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      06-27-2022, 02:02 PM   #1
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GM vs ZF6HP21

Hi, I just got 578E (transmission oil wear) error on my GM transmission and thinking maybe it's not worth changing transmission oil and filter? Maybe its good time to swap GM with 6HP21? (although my GM is working ok)
I am wondering how big is a difference?

According to this post:
https://www.xhpflashtool.com/blog/co...m_gm_to_zf6hp/
Quote:
... it's pretty easy to swap your GM unit for a ZF and take advantage of an immense performance and fun gain. We're not shy to say, you won't recognize your car again. Some might disagree, but even if not "xHP-ed" the ZF is superior to the GM in many aspects ...
Anyone swapped GM to ZF6HP21? Was it worth it?
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      06-28-2022, 08:06 AM   #2
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Sounds like a ton of headache instead of just changing the fluid/filter.

My transmission shifted way better when changing the fluid with the Valvoline MaxLife ATF and a filter kit from FCP Euro.

FWIW, it shifts better than my friend's 2006 325i with the ZF.
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      06-28-2022, 11:44 AM   #3
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Quote:
Originally Posted by Hotter View Post
Hi, I just got 578E (transmission oil wear) error on my GM transmission and thinking maybe it's not worth changing transmission oil and filter? Maybe its good time to swap GM with 6HP21? (although my GM is working ok)
I am wondering how big is a difference?

According to this post:
https://www.xhpflashtool.com/blog/co...m_gm_to_zf6hp/


Anyone swapped GM to ZF6HP21? Was it worth it?
It's all the same unless you're planning on tuning with xhp in my opinion.

I would prefer a manual swap over either, that's for sure.
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      06-28-2022, 12:23 PM   #4
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Quote:
Originally Posted by Leucosticte View Post
It's all the same unless you're planning on tuning with xhp in my opinion.

I would prefer a manual swap over either, that's for sure.
Yes, I am planning tuning with xhp (or Alpina b3?).

Though the cost would be quite high with xhp...:
Code:
250e gearbox
150e other parts
200e new oil/filter (similar to GM)
~200e work (50e GM)
220e xhp flash.
so it would be 770e investment compared to GM, not sure if that would pay of for what fun it would give. My engine is tuned to 330i, so maybe I would gain more from transmission than regular 328i?

Would be interesting to hear if anyone done this, and regret it or recommend it. Or no one done this?
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      07-01-2022, 05:00 AM   #5
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I haven't done the swap, but did a lot of research into it.
Stock, 6HP21 shifts a little better than 6L45, but the real difference is in xHP - no tuning is available for the 6L45. Make sure the mechatronics are 03/2007 or newer so it's fully compatible with xHP.

Avoid the 6HP21 from 335i; the torque converter and gear ratios are different. Here in Australia, I could only find N52 6HP21 in late 2006 cars. In Europe, I believe it was also used from ~2010 until the end of production. Hopefully you can find one that's suitable.

Wow, parts are so much cheaper for you! Used 6HP21 costs around 5x higher here.
If you can't find 323/5/8i 6HP21 newer than 03/07, perhaps you could swap in the mechatronics from a newer one, for full xHP support.

You will most likely want to change the mechatronics sleeves to ensure good shift quality; it's especially important for xHP with increased line pressure. If you have the budget, replacing the shift solenoids should improve things a little further, but it's not strictly necessary.

Hope this helps.
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      07-01-2022, 01:50 PM   #6
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I guess I will need to attempt to do this retrofit, since not much people have done this and I will be able to tell if it was worth it or not. If it would improve kick-down once you step on the gas pedal while cruising that would be well worth it for me. As I really hate that lag. Faster manual shifting is also welcome.

Quote:
Originally Posted by cheerio View Post
Wow, parts are so much cheaper for you! Used 6HP21 costs around 5x higher here.
Maybe thats because I have found 6HP21 from 2010 E90 320i N43.
https://www.realoem.com/bmw/enUS/partxref?q=24007592522
Does 6HP21 from any engine is ok? 116i/120i/320i from N43/N46?

Quote:
Originally Posted by cheerio View Post
If you can't find 323/5/8i 6HP21 newer than 03/07, perhaps you could swap in the mechatronics from a newer one, for full xHP support.
Or I must use 6HP21 323i/325i/328i/330i from N52 only?
If so its much harder to find, I find only old gearboxes of 2006 or similar...

P.S. Interesting find that in Europe E92 N52 had GM GA6L45R gearbox not ZF:
https://www.realoem.com/bmw/enUS/par...BMW-330i&mg=24

Last edited by Hotter; 07-03-2022 at 10:28 AM..
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      07-02-2022, 08:40 AM   #7
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Physically, I'm not sure if the N43/46 transmissions will bolt up to the N52. Perhaps it may need some modification.

They will have different torque converters, and possibly different gear ratios. You could try flashing the 330i software to it, but it will not perform exactly how it should, because of this.

I think for it to work perfectly, you'll need to use an N52B30 6HP21. (torque converter differs between N52B25 and N52B30).

If you can't find an E9x N52 6HP21, another option is to take one from the LCI E60 530i. That gearbox is electronic shifter only; the E60 e-shifter will definitely work but might need modification to fit. E89/E84 8HP shifter will bolt into the E90 chassis, and should be electronically compatible. I don't recall if the E60 6HP21 needs a the parking lock cable, you would have to find out. With this combination you could use the E60 Sports Automatic Transmission coding, which gives much faster shifts than the stock E90 6HP21.
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      07-02-2022, 10:21 AM   #8
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Quote:
Originally Posted by cheerio View Post
They will have different torque converters, and possibly different gear ratios.
I have asked xHP support if 320i N43 gearbox will work:
Quote:
Yes, all BMW derived 6HP21 HW#7591971 are compatible. The overall gear ratio is changed by a different Rear differential ratio, so you can take the transmission itself from any model.
So it seems that 6HP21 HW#7591971 320i N43 should be suitable too.
Should I completely trust them?
I will ask them does torque converter differ if the gearbox would be from N52.
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      07-02-2022, 11:52 PM   #9
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This may be of use.
https://www.xhpflashtool.com/blog/co...m_gm_to_zf6hp/

For my swap on my 06 330xi (6HP19), I'll be using a 6HP19TU (6HP21) from an 10/07 335xi. I can't wait remember the HW# off the top of my head. I then bought a separate torque converter from an 07 E60 (not sure if 535 or 530) with transmission part HW 7573661, which is to be compatible with the N52 according to the link I posted above. Main reasons being I wanted a straightforward swap with as little surprises as possible and as cheap as possible. That eliminated the 8HP and DCT. Couldn't find an LCI 530 trans for the life of me. Got the 60k mile 335xi trans for $400, torque converter for $175. Unfortunately not much info on the TC. It was the only one I could find with that HW number and matching splines. It looked to be good condition so I'm not sweating it. Might sell the tc from the 335 or keep it as a backup.

Cool thing is the 2nd gen is more tuneable than the 1st gen. To get xHP to have the correct maps for the 330 with the new trans, coding the DME/CAS/TCU with a vin of an N52 E9X w/ the 2nd gen trans is necessary. I found a proper vin from an EU car. Will have xHP swap my old vin from my account for the new one once everything is good to go. Finally after that I'll see if Rod Sutphin can help me out with a custom tune. Going down the rabbit hole and see how smooth I can get D mode and how sporty I can get sport mode..

Having the guy who's replacing my engine do my trans swap as well so he'll be undertaking everything but it should be fun. I'll update yall as much as I can. With that being said my 1st gen will be up for sale once everything is complete
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      07-03-2022, 03:30 AM   #10
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Quote:
Originally Posted by Hotter View Post
I have asked xHP support if 320i N43 gearbox will work:


So it seems that 6HP21 HW#7591971 320i N43 should be suitable too.
Should I completely trust them?
I will ask them does torque converter differ if the gearbox would be from N52.
Very interesting! I'd been going by some tables of gear ratios I'd found floating around, but perhaps I've been misinformed. The 6HP21 parts catalogue here only lists a single set of ratios, so seems like you're right.

The torque converter is definitely going to be different. Mechanically, I don't think the N46 converter will bolt up to the N52 flywheel. As for lockup and other characteristics, I can't say for sure. This catalogue lists ZF part numbers for torque converters (unfortunately only for US), and there seem to be lots of variants. You could also check part numbers on RealOEM.

Perhaps you could use a torque converter from early 6HP21 (late 2006)?
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      07-04-2022, 05:06 AM   #11
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Quote:
Originally Posted by cheerio View Post
The torque converter is definitely going to be different. Mechanically, I don't think the N46 converter will bolt up to the N52 flywheel.
This is response from xHP:
Quote:
the torque converters are pretty the same, the diesel ones have higher stall compared to the petrol ones but its barely feel-able.

The best would be to search for an 335i 6HP21 with HW#7591971, as they would fit perfectly to your 6 cyl N52.
335i 6HP21 will be too hard to find and much more expensive...
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      07-04-2022, 06:46 AM   #12
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Quote:
Originally Posted by Hotter View Post
the torque converters are pretty the same, the diesel ones have higher stall compared to the petrol ones but its barely feel-able.
This is interesting and great news for me, because it means that perhaps my 8HP45 swap will be usable before I figure out the tune

Perhaps you could look at using the torque converter from an early N52 6HP? If 6HP19 torque converter fits the 6HP21, that should open up some options.
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      07-04-2022, 11:09 AM   #13
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I have found 6HP21 from 2006 E90 325i N52B25, but I guess Torque converter is still different between N52B25 and N52B30?
You said something about mechatronics, it wont be fully supported by xhp if its 2006?
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      07-04-2022, 03:43 PM   #14
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I guess you can use RealOEM to check if N52B25 uses the same gearbox P/N as the N52B30. If they're the same you know 100% it will work, if they're not the same, there's still a chance the gear ratios will be the same and they just used a more beefed up torque converter.

Cheers.
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      07-05-2022, 05:19 AM   #15
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Quote:
Originally Posted by zEDo View Post
I guess you can use RealOEM to check if N52B25 uses the same gearbox P/N as the N52B30. If they're the same you know 100% it will work, if they're not the same, there's still a chance the gear ratios will be the same and they just used a more beefed up torque converter.
Its not the same on RealOEM, even gearboxes have many different codes...

Quote:
Originally Posted by Hotter View Post
I have found 6HP21 from 2006 E90 325i N52B25, but I guess Torque converter is still different between N52B25 and N52B30?
You said something about mechatronics, it wont be fully supported by xhp if its 2006?
I just got last question answered from xHP:
Quote:
it would be better to use the Gen2 6HP21 from at least build year 2008 and up. The Gen1 transmissions are not recommend. Torque converters are pretty similar for all 6HP21 transmissions.
So 2006 gearbox is "no go". They say that torque converters are pretty similar, so wondering should I go with 6HP21 HW#7591971 320i N43?

P.S. Maybe its time to switch to N54/N55 which comes with ZF already But its not that easy to find good one with good options... but its a possibility.
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      07-05-2022, 07:49 PM   #16
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Sounds like you could use the N43 6HP21 transmission, with the torque converter from the 2006 N52B25 6HP21.
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      07-17-2022, 07:09 AM   #17
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I have found 6HP21 from 325i N53, will need to go check codes in junkyard, so I might attempt the swap (the price is ok, but that transmission has ~100000miles which is not very good).

What about transmission VIN code? Will I be able to change it to my car VIN?
How will xHP work, by TCU VIN or DME VIN?
If I will want to use 330i maps of xHP I will need to get 330i VIN?
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      09-10-2022, 08:59 AM   #18
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So I have completed my 328i GM to ZF (6HP21 part 7592503 with HW 7591971 from 2008 325i N53 with 84k miles on it) transmission swap.

Swap went ok, needed few extra parts:
Trans-Support-Bracket: 22326795972
Oil-Cooler Line Feed: 17227577635
Oil-Cooler Line Drain: 17227577636
Muffler Holder Bracket: 18207521602
Transmission Heat shield: 24007535232? (bought by mechanic)

After transmission swap there were no errors and everything worked perfectly. My butt'o'meter noticed that ZF transmission is a bit more aggressive compared to GM, but not that much in stock form. GM transmission seems to prioritizes smother shifts. After few weeks of driving I have flashed my VIN to it with WinKFP and then used xHP stage 3. Transmission is even more aggressive, I guess faster shifts and it does not lose much engine power during shifts as they rev match better?..

D mode is almost perfect, the only thing I have noticed that Kickdown does not work sometimes if you hit it too slow, you need to hit it very fast.
S mode has quite aggressive throttle blips which I am not a fan of when I come to a full stop and it throttles almost at the end of the stop.
M mode has quite aggressive throttle blips also. It seems to respond much faster to peddle shift compared to stock.

Overall I like it, but I have spent ~1500eur (for used transmission, complete filter/oil/seals change, parts, labor, xHP) which is more than I anticipated.
I would say it was worth it if I will keep this car. Although I am thinking about buying 335i - than it was kind of waist of money. But still I am enjoying it.

Last edited by Hotter; 09-11-2022 at 04:22 AM..
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      09-10-2022, 06:52 PM   #19
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Great to see! How was the wiring part of the swap?
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      09-11-2022, 04:16 AM   #20
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Quote:
Originally Posted by Leucosticte View Post
Great to see! How was the wiring part of the swap?
Removing wires from the plug was the hard part. After that there were no problems with a given pin-out table. Everything worked and no errors.
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      09-14-2022, 06:42 AM   #21
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Nice! Sounds like xHP worked fine by simply coding your vin to the new transmission?
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      09-14-2022, 07:09 AM   #22
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Quote:
Originally Posted by colorado.e9x View Post
Nice! Sounds like xHP worked fine by simply coding your vin to the new transmission?
Yes, they support "328i (ZF auto)" with VIN 4-7 numbers WB33. But I had an issue that it did not work at the start, because my VIN is WB3C (I think C is for USA market). They added this in their backend and it started to work. Good support from their side.
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