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  #1  
Old 10-31-2010, 12:50 AM
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Location: Oceanside CA
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w126-M103. Overdrive trans swap question.

Ive been combing various forums for about a week trying to find some concrete information on swapping a later model overdrive transmission into my 1991 300SE.

From what I have gathered, swapping a 722.5 wouldn't be too difficult, except that it is a relatively weak transmission and there are issues with the trans skipping 4th.

I've read a little bit about the possibility of swapping a 722.6 into it. I have been able to get even less info on this.

Does anyone have a good source for technical data on MB transmissions?

Does anyone know exactly what this kind of swap would require, and which swap would be "better"?

I am a former MB Tech, unfortunately, most of my experience is with later model cars, and i have done very very little modification to MBs.

I have done a couple v8 swaps into datsuns...

Thanks guys.
Joe

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  #2  
Old 10-31-2010, 06:38 PM
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Well, can't really help you there; don't know of anyone who's done it.
But, taller gearing on the 300SE would surely be welcome.
I toyed around with the idea of installing a taller differential, then tweaking the existing tranny to always start in 1st gear - effectively getting the same results as installing an OD tranny. But, I haven't ever pulled the trigger after 20 years, so it likely won't happen.
I would assume, to install a later OD transmission, you would face all the problems (and more) that we do in Chevys and Fords: the bellhousing wouldn't mate to the engine, the starter wouldn't fit, the flywheel wouldn't bolt to the tranny, the driveshaft would require a length change, the later tranny would be computer controlled and require major electrical tweaking and interface with the engine controls to behave properly, clearance under the body would rear it's ugly head, tranny mount wouldn't adapt.
And, if nobody's done it, then you'd get to learn all these little tidbits on your on. I'd say if you can't find info from a previous successful job, look closer at the option I mentioned above.

But, I'll hide and watch with keen interest; keep us posted.

DG
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  #3  
Old 11-01-2010, 01:16 AM
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Well... You did manage to point out nearly every concern I have with the swap.

The other concern i have is results. The 722.5 appears to have about .81 overdrive. (At least the ones attached to m104s.) That is only a 19% improvement. Which is not exactly enough. I would like to get something in the .68 range. Which is a more common ratio in the 722.6.

The 722.6 IS computer controlled, while the 722.5 is hydraulically controlled.(as i recall)

So... I could go ahead and put a 2004r from a chevy in there. and have an adapter plate machined, and a custom ring gear made.. and modify the driveshaft, the cross-member, and set up a TV bracket... Basically a real hassle.

The easy route is swapping in a diff from a 560se/l. Going from a 3.48 rear to a 2.47 rear will garner the results i am after. However, I do fear that the already lackluster performance will become downright 240D like.

I could go bat feces crazy and pick up a 2001 chevy van with a 5.3l v8 and 4l80e and swap that in there. The 2001-2003 chevys have pretty simple emission control devices, so getting the car reffed should be "easy". And By "easy" I really mean "nearly impossible".

But it would be nice to get 325hp and get somewhere near 30mpg on the highway.

I do plan on picking up a 300SD at some point and doing that swap(Bi-turbo just to be ridiculous too). I think I will do some more research on the trans swap... and then buy a differential.

If anyone has any input I would appreciate it.
Joe
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  #4  
Old 11-01-2010, 01:46 AM
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I have found something odd.
Its probably nothing. But the 722.5 transmission is listed in all-data as a transmission for the 91 300se.

It also shows a .75 od.

Weird.
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  #5  
Old 11-01-2010, 01:00 PM
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Thumbs up

All-Data does not always have it 'all'

2.88:1 diff from an SDL will work fine...... it will make actual use of first gear instead of just a creeper gear.

A member here has that done, but his user handle slips my mind. I will try to remember.

I am going to do it once my head is rebuilt and I get my converter restalled so the car can actually get a decent launch around town.
__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.
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  #6  
Old 11-01-2010, 01:41 PM
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I guess I bought a diff last night.

Thats what I get for mixing gin and Ebay.
Now to figure out how to make first gear starts happen in D. Anyone know which check ball to add/omit? Or a simpler solution?

Thanks
Joe
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  #7  
Old 11-02-2010, 01:53 PM
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Allright, let's get this ball rolling.
Yeah, the 2.88 was the diff I was considering a few years back.
I hung around on eBay, and noticed from various photos of diffs that there appears to be several mounting configurations over the various years and model types. Different axle mounting configurations also. So, be careful to get one that will bolt up.

I was worried about the speed/torque in each gear with the new dif also. An easy way to see how it will affect the various gear speeds and power is to check your speedo - there are little dots printed on the speedo face that correspond to the max speed in each gear. All you have to do is multiply those speeds by 83%, and that will be the new max speed of each gear when converting from the 3.45 to the 2.88 dif. From there you can find max torque range, etc, etc.

Somewhere here in the achives I saw a diagram of the valve, with the directions for converting to the 1st gear start. seemed really easy; but in the thread, someone commented it would make really weird shifting things happen, so I never tried it. I'll try a search.

I always wanted to put the 350 TPI from my old '87 Vette in the MB. that thing had no top-end, but made amazing torque below 3000. Imagine that engine with a first-gear start in the MB!

DG
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  #8  
Old 11-02-2010, 02:22 PM
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Check the thread title below. There is a diagram of the spring swap.
Can't be that easy can it?

1991 300sl...first gear start
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  #9  
Old 11-02-2010, 06:31 PM
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'Motor Head' is the guy that did the 2.88:1 swap, said he loves it.

A place in CA apparently does the VB first gear start, Silver Star is the name. When I have my car down this fall for the head R&R I am sending my VB there.

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.
[/IMG]
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