8 Speed ZF8hp from C7 RS7 and RS6 retrofit.

[IanH755]

Does anyone know if it's possible to split the prop-shaft into two?

Nick

The C6 propshaft can be as it's a two piece design with a bushing/support at the mid-way point which is the join between the two pieces. I've not seen a C7/8 one yet but would imagine it's a similar two piece design as the C6 one, just different lengths.

[welwynnick]

The C7 prop-shaft is very similar to the C6.

How do you separate the joint? Can you just pull it?

I was just wondering if I could use a hybrid prop-shaft with the front section from the C7 and rear from the C6?

It's not not necessarily a solution, but it might be an option.

There's quite a lot that can be done with VAG parts engineering.

[IanH755]

You'd have to measure the two parts of each prop shaft as the position of centre bushing/support may need to be adjusted to suit a mix/match of C7/C6 prop parts. Splitting is relatively easy but these are balanced so keep note of where bits are fitted. IIRC there's a least 1 person on here which has the split the shaft themselves, maybe worth a search as I know I originally though that, as Audi sell it as a single piece unit, only a specialist could take these apart but I was wrong.

[welwynnick]

By a strange fluke I just found out that one of the UK's biggest prop-shaft manufacturers is walking distance from work. Never knew they existed, but I might be beating a path to their door in the near future.

[welwynnick]

I also found out that the RS7 propshaft has a 25 spline coupling at the gearbox end. With some apprehension I counted the splines on my gearbox. 25. Phew!

The 8HP torque convertor fits the C6 flywheel / flex plate as well. Phew 2.

[welwynnick]

Most of my attention turned to property this year, rather than cars, but I've made a bit of progress with gearboxes.

It was a shame that the RS6 C6 came to the market just a little bit too soon for the 8HP, but there are some striking similarities. I think ZF tried to make the 8HP as close to a drop-in replacement for the 6HP as possible. It's used on a similar range of up-market RWD and AWD cars, with similar torque capacity, dimensions and weight. Apart from the rear output shaft, the mechanical interfaces are similar as well.

So is the architecture. The 6HP uses an internal (not external) TCU that sits in the sump and controls the solenoids that actuate the valves in the valve block. The external interface to the engine etc is by CAN bus, which is a standardised interface. The point is that unlike some slightly older cars, the electronic control of the transmission - the logic and control laws and adaptations for the actuation of the clutches and brakes - is performed internally. The 8HP is the same. I don't know yet if they use the same CAN version, but I think the architecture does lend itself to transplants a bit more easily than you might think.

The mechanical interface is also similar, but as Ian has previously explained, Audi tried hard to reduce the nose-heaviness of their cars, and the C6 benefitted from having the clutch or torque convertor behind, rather than ahead of the axle as before in the C5. The 8HP took things a little further, and packaged things more tightly to move the bellhousing plane a few cm further back, and that does make transplanting a bit more difficult, but not much.

Back soon, Nick

[VaudiGR]

Good stuff! Thanks for the update

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[welwynnick]

For most cars, the 8HP was introduced as part of a major change in the model line-up, but there were a few mid-life updates, including Aston Martin Vanquish & Rapide, Audi Q7 4L, BMW 7 series F01, Range Rover L322 and Range Rover Sport. In each case, there were big improvements in both acceleration and economy compared with the 6-speed.

The most interesting update is perhaps the Audi A4/A5 B8, which had the most bizarre transmission choice: FWD & 4WD; auto & manual; 6, 7 and 8-speeds, and epicyclic (Tiptronic), dual clutch (S-Tronic) and CVT (Multitronic) automatics. I'm not 100% certain, but I believe that some versions of the A4/5 TFSI 2.0 Quattro were originally designed around the 6HP, but later fitted the 8HP transmissions, indicating that the 8HP might actually retrofit the A6 C6. With some work.

The Audi Q7 4L upgraded from 6HP to 8HP mid-life, so that might be a doner candidate. Unfortunately, the centre and front differentials are in separate units in the 4L, rather than integrated like the A6/A8/RS6. The later Q7 4M did have an integrated transmission, but I'll skip that for now. At this point you might be thinking that the 4M or C7 could be a good doner for the C6, but I think there's a better option.

Back soon, Nick

[welwynnick]

I spent a lot of time looking at the back end of engines and the front end of gearboxes, and noticed that most of the V6, V8 and V10 engines in the VAG group looked remarkably similar, and so did their mating transmissions. Things like the bolt hole positions and the stater motor location all seemed the same. Here's the back end of a V10, and the front end of a 6HP26A:

When the 8HP came along, it looked rather like the 6HP28, with a different casing, a circular torque convertor drive plate, a straight prop-shaft tube, the bell housing is shorter (allowing the engine to sit further back), and the LHS output shaft was moved outboards, so the drive flanges were symmetrical with the centreline, and the front drive shafts could be made equal length. It's fairly clear this won't bolt up to the V10:

However, the 4.0 V8 and the ZF 8HP were fitted to the Bentley Continental GT and Flying Spur mid-life update around 2012, and the 4.0 was based on the V10 - same bore, same stroke, and similar bell housing as it turned out. The rest of VAG moved on, but Bentley ALSO wanted the same 8HP to fit the old W12, which had only been partnered with the 6HP26A before (same as the RS6 C6). Therefore they needed a bellhousing adapter for the new gearbox so the engines could stay in the same position. There were other accommodations to minimise changes, too. Here is the back end of the Conti GT 4.0 engine with the adapter fitted, and the front end of it's transmission:

Notice that the LHS drive shaft is very short, like the 6HP26, and the flex-plate to torque convertor mounting bolts are grouped in three places, not six.

I think it's worth mentioning that the 'box fitted to both the V8 and W12 Bentleys is the 8HP90, which is the version with the highest torque capacity of all, 900 or 1000 Nm, depending on where you read. Apart from Mercedes/Aston/Tesla, this 'box is also fitted to all the biggest and most powerful European and American SUVs. Many of them are tuned and raced with huge power and torque.

Are you thinking what I'm thinking?

[VaudiGR]

Looking at Bentley continental GT gearboxes as we speak... hahaha

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[welwynnick]

Here's the bell housing adaptor, Bentley part number 3W0 103 546 A:

I also happened to acquire some other interesting parts:

Audi RS6 flex plate (connects torque convertor to crankshaft)
Audi RS6 rear timing cover (provides mechanical interface between 6HP and the back end of the following engine:)
Scrap Audi R8 V10 engine

It got quite exciting at this point. Wanna know what happened when I tried to mate the greatest engine ever made to the greatest gearbox ever made to fit them in an RS6?

[welwynnick]

See the pictures on 05 June 2021, comparing the back end of the RS6 and the R8 V10 & Huracan:

viewtopic.php?f=47&t=129086&start=15#p955439

They look very different, but having taken the R8 engine apart, I now have a clearer idea about the differences between them. I'll write it up in the "How high can the V10 rev?" thread, but as far as gearbox compatibility is concerned, the differences are quite simple:

The R8 has a dual clutch gearbox, so the rear timing cover is completely different to match (but is interchangeable with the RS6 timing cover).
The R8 has bearing housings for the prop-shaft to the front axle, and the RS6 does not.
The R8 starter motor and power steering pump are BEHIND the engine, but they're down the side on the RS6 (so a stiffening web is deleted).
There's also a foul between the bell housing adaptor starter motor shroud and the gearbox, but that's a quick hacksaw job, and there's a couple of other minor fouls.

On the upside, with the adaptor in place, the RS6 flex plate simultaneously bolts onto both the R8 crankshaft, and the 8HP torque convertor.

So in short, no, the Bentley 8HP doesn't quite bolt straight up to the Lambo / R8 engine, but .......

[welwynnick]

Here's the money shot:

I believe this the first time an 8-speed gearbox has been mated up to a 10-cylinder engine. Long way to go, but very happy with that!

[RS6-T28RPH]

awesome work! would me mega to have an "upgrade kit" for the C6 although just spent a lot on my existing 6 speed to make it take 1200nm torque every day of the week

[welwynnick]

One of the other advantages of going the Bentley route is that their versions of the 8HP have a cable-operated mechanical control lever down the left-hand side, in the same place as the 6HP.

The front wheel drive shaft flanges are also the same size and position, so I believe the RS6 C6 driveshafts should hook up directly.

Apart from the rear output spline drive, Bentley & ZF seem to have gone to some efforts to make these particular gearboxes a close retrofit as possible.

Nick