6HP to 8HP conversion feasibility
 

Has anyone looked into if swapping an 8 speed transmission into the earlier E70s is feasible? I'm doing some preliminary research using my 4.8 as a reference and a 50i as the donor. So far, the bellhousing bolt pattern appears to be the same. I'm guessing overall length is close as well - the rear driveshaft is 1mm different in overall length, but of course that incorporates the transfer case as well. The E70 has seen a number of transfer cases over the years and it's another challenge to address - is the bolt pattern and splined output shaft the same between the 6HP and 8HP? If not, then the transfer case that goes to the transmission would have to be used and I'm not sure how that would integrate. From what I can tell, it does look the same (at least the bolt pattern). Once the mechanicals are addressed, we need to worry about the electronics. I've heard, though not confirmed, that the 8HP uses FlexRay for communication whereas the 6HP uses CAN bus. My suspicion is that while this might be true on F-series vehicles, the E70 likely retained CAN bus for its 8HP implementation. Assuming the message format is the same, I suspect the transmission may be plug and play... As far the immobilizer is concerned, I have the equipment to synchronize the ISN on both the 6HP & 8HP. It looks like the CAS is shared between 6HP & 8HP, so I again suspect it can be made to work. Any thoughts? Does anyone have wiring diagrams for the 8HP by any chance?

Wiring diagrams are available here. https://www.newtis.info/tisv2/a/en/

The 8 speed transmission uses a different transfer case than the 6 speed. I checked out some ebay photos and the output of the 6 vs 8 speed does not look the same. The best way to do this may be a complete donor vehicle so you can get trans, tc, driveshafts, diffs and mounts. I would love to have the 8 speed in my diesel. E70 LCI diesel in Europe has it.

Interesting. I did not know that.

Thanks. According to that, both use CAN bus and appear to be wired about the same. There's a couple pin differences and an additional connection on the 8HP for tiptronic (I think). None of that seems particularly worrisome.

At least on the V8, the transfer case bolt pattern does look the same to me? See these two auctions
https://www.ebay.com/itm/2007-2010-B...cAAOSwEFhdpLeq

https://www.ebay.com/itm/2011-2013-B...8AAOSwz~RdI3SR

On the plus side, I can reference a 2013 xDrive35i. I'd like to keep the swap as simple as possible so if the original transfer case can bolt on and the original driveshafts can be reused, that would be fantastic. If I ask really nicely, I might be able to get a CAN bus capture from the 2013 which would definitively answer if the messaging is the same.

To me, it looks like the output shaft is longer on the 8 speed.

You're right. I think the ATC450 could also be swapped in or it looks like it may be possible to put a spacer between the 8HP and the ATC700. In the pictures, the ATC700 has an almost flush mounting flange whereas the ATC450 is recessed. I also did some more research and found the diffs are different. Early models received very short gears - 4.44 in the gas 3.0 and 3.91 in the 4.8. Later models received 3.15s which works with the broader torque curve of the turbo engines & the wider ratio spread of the 8HP. While the 8th gear is about comparable to 6th in the 6HP, first gear is much shorter on the 8HP. I think I'd prefer to try and keep the 3.91s if possible.

I also looked up the immobilizer and it looks like the CAS3 was used for the entire production run of the E70. That means it should absolutely work with the 8HP.

Very interested in how this goes. I know the ATC700 is thought to be much stronger- and my 35d would need it. I've looked into swapping to the 3.15 of the 50i, to bring my final drive down from 3.64, to increase fuel efficiency, run at lower RPM, and generally run quieter/ more efficiently on the long haul. If you were to swap the 8 speed, I'd be very curious to see a what it takes!

Regarding the rear difs, the 3.15 for the 5.0 has a different part number than the 3.15 for the 3.0. This may be due to the larger axle shafts on the 5.0. Some of us diesel people are considering a 3.15 dif swap to get better highway MPG. A 3.15 dif alone will lower the highway RPM 50% compared to the 8 speed/3.15.

Where are you getting the 50% number from? By my math, it’ll lower our highway rpm by 15.5% from our 6 speed/3.15. I was under the impression that that would put us very in line with the engine speed of the f15/ 8 speed.

Sorry for the confusion on the 50% number. I was using a final drive calculator on some website a while ago. If I remember correctly, my current 2600 rpm at 80 mph would go to 2300 with the 3.15 dif. Then 2000 with the 8 speed.