6HP28 Swap mated to N54

doublespaces

Administrator
Oct 18, 2016
6,758
AZ
#1
After many years of speculation, a 6HP28 has successfully been swapped in place of a 6HP21. There have been countless times this idea was brought up, only to be shot down as a stupid idea because the TCU was always locked down. Once XHP Flashtool came about, a few people became interested, myself included.

The 6HP28 is found in several vehicles, mostly V8 applications but is also found in the 335d, which is what makes this swap interesting. The 6HP21 and 6HP28 are exactly the same length, same general shape and even share the same TCU security.

The bellhousing looks incredibly similar, but it is infact different. The bolt holes in the housing are so close in some cases they overlap each other and this makes some types of bellhousing adapters more difficult to make. The solution is cut the bellhousing off both transmissions and weld the 6HP21 bell onto the 6HP28, which allows you to keep the original starter since it won't fit the 28 bell anyway.

You need to keep the 21 flywheel and modify it to fit the 28 torque converter holes. The 6HP28 does use the E-Shifter, so some wiring changes will be involved but nothing serious. The stock drive shaft and differential are used, but keep in mind the 6HP28 uses a 2.81 rear end. If you're coming from a 6AT, you've got a 3.46 final drive and this is probably unusable.

As for the software, it has been taken care of thankfully by XHP, if anyone does this swap you'll just contact them and they can provide you with a custom tune that will accomodate the hardware combination, but the transmission seems to be fine spinning to 7500 rpm.

Thanks to Taron for the information and photos:

full.jpg

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This swap was done on an xDrive vehicle as well. More information and videos to come.
 

Jeffman

Lieutenant
Jan 7, 2017
876
#3
Potentially lots cheaper and lots better than the PD solution, I surmise. Somebody’s going to have a nice little business swapping and coding in 6HP28’s and rear diffs.
 

doublespaces

Administrator
Oct 18, 2016
6,758
AZ
#7
Here is an older video of it shifting with the electronic shifter:

And thanks to Nizpro for these photos, but this is a comparison of the 6HP21 vs 6HP26. Keep in mind, this swap entails the 28 which is even stronger:

image_zpsqak5lgll.jpeg
image_zpsrlnlkn05.jpeg
image_zpsxso390qf.jpeg
6-speedupgrades.jpg


And its been shown the 21 input dhaft can break at higher power levels:
Input Shaft.JPG
 

doublespaces

Administrator
Oct 18, 2016
6,758
AZ
#9
Also according to @[email protected] the 21 in 6HP21 stands for 21mm input shaft. So the photos compare 21mm vs 26mm. The 6HP28 is a 28mm input shaft of course. And I'd suspect all other components are equally upgraded.

So if ZF rates the 21 at 450nm which is 330 ft lbs, and we religiously double that. The 6HP26 is rated at 600nm and if the 6HP28 is 700nm, then doubling this is 1032 ft lbs tq... .
 
Last edited:
Jul 3, 2018
331
#10
Congrats again to Taron for finally figuring this out! Definitely a route I plan on traveling down. Was kinda hoping there'd be a *little* more detail considering most of the first post was already public about his swap but there's still some good info in there that'll help the local shops.

Here is an older video of it shifting with the electronic shifter:
]
So did he end up having to switch shifters too then or...? That's what I'm most confused about.

It seems for the swap we need to:
A: Cut 21 bellhousing, weld to 28 trans (for sure)
B: Custom xHP tune (for sure)
C: Keep recipient car's original DS and pumpkin + axles? No modifications to DS like for an M3 swap right? And anyone who's done an LSD upgrade didn't throw money out the window correct?
D: use the OLD style auto shifter (pre-lci, no individual buttons on shifter) with some custom wiring work? Or do we retrofit the newer auto shifter if we have the old style shifter? If the latter, does it take additional wiring other than what the current shifter retrofit kits have?

I have an 06 build with the beefier pumpkin and axles so I'm hoping my understanding is correct and that we don't have to retrofit the fancier shifter, but I'm still obviously a bit confused.
 

doublespaces

Administrator
Oct 18, 2016
6,758
AZ
#11
Congrats again to Taron for finally figuring this out! Definitely a route I plan on traveling down. Was kinda hoping there'd be a *little* more detail considering most of the first post was already public about his swap but there's still some good info in there that'll help the local shops.


So did he end up having to switch shifters too then or...? That's what I'm most confused about.

It seems for the swap we need to:
A: Cut 21 bellhousing, weld to 28 trans (for sure)
B: Custom xHP tune (for sure)
C: Keep recipient car's original DS and pumpkin + axles? No modifications to DS like for an M3 swap right? And anyone who's done an LSD upgrade didn't throw money out the window correct?
D: use the OLD style auto shifter (pre-lci, no individual buttons on shifter) with some custom wiring work? Or do we retrofit the newer auto shifter if we have the old style shifter? If the latter, does it take additional wiring other than what the current shifter retrofit kits have?

I have an 06 build with the beefier pumpkin and axles so I'm hoping my understanding is correct and that we don't have to retrofit the fancier shifter, but I'm still obviously a bit confused.
More information is coming. The pumpkin will need changed, because the final drive for the 28 is 2.81.

You need the new electronic shifter, I don't know whats involved but there is a little harness work I'd presume, nothing that you couldn't figure out using the wiring diagrams with the help of https://www.newtis.info/

You have an 06 build date 6AT? That means you have a 215mm 3.46 bolted differential. The axles aren't beefier. The main difference is that it is needlessly larger (because the 188mm pumpkin has never broken) and the pumpkin is slightly longer with a slightly shorter drive shaft. But none of that really matters because the driveshaft is telescopic. I switched from the 188 to the 215, and I wish I didn't because it was pointless and just gives less clearance for your exhaust system.
 
Jul 3, 2018
331
#13
More information is coming. The pumpkin will need changed, because the final drive for the 28 is 2.81.

You need the new electronic shifter, I don't know whats involved but there is a little harness work I'd presume, nothing that you couldn't figure out using the wiring diagrams with the help of https://www.newtis.info/

You have an 06 build date 6AT? That means you have a 215mm 3.46 bolted differential. The axles aren't beefier. The main difference is that it is needlessly larger (because the 188mm pumpkin has never broken) and the pumpkin is slightly longer with a slightly shorter drive shaft. But none of that really matters because the driveshaft is telescopic. I switched from the 188 to the 215, and I wish I didn't because it was pointless and just gives less clearance for your exhaust system.
Sorry, I'm still a bit confused. Are you saying the pumpkin will need changed from what I currently have on, or that you can't use the 28's pumpkin? I was hoping to go for a LSD, lock down bracket and then swap one of these in when the time comes but will hold off if my stock pumpkin won't cut it. Regarding the beefier axles thing - I thought I had read some years ago that the 06 builds had a few MM thicker of axles than the newer builds, I think it might've been in one of the M3 LSD swap threads but I could very well be mistaken. And I know the smaller diff has yet to break but bigger is always better right? Lol.
 

doublespaces

Administrator
Oct 18, 2016
6,758
AZ
#14
Sorry, I'm still a bit confused. Are you saying the pumpkin will need changed from what I currently have on, or that you can't use the 28's pumpkin? I was hoping to go for a LSD, lock down bracket and then swap one of these in when the time comes but will hold off if my stock pumpkin won't cut it. Regarding the beefier axles thing - I thought I had read some years ago that the 06 builds had a few MM thicker of axles than the newer builds, I think it might've been in one of the M3 LSD swap threads but I could very well be mistaken. And I know the smaller diff has yet to break but bigger is always better right? Lol.
You cannot use your current pumpkin, if you're 6AT then you have a 3.46 FD. That will not play nicely with the 2.81 FD the 6HP28 is normally paired with.

There is nothing physically blocking you from using your current pumpkin, however you might find yourself in 6th gear all the time and doing a ton of shifting.
 
Jul 3, 2018
331
#15
You cannot use your current pumpkin, if you're 6AT then you have a 3.46 FD. That will not play nicely with the 2.81 FD the 6HP28 is normally paired with.

There is nothing physically blocking you from using your current pumpkin, however you might find yourself in 6th gear all the time and doing a ton of shifting.
Alright, it was the physically using it thing I was hung up on - I don't mind ordering a different FD LSD if need be but I'd rather not pay an extra $4-600 for a shell if I don't have to. Do you know what Taron ended up going with for the FDR?

I mean, I did swap a 4.11 FD into my 6AT LS3 Camaro when I had it so I'm not a stranger to short shifts and stupid amounts of torque but I'd like to do it right and have a car that's (very) competitive at the strip lol. I have connections and/or access to some excellent mechanics, machine shops and even trans shops being in Chicagoland so I really think this is a feasible solution for me down the road. Just dying to know the kind of details we got when someone first did the M3 drivetrain swap ten years ago :(
 

doublespaces

Administrator
Oct 18, 2016
6,758
AZ
#16
Alright, it was the physically using it thing I was hung up on - I don't mind ordering a different FD LSD if need be but I'd rather not pay an extra $4-600 for a shell if I don't have to. Do you know what Taron ended up going with for the FDR?

I mean, I did swap a 4.11 FD into my 6AT LS3 Camaro when I had it so I'm not a stranger to short shifts and stupid amounts of torque but I'd like to do it right and have a car that's (very) competitive at the strip lol. I have connections and/or access to some excellent mechanics, machine shops and even trans shops being in Chicagoland so I really think this is a feasible solution for me down the road. Just dying to know the kind of details we got when someone first did the M3 drivetrain swap ten years ago :(
He used the 2.81 rear end from what I recall.
 

taron

New Member
Mar 20, 2017
6
#17
Hi Everyone

sorry for getting in late but i was stressed out with my work last month.

here is what everyone needs to know about the swap.

1. cut bell housing from 21 and weld on 28
2. use e shifter from e60(shifter from dct e92 will work as well but I had not one around to test)
3. wires are not hard to do, look around for AT to DCT swap its almost identical.
4. Torque converter is hard part to figure out. I have used TQ from 335d but its very lazy on this engine with single turbo, my goal is to go with another tq from x6m. I will update everyone once this is completed.
5. you need to figure out codding of CAS, unfortunately it seems without professional software/hardware like bmw explorer and KTAG its not possible to do, but i cant confirm 100% as i'm not professional on this area.
6. Contact XHP for custom flash file. They have prepared file for me which makes my car daily driven. It still can be improved but Thanks to @clemens for hard work and getting flash together from so much components.


for anyone having specific questions to me I would suggest collecting them all in this thread in one post and I will answer them all.

regards
taron
 
Jul 3, 2018
331
#18
Hi Everyone

sorry for getting in late but i was stressed out with my work last month.

here is what everyone needs to know about the swap.

1. cut bell housing from 21 and weld on 28
2. use e shifter from e60(shifter from dct e92 will work as well but I had not one around to test)
3. wires are not hard to do, look around for AT to DCT swap its almost identical.
4. Torque converter is hard part to figure out. I have used TQ from 335d but its very lazy on this engine with single turbo, my goal is to go with another tq from x6m. I will update everyone once this is completed.
5. you need to figure out codding of CAS, unfortunately it seems without professional software/hardware like bmw explorer and KTAG its not possible to do, but i cant confirm 100% as i'm not professional on this area.
6. Contact XHP for custom flash file. They have prepared file for me which makes my car daily driven. It still can be improved but Thanks to @clemens for hard work and getting flash together from so much components.


for anyone having specific questions to me I would suggest collecting them all in this thread in one post and I will answer them all.

regards
taron
Thank you! I do have a couple questions:
1. Can you confirm you used the HP28's differential? If you did, would you recommend sticking with that or would a different final drive ratio benefit the car better? I'll be using large twins rather than a single turbo
2. Any modifications need to be made to the cars tunnel to fit the trans? Or even a brace removal or anything?
3. I'm assuming you were Auto before too, did you reuse your driveshaft etc?
4. What issues will happen if the CAS isn't coded properly before the TCU? I wouldn't have expected the CAS would need coding tbh, do you know all what needs changed? Just a VIN re-write or something?

Thanks!
 

taron

New Member
Mar 20, 2017
6
#19
Hi

1. I have used both my stock transfer case and rear differential. 6hp21 and 6hp28 has the same gear ratio so no need for swapping the rear diff.
2. no modification to tunnel or any cut operations besides bell housing
3. i was awd auto and i stay awd auto and I reused everything
4. not sure about issue but big gear fault icon will flash all the time and probably dsc/dxc will not work. I dont remember exactly whats need to be done but the route I went is I read all dxc data from x6 and compared with dxc data from my car and codded missing items one by one.

regards

Thank you! I do have a couple questions:
1. Can you confirm you used the HP28's differential? If you did, would you recommend sticking with that or would a different final drive ratio benefit the car better? I'll be using large twins rather than a single turbo
2. Any modifications need to be made to the cars tunnel to fit the trans? Or even a brace removal or anything?
3. I'm assuming you were Auto before too, did you reuse your driveshaft etc?
4. What issues will happen if the CAS isn't coded properly before the TCU? I wouldn't have expected the CAS would need coding tbh, do you know all what needs changed? Just a VIN re-write or something?

Thanks!
 
Jul 3, 2018
331
#20
Hi

1. I have used both my stock transfer case and rear differential. 6hp21 and 6hp28 has the same gear ratio so no need for swapping the rear diff.
2. no modification to tunnel or any cut operations besides bell housing
3. i was awd auto and i stay awd auto and I reused everything
4. not sure about issue but big gear fault icon will flash all the time and probably dsc/dxc will not work. I dont remember exactly whats need to be done but the route I went is I read all dxc data from x6 and compared with dxc data from my car and codded missing items one by one.

regards
Awesome, thank you man. TBH I was worried this would be a bit of a challenge in every regard, but it sounds pretty straightforward honestly. I'm pretty confident the local shop I use for things I can't/won't/don't want to do could definitely accomplish the vast majority of this for me.
 

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