2.88 diff in manual trans 240D - first hand impressions
 

Hello all, I thought I would take a few minutes to write up my experience with this over the last few thousand miles. I found little to no info and lots of speculation trying to research before going forward with it.

Car is a 1980 240D all stock with 4 speed manual trans. Original ratio was 3.69. I am running 195/70R14 tires which are about 24.8” tall. Car is in good tune with adjusted valves and timing bumped 2-3 degrees.

I use this car to commute on the highway 70-80 miles round trip everyday. I am in Connecticut which I would say is moderately hilly. Very little of my drive is flat, but there are certainly no long steep grades to worry about.

Initially, I tried a 3.46 diff which I would not describe as much more than noticeable. It felt and sounded a hair happier at 70mph, but not much else.

I started searching for a 3.07, thinking a 3.27 would be perfect but would be like finding a needle in a haystack. A local guy I know had a 2.88 already out of the car for $50. 3.07 was a 13% decrease from 3.46 and 2.88 was 18%. Meh. After tackling the first diff swap I felt like I had a good handle on it and it was worth the $50 and 1.5hr experiment.

Somewhat unorganized thoughts/impressions:

By myself doing 70-80 with left lane traffic it works really pretty well. There are a couple hills that have me foot on the floor losing speed if I didn’t hit it at 75-80, but honestly... that’s not hugely different from how it used to be. Starting off requires a little clutch work to get going, and takes a slippage to get moving if starting going up a hill. Once you’re moving, it doesn’t feel much slower than before. Low HP motor with good torque and flat torque curve. I merge every morning on a long uphill on ramp and I am able to use the torque from 40 to 60mph in 3rd gear which actually feels easier than trying to make it rev higher than it really wanted to in 3rd and bogging and merging in 4th like it with the 3.69. 4th gear is not used for anything other than flat ground below 50mph and 3rd still feels good up to 50-55 for cruising if needed, not winding out. Having a second person and their stuff in the car does slow it down a wee bit.

I am getting right about 30mpg doing 70-80mph. About the same mileage I was getting doing 65-70 with the 3.46 diff. For my long commute, I really enjoy traveling at faster speeds without losing any fuel economy.

Overall, I think the 3.07 would be ideal. I may very well swap one in at some point. But, I haven’t been able to find one easily and the 2.88 is really pretty good. It has not caused the car to be undriveable or caused any of the other catastrophic things I read lots of people speculating would happen, at least not in the last ~3000 miles. I much prefer it for MY driving over the 3.69 or 3.46, YMMV.

Here is a thread from 2015 on this subject:


http://www.peachparts.com/shopforum/diesel-discussion/373896-240d-differential-swap.html

Post your location for members nearby to help ya find a 307...

I have a few I'm pretty sure...

Just updated my info. I’m in Eastern Connecticut.

I would love to do something like this on my 240D but am apprehensive as its automatic. Unfortunately the auto trans seems to like to lug around 25mph in 4th gear which makes it a REAL slug unless I use the kick down switch or shift it manually. Would be willing to sacrifice some acceleration (not like theres much to begin with) for having the engine be a bit calmer at freeway speeds. I suppose my hope would be that I would get more use out of third gear. And of corse a bit less nosey top highway cruise.

if you are in 4th going 25mph, you have other (most likely vaccum) issues to address before worrying about swapping differentials.

It's all about relative discomfort at 70 mph, which is about average for freeway traffic:




Differential RPM's at 70


3.69 3514
3.07 2923
2.88 2742


IMO the 2742 is about the limit

What would the effects be like at a 2.47 ratio?

It'd be like taking off in second or third and fourth gear would be useless unless going down a mountain. ;)

Great Thread, and thank you ! A lot of folks commented over the years that a 2:88 would be about impossible to use in a OM616. I guess your experience proves them wrong.

I have access to a 3:07.

If you end up getting a 3:07 and do that swap, please continue this thread.

I have a 2:47 also and was thinking of mating that with a OM616 but with 5 or 4 speed manual tranny.

If you spend any time on the highway and your car is running good, I wouldn’t hesitate to install the 3.07.

I personally would not bother with 2.47 at all in an otherwise stock 616 powered car. Without having tried it myself, I feel 2.88 is probably the reasonable limit.

I'm in Maine and my '84 300td diff has a nasty whine and needs to be replaced. If you find another diff you want to use, I'd buy the 2.88 off you. I think the 617 turbo would be really happy with a 2.88.

I have two 617's with 2.88's and 5 speeds and I believe its the car Mercedes should have offered.

The 240d, if you look at the speedo, has one number highlighted and that is 55. This was of course the speed limit at the time. Well with a 3.69 diff at 55mph the rpm was 2761. This is probably the rpm the Mercedes engineers were comfortable with for that motor.

I don't think that is the comfortable rpm, but it is the powerband peak efficiency area.

Now that road speeds are higher, taller ratio diff gets the powerband into a higher road speed.

The problem would be with an auto the slippage would be high... external trans cooler would be strongly recommended...