Diffs and driveshafts/output shafts

[DaveP]

If you haven't read my ownership thread, for which you'd be forgiven, you might not know that I'm trying to fit a 3.0si auto diff to my 3.0i manual.

I had read a post, either here or on the Faceache group, where someone said they had done the same swap plug-and-play. Brilliant, thought I.

3.0si auto:


3.0i manual:


That looks to me like the output shafts are a different diameter (27mm vs 38mm), but that the attaching points to the output flange on the diff is the same 86mm. Does anyone know if this difference in diameter affects the way they attach to the diff output flanges? Looking at the diff part #s, it appears that the output flange on the two diffs is identical (#33131428124), which suggests that the output shafts should be interchangeable.

Additionally, the 3.0si auto appears to have a shorter driveshaft than the 3.0i manual. That alone wouldn't bother me (assume it's a gearbox length thing), until I spotted that the 3.0si manual also has a 7mm shorter driveshaft than the 3.0i manual. They share the same gearbox (GS6-37BZ) and diff casing (188K), so why would they have different length driveshafts?

All terribly confusing stuff.

 

[DaveP]

Anyone? Or am I going to have to suck it and see?

 

[enuff_zed]

Anyone? Or am I going to have to suck it and see?

Purely so you know someone has read this, I have no idea, sorry. :?

 

[Liam22]

The 7mm driveshaft length difference may be a result of slightly different engine lengths (or mounting position) between M54 and N52. Presumably the N52 bell housing sits 7mm further aft than the M54.

 

[feket663]

I changed the 3.0i original differential to 3.64 ratio diff from E83 X3 3.0i automatic. Even though the differential was exactly the same as the original, the drive flange output was completely different, but I pulled the new diff's drive flange output (marked in blue circle) and replaced it with the original part.
In my opinion, the 7 mm difference in the length of the prop shaft is nothing to worry about, if it causes a problem, then the part circled in the picture should be replaced as a last resort (making sure that the differential settings are maintained, of course)

 

[DaveP]

Anyone? Or am I going to have to suck it and see?

Purely so you know someone has read this, I have no idea, sorry. :?

Appreciated as ever.

 

[DaveP]

The 7mm driveshaft length difference may be a result of slightly different engine lengths (or mounting position) between M54 and N52. Presumably the N52 bell housing sits 7mm further aft than the M54.

It's definitely possible, but would be weird to shift the entire engine and gearbox setup 7mm back when the packaging is essentially the same. I'll look into some part numbers.

 

[DaveP]

I changed the 3.0i original differential to 3.64 ratio diff from E83 X3 3.0i automatic. Even though the differential was exactly the same as the original, the drive flange output was completely different, but I pulled the new diff's drive flange output (marked in blue circle) and replaced it with the original part.
In my opinion, the 7 mm difference in the length of the prop shaft is nothing to worry about, if it causes a problem, then the part circled in the picture should be replaced as a last resort (making sure that the differential settings are maintained, of course)diff.jpg

Excellent insight, thank you. I hadn't considered differences in the input flange, so I'll go and compare part numbers now.

Did you actually need to change the input flange when you installed?

Edit:
RealOEM doesn't list the input flange as a separate part number annoyingly, but from the diagrams the input assemblies appear to be identical on both diffs.

Guess I'm going to have to suck it and see on this one. From reading back through forum threads where others have done the same swap, nobody seems to mention this issue.

Perhaps it is, as previously suggested, just a case of accommodating a charge in length of the engine and gearbox between the M54 and N52. That's the best possible outcome here. Hoping the car goes in for the work this week, so we'll know soon enough.

 

[feket663]

Excellent insight, thank you. I hadn't considered differences in the input flange, so I'll go and compare part numbers now.

Did you actually need to change the input flange when you installed?

No, That is why I wrote that it should be done as a last resort.

Here is the difference, the original and the X3's differential ratio:


With the new differential, 6th gear is almost identical to the old 5th gear.