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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 expl