Retrofitting a 6HP21 into a 2006 330i

I recently installed a ZF 6HP21 from a 2011 335i into my 2006 330i that came from the factory with a 6HP19. I have been wanting to do this swap for awhile, but never found concrete information on what all needed to be done.

This swap should also be achievable on an 07+ 328i as they came with MSV80 DME from the factory. I have not tried this myself, so don't send me angry messages if it does not work on your 328i. I do know the transmission harness will need to be repinned. If I were doing this swap on a 328i with GM, I would start here

My original 6HP19 had around 230k miles when removed, and the torque converter was not transferring all of the power. With the supercharger adding an additional 200HP, sometimes I could not shift at redline (full boost). The only maintenance I did on this were fluid + pan change 3 times, and mechatronic seals once. What a champ.

I decided on the hp21, as I remembered how well it shifted in my 335i, and it would easily tolerate abuse from the extra torque produced by the 22RPD Upgraded ESS Supercharger Kit.

Onto the new transmission, I purchased it from a 335i partout with 130k miles including the torque converter, And also a full 335i driveshaft (I only needed the front half). Other parts I found out I needed during the swap was a 335i flex plate, 335i shifter assembly, and 335i flexplate bolts.

One thing I knew from my friend Tyler, is that I would need to retrofit an MSV80 DME to communicate with a 6HP21, as he had tried with an MSV70 and it did not work.

Retrofitting the MSV80 was easy, with this guide provided by SpecE9x here

I paid $200 for a matching set MSV80 DME, CAS3, and key. All 3 must be matching or I would have to spend more on progamming, The MSV70 set can be sold for the same amount or more, so this is basically free. I had 22RPD make a new tune for MSV80, as an MSV70 tune would not work.

 

All parts I needed for the swap-

335i 6HP21 transmission $350

335i Driveshaft $50

335i Shifter Assembly 25167563655 $40

335i Flex Plate 11228604045 $90

Replacement stock flexplate bolts 24407561393 $28

335i flex plate bolts BMW-11227522125 $24

Replacement 335i transmission pan 24407561393 $87

Replacement bellhousing bolts CRP-HWK0002 $10

335i mechatronic seals $84

Redline D4 2 gallons $120

335i front giubo $free

Total Cost- $883

 

The first thing I did is dump the old fluid out of the new transmission, and replace the mechatronic seals, then bolted the old pan back on in case it was damaged while moving it around.

I wish I did a Sonnax Zip Kit and new solenoids while everything was out, but I didn't want to invest too much money not knowing if this swap would work.

I then removed the old transmission and took off the rear engine 'shield plate' to use as a stencil on the new trans. The top two holes need to be slotted with a dremel to line up with the N52 engine block. I ended up having to dremel more afterwards as shown in the second photo.

 

After this, I removed the N52 flexplate, and then installed the 335i flexplate. I found out here that the 335i bolts are different and had to order them. I made sure to brake kleen and dry them then apply red loctite. These bolts need to be replaced every time and clean or they can back out.

The 335i flexplate has different dimensions to accomodate the dimensions of the 335i torque converter.

Then it was time to install the transmission. I had to line it up with the engine then drop it a few times to get the bellhousing holes dremeled enough to line up.

The next step was to remove the front driveshaft from my car, and slide on the 335i front driveshaft. I added some graphene grease on the splines. The 335i front giubo is different, and will need to be installed in place of the N52 one. The front giubo bolts are also different, and should only be used once, along with the locknuts.

After that, it was as simple as reinstalling the heat shields, exhaust, etc.

I then filled the transmission as much as it could take with the car off, then did a transmission fill procedure with the car running, of which guides are available online.

After putting it into gear for a test drive, I found that the shifter will have to be swapped for the 335i part, as the N52 one is not the correct length. It will bolt up and "fit", but when going into drive, it had to be forced in, and didn't feel seated fully. Upon pulling the N52 shifter, the cable was bent.

I could not get sport mode to work, even with the new shifter, until I coded EGS paddle in protool in the transmission to "wert 1". It was on an old e90post thread where BimaExtremeRetro mentioned trying all 3 wert options until one works. Then it worked perfectly.

The final step was to get XHP working. I could not flash the transmission with XHP as-is, because the XHP app thought the car was a 328i with the GM transmission as that is what the DME is from. I messaged XHP, and they recommended changing the VIN on the DME and CAS to match the donor transmission and then it would work properly.

I didn't want to wait to send my set out to be programmed, so I bought a matching 335i N55 CAS and key (with the same type code in the VIN as the transmission, this is what XHP checks), and installed the 335i CAS and key to flash XHP, then went back to my original CAS and key afterwards to drive it.

 

It shifts like a completely different car now, and I am more than happy with the results. I would say it shifts at least twice as fast as my 6hp19, both tuned on XHP stage 3. The car also feels faster now while in gear with the newer torque converter.

I will probably replace the solenoids and install a zip kit at some point, but it is working great as of now.

 

Some helpful pages from the Bentley with torque specs/sequence