or a little more than 3k RPM. The engine noise is still down but the engine is starting to get up on the power curve. This ends up being 80-85 mph. With the old diff it was about 60-65 mph. On the L.A. trip this is about where traffic flows most of the time.

While the car is great at those speeds it is on the edge of blinking lights in the rear view territory. Given what I know now I would reccomend the 2.87 gears.