W115 240D Manual rear end swap: 3.69 to 3.46? - PeachParts Mercedes-Benz Forum

Screwdriva · #1 10-31-2019, 03:33 PM

Has anyone done this swap from the OM617 to the 616? If so, I was looking for feedback on the difference it would make the noise level, acceleration and fuel economy.

Many thanks in advance!

Usaguy · #2 10-31-2019, 06:59 PM

3.69 to 3.46 will not make that much of a difference.

3.07 from a turbo om617 will make the ride quieter and give you a few mpg's more on the highway and 1st gear will actually be more usable.

the downside is that you will have less pull when going uphill on the highway and you will have to modify your speedometer

Screwdriva · #3 11-01-2019, 05:25 AM

I'm quite familiar with the 3.07 swap and have heard good things about it. Alas, I live in a hilly area and will need to preserve that "pull" you speak of, and am trying to shift the needle slightly towards drivability but not as much as the the 3.07. Hence my question about the 3.46 swap.

t walgamuth · #4 11-01-2019, 08:10 AM

It sounds like a good compromise ratio. The 307 with a 240 would be very very lethargic coming off a stop.

Shortsguy1 · #5 11-01-2019, 11:40 AM

What is your main goal? Higher top speed? Easier freeway/motorway driving? Lower noise on the freeway? Etc.

I ran the numbers for my 220d and there is no point in changing the diff ratio. The only benefit would be a slight reduction of rpm and noise at 55 mph. Acceleration would be slower and (perhaps confusingly) the top speed would be less. Mercedes did a good job at picking the diff ratio for our cars by matching the power required at high speeds to the power produced at redline, so I really wouldn't think it is worth the effort. Maybe a 240d with a little bit more power would be a different answer, so you might want to run your own calcs, but it didn't make sense for a 220d.

Screwdriva · #6 11-01-2019, 02:44 PM

Goal would be less noise at highway cruising. Anything above 55mph with the 3.69 can get loud as you're well aware. Increased fuel economy would be a secondary benefit. The car is VERY peppy for a 240D, probably because its a manual with no AC drag. Engine compression is also really good! I wouldn't attempt this without a manual tranny.

I believe the 3.47 ratio was only available on the W115 300D and the R107 so I'm hoping someone somewhere has done this swap and can share their experiences.

Usaguy · #7 11-01-2019, 02:50 PM

Quote:

Originally Posted by Screwdriva

I believe the 3.47 ratio was only available on the W115 300D and the R107

3.46 was also on the N/A w123 300D and a first gen W126 280S/SE/SEL (second gen w126 300se won't fit)

mabbonizio · #8 11-03-2019, 01:39 AM

I did this swap! Although my 115 240d started life as a w115 220d so the rear end ratio was something like 3.96.

The car is definitely peppier and the gears more usable. I live in a very hilly area and was worried that the 3.07 would be hard starting on hills.

Biggest difference is the car cruises more comfortably at 65/70 mph.

My goal is to add some sound deadening and see if it will come close to how my 123 sounds on the highway.

Screwdriva · #9 11-03-2019, 05:15 PM

Quote:

Originally Posted by mabbonizio

I did this swap! Although my 115 240d started life as a w115 220d so the rear end ratio was something like 3.96.

Biggest difference is the car cruises more comfortably at 65/70 mph.

Wonderful to hear! Did you notice a difference in fuel economy? What do you get in the city or the highway?

Do you have a manual? If so, was low speed acceleration impacted?

mabbonizio · #10 11-03-2019, 07:54 PM

Sad to say I haven’t noticed much of a difference in fuel mileage - neither my fuel gauge nor odometer work reliably enough to track fuel millage.

Best I can say is I get between 400 - 425 miles a tank.

My car is a four speed manual - and I haven’t noticed any difference in acceleration in lower gears. At low speed. Honestly the car feels quicker to me. I think I get a little more use out of the engines torque.

I got my diff from a w123 300D non turbo.

To be honest, between the suspension rebuild I did, new “stiff” bilstein shocks all around, shift bushing rebuild, throttle linkage rebuild, and diff swap - the car feels “dialed in” to my tastes. It’s fun to zip around curves with and while it’s not fast at all - I have more fun driving it fast than driving my “fast” cars slow.

Screwdriva · #11 11-04-2019, 07:26 AM

Quote:

Originally Posted by mabbonizio

To be honest, between the suspension rebuild I did, new “stiff” bilstein shocks all around, shift bushing rebuild, throttle linkage rebuild, and diff swap - the car feels “dialed in” to my tastes. It’s fun to zip around curves with and while it’s not fast at all - I have more fun driving it fast than driving my “fast” cars slow.

Super useful information! If what you're saying is true, i suppose the reason Mercedes didn't install this differential on the 240D, was any impact on the automatic transmission equipped models.

I will proceed with this modification when I change my axle boot covers next year, based on your feedback. I'm in the midst of renewing all my rubber suspension bits with OE Mercedes components and Heavy Duty Bilsteins, but even before all this I would agree that I love driving the 240D Manual more than any other Classic Mercedes, regardless of power. It just "feels" great to drive and shows you how little horsepower figures really matter!

I have a thread on my rolling restoration here in case you're interested.

Your fuel economy seems a bit low at 25mpg? I netted 30 mpg US when I drove the car home (partially seized front calipers, wheel bearing grease turned to gunk, budget tyres etc.)