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  #1  
Old 03-19-2010, 04:55 PM
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A 2.24 Rear Differential on a 1984 300SD - so far I'm a believer!

Even though my –84 SD always has gotten a fairly good fuel economy (especially in comparison to similar sized vehicles of its own age), I (too) wanted to improve it. Having done all the usual stuff (setting engine internal timing, replacing injectors etc) I wanted to go out a little further in “ecofriendliness”. What next?

So I started thinking about a rear differential swap especially since several folks here have reported good results with a 2.47 rear. Unfortunately, obtaining a suitable 2.47 (for the early W126) over here in Finland was going to be difficult (in my pricerange, at least)… What I did happen to find was an affordable 2.24 (early euro 500SEL/SEC) with a matching speedo so I decided to go for it, even though I was really concerned that this would be too much for poor old 300SD accelerationwise. WELL – having now testdriven the car for a few days, I’ve become a believer in the good things that such a rear swap brings along:

a) As one would expect, the RPMs drop greatly! For example, whereas before at the speed of 120 km/h (75mph) the RPMs were up to about 3400 (IIRC) now they are around 2500 – this surely has an effect on the fuel economy. (BTW, it was nice to notice that math didn’t fail us here as: 3400 x 2.24/3.07 = 2480)

b) With the RPM drop the noise level in the car has dropped also greatly

c) I was also worried about in town driving – not any more – the SD pulls fine with ability to accelerate even on 4th gear in lowish speeds. (at 45 mph the RPMs are somewhere around 1400RPM

d) The most amazing thing is the accelaration off the line – it is NOT that bad. Really, I do notice a change, but much less so than what I expected. It still moves nicely even with this 2.24 diff. (We are not talking about a rocket here to begin with )

e) Others have reported the same thing – the shifting (auto tranny) has improved. Yep, didn’t believe what others claimed, now I do. (Maybe the old worn tranny just simply benefits from the change. My tranny certainly was tuned to perfection with the 3.07 (Probably NOT!) - I dunno what the reasons for this are)

The only other downside (besides the acceleration) that I’ve noticed is coasting to a stop (engine breaking) – this is more difficult – the car just wants to roll and coasts to a stop much more slowly (makes sense, of course, just didn’t see it coming). So, one has to plan ahead or just use more brakes…

Well, the most important thing is still missing – the hard numbers concerning fuel mileage. I will report on this thread once I get some driving and fueling done on the car. Like I said, just got the car back on the road so… And beware, all of what I’ve said is still “initial experiences” based on relatively little driving.

The terrain around here is not very hilly, so if that is the case where you are at then I would not hesitate doing this swap… Probably a different story if you live in the Alps or some other mountain area (I don’t think the climbing would be the problem - but not being able to engine break could cause…well…discomfort).

Anyways – my point is – it is not a terribly bad idea to swap the 3.07 for a 2.24. Furthermore, based on what I’ve read from others, I don’t think the difference between a 2.47 and 2.24 is that noticeable (in a diesel). So if you happen to get your hands on a 2.24 go for it… makes you love the SD even more, I think!
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Old 03-19-2010, 05:40 PM
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My observations with the 2.88 confirm that I cannot go any taller on the diff. It can barely hold 65 mph on some decent hills. The horsepower at 2800 rpm is just not there.

Yes, you can enjoy the 2.24 diff and the resulting leisurely acceleration provided that there are no hills to climb.
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Old 03-19-2010, 06:12 PM
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I see. Haven't tried any real hills yet (ain't got any available right where I live).

Help me out with this question, though. IIRC, the max torque of the OM617 occurs at 2400rpm, no? So at that speed with a taller rear diff you wouldn't be perhaps worse off than with the 2.88 as your engine RPMs would be closer to the torque max giving the car more "climbing abiliy"??? Hmmm I don't know. I am only talking about the situation you presented - obviously, in general, 2.88 revs up faster.
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Old 03-19-2010, 06:26 PM
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Quote:
Originally Posted by Johtotahti View Post
I see. Haven't tried any real hills yet (ain't got any available right where I live).

Help me out with this question, though. IIRC, the max torque of the OM617 occurs at 2400rpm, no?
This has been the claim from M/B..........forever.

However, I cannot duplicate it in any manner. The SD has almost nothing at 2400 rpm and it continues to build torque all the way to 3200 rpm. It's pulling much stronger at 3000 rpm than it does at 2400 rpm.

It's also evident on hills. If I can maintain 65 mph in traffic, it will hold that speed..........but, if it falls to 55 mph, the severe dropoff in torque results in an inability to hold 55 mph...........even though much less torque is required.

I conclude that the specification of 2400 rpm must be false.
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  #5  
Old 03-19-2010, 06:43 PM
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Quote:
Originally Posted by Brian Carlton View Post
This has been the claim from M/B..........forever.

However, I cannot duplicate it in any manner. The SD has almost nothing at 2400 rpm and it continues to build torque all the way to 3200 rpm. It's pulling much stronger at 3000 rpm than it does at 2400 rpm.

It's also evident on hills. If I can maintain 65 mph in traffic, it will hold that speed..........but, if it falls to 55 mph, the severe dropoff in torque results in an inability to hold 55 mph...........even though much less torque is required.

I conclude that the specification of 2400 rpm must be false.
Maybe yours is weak, mine makes tons of torque in the 1500-2700 range. My dad's SD has a 2.47 in it and it climbs hills with no issues in 4th gear. I am considering a 2.47 swap for my car this summer as I drive 80 miles a day on the highway and the rpm drop would be nice.
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  #6  
Old 03-19-2010, 06:54 PM
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Quote:
Originally Posted by pawoSD View Post
Maybe yours is weak, mine makes tons of torque in the 1500-2700 range. My dad's SD has a 2.47 in it and it climbs hills with no issues in 4th gear. I am considering a 2.47 swap for my car this summer as I drive 80 miles a day on the highway and the rpm drop would be nice.
I doubt it. The SD does 0-60 in 12.7 seconds. No, you cannot accelerate on a decent hill (6% or better) with a 2.47 with 120 stock horsepower. It might hold speed on such a hill...........but there is no reserve.
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  #7  
Old 03-19-2010, 07:00 PM
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Hills are what gears are for!
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  #8  
Old 03-19-2010, 08:09 PM
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Whats the highest speed you can get to in first gear now? If I went with a 2.47, I'd probably get the newer tranny with the lower first gear swapped in, that way you can keep some more of the off the line acceleration. I can also hit 70mph in 3rd with the stock diff, so I wouldn't mind downshifting to 3rd to climb hills with a 2.47 diff...
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Old 03-19-2010, 08:19 PM
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It'd be awesome if you got the 2.47 with limited slip, that sounds ideal!
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Old 03-19-2010, 08:44 PM
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Would be interesting to see the results of a couple of these cars on a dyno. That way we could see when max torque occurs & what different loads do to the operation. It would remove the theorizing and opinion.
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