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Transmission Upgrade 6HP28 on N54

Hello everyone, My name is George Gelashvili I'm from Georgia Tbilisi

I'd Like to share this info for forum members here, i think it will be useful for N54 Enthusiastics who love DIY stuff and non ordinary approaches for improving car performance.

Anyways: Everything started from E90 335i AT (2007 model), that i bought for my Wife on 2019 new year. it was absolutely stock, when i got it for her. I have had tuned a lot of N54s previously, but never had personal one, so i was very excited to modify it and push it as far as possible with help of my team members.

We did everything on our own and custom, such as:

  • Double stock intercooler

  • 3" piping

  • Tial bov

  • Custom 3" downpipes

  • Dual 2.75 exhaust with dual cutouts at diff area

  • Custom exhaust muffler

  • E60 M5 Diff 3.64:1

  • Single Walbro 450 intank

  • PCV valve mod.

  • N55 Ignition Coils

  • 1 Step colder spark plugs gapped to 0.5mm

  • Water/meth injection before and after cooler.

  • Relocated intakes on passenger side with chinese air filters

  • Upgraded wastegates and adjusted actuators to sit tighter

Same time, i bought billet compressor wheel 39mm but have not installed it YET, as i wanted to push and see stock turbo limits what it was capable of and see 100-200 times. so i left billet wheel on the shelf.

  • Custom tuned car to 22psi and flashed with MHD Flash tool

  • Ran 100 octane fuel with 15% ethanol in tank

Stock 6HP21 AT Trans w(235000km on it) XHP Stage 4 Flash with my custom torque adjustments car ran 7.41 100-200km/h but transmission used to slip badly past 200km/h in 5th gear once AT Fluid would warm up.

I was able to do few 80-250km/h runs (maybe 2-3) and then it would start to slipp badly.

Long story short, it was time to think about upgrade. Clutch pack upgrade was pretty costly, especially when it comes to shipping, import taxes, etc. (i'm far from USA) and overall there was no warranty that trans would hold abuse and would operate smooth and just like stock.

So i did my own research over internet and thought why not give a try to 6HP28 from E90 335D. Some "experts" said not all holes line up BUT! if u do some grinding and enlarge bolt mount holes trans will bolt on without issues, as well as torque converter.

I found Taron's thread where we said that for 6HP28 you will need:

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.

I was like... So much work and somebody said you just need to enlarge holes and it lines up...

So i went and found trans from 2011 335D which had, same shifter mechanism and i didn't need any other shifter than what i already had. - I was like.. hmm task list got shorter now..

When we removed stock transmission and tried to fit Diesel one on engine.. figured out that:

  1. 6HP21 Torque converter (TC) has 6 bolts and 6hp28 has 4

  2. 6HP21 has smaller diameter TC than 6HP28

Well good news was that, input shaft and splines were same so torque converters turned out to be interchangeable, so we quickly "solved" this issue by deciding to leave 6HP21 TC on 28 Trans.

  1. None of the mounting bolts of 6HP28 lined up to block.

  2. Thought about enlarging them, but turned out only top two bolts can be mounted but none of rest because, Transmission bellhousing is wider itself and does not line up at all.

  3. Even if we managed to mount transmission somehow another issue was that transmission sump was angled... it could not stand straight when mounting top two bolts.

So i was like WTF... Who said you need to enlarge bolts and it lines up?

It was an interesting challenge... decided to cut both transmissions and weld to each other. So this is what we did:

Cut both 21 & 28 front side of bellhousing and decided to weld 21 front side to 28 rear end.

When we lined them up.. turned out that 21 is a bit smaller diameter than 28 so was torque converters.

As long as i went for such hardcore direction, i decided to use 6HP28 Torque converter as well as i was told that it is stronger than 21.

Took measurements of N54 flywheel and figured out there was room to CNC drill holes for new mounting points.

After 5 days of measurements, cuts, welds, machining and F***ing with transmission we did it... Got damaged engine block, crankshaft, machined guide from crank to trans to line it up and fit straight. Measured length of both transmission and everything was fit within 1mm clearance. Lined up transmission sump to engine sump so they were on same level and not angled.

Outcome and look was not shiny and perfect but as long as it works i don't give a **** what others will say or think.

If somebody want shiny parts on a car be ready to pay few thousand dollars for shiny adapter plates.

From beginning of project after 1 week transmission was ready to mount and check rest things such as mechatronic, cooling lines, etc.

Modified flywheel bolted to the engine. 6HP28 torque converter bolted to it perfectly. Transmission mount process was real b**ch because tools briefly fit with bolts. There was no room to put hand and tight bolts so mounting took very long.

Once mounting was finished it was time to check mechatronic and compare 6HP28 to 6HP21

I thought they would be interchangeable and i would just swap TCUs so there would be no programming issues.

They looked very very similar - BUT turned out they are different and not interchangeable

Sensor height was main difference so we assembled trans back. Also compared 6HP21 and 6HP28 sump / filter to each other to see if they are interchangeable - they are different as well.

Once everything was ready, trans was filled with new oil, started engine and oil came out from trans cooler pipes mounting pint. We have not paid attention and it turned out that 6HP28 uses larger diameter pipes so test drive was postponed for a while..

I could not find OEM cooler pipes so had to think what to do, but it didn't took me long... i drew adapter for 6HP28 to accept 6HP21 cooler lines without leakage and modification. Took me few hours to machine them with appropriate size washers to fit pipe holding bracket properly and not angled.

Anyways, installation of cooling pipes with adapters and boring was easy as 1,2,3

Filled completely with new oil and took car for a test drive.

First impression was - no kicks and no strange behavior. No vibrations car runs smooth and nice... it was really great.

BUT! now it was time to do some programming as diesel does not rev past 5k

Read stock flash, checked maps.. but i thought instead of wasting time on research and investigation i would pay to XHP to make custom flash for my trans and let them do all that stuff.

Took about 2 week back and forth communication with Clemens from XHP to develop flash for my Transmission which is still under investigation and development. He did many revisions as it is something that u will not see in everyday live and big shout to him for that!

So far car runs perfectly fine as cruise, or moderate load or full load. We are still in development and testing process, flash is not 100% complete yet but i love how car runs and holds power without slippage.

Meanwhile when car was apart i did not waste time and prepared turbochargers with billet 39mm compressor wheel and 9 blade turbine wheels.

Now it is time to tune and check what car will be capable of on new trans and compressor wheels.

I'll be posting more info on my INSTAGRAM: @Geptuned and Facebook: Gelashvili Performance

I thought i would share this info for somebody struggling like me and find useful for future.

I'll be uploading pictures whatever i have taken from trans upgrade process feel free to ask questions

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