Using an AWD transmission in place of a transaxle

Hey guys, I'm new here. I've got a question that I have not been able to find an answer to anywhere else and you guys seem to know your stuff so let's get right into it.

I own two Audi S8 sedans, both with the Lamborghini based 5.2 V10 with AWD. One is my weekend/fun car and the other is just a spare parts car. I've decide to part out what I can and then use the drivetrain in my new project car. Both cars have auto trans, there were no other options for this car as it is their top end luxury highway boat.

The project car is a planned to be a tube frame car with a mid engine rwd layout. I'd like to use the v10 and AWD trans. Partially because I already have it so why not, and partially because the Audi electrical is scary and I don't want to mess with the working pair. 

Anyway, I haven't been able to find a straight answer on what would happen if I was to use the front drive output from the AWD trans to power the rear wheels of my mid engine car? Without the shaft going to the rear diff, would it spin free with no issues or would I need to lock it out to transfer power to the front? Or do neither of those solutions work? Is there any possible solutions that would work?

Before we get into how I should buy a real transaxle or whatever, I just want to thoroughly vet this idea before I throw it out. Ideally I can make this AWD trans work because the v10 has a few systems that require communication between the engine and this trans specifically. 

Is it manual or automatic?

Either way, you'll need to lock up the center differential (the output goes to the rear axle)

Or see if there's a FWD version of that transaxle you can nab from overseas to swap in place.

In reply to Stefan (Forum Supporter) :

It's an auto, I'll add that to the post. Sadly, there were no other options for this car. It's their top end luxury highway boat so no manuals available. Although, for an auto, this thing can shift really quick.

So you think locking it out is all it takes to make it work? I'm not going to melt any spider gears or brick the trans?

This is done with Subaru transmissions for the Factory Five 818 and for Subaru-geared Vanagons. I've built one but I have to admit I've forgotten the details. Look up Subagear installation instructions as a guide. 

Deric said:

In reply to Stefan (Forum Supporter) :

It's an auto, I'll add that to the post. Sadly, there were no other options for this car. It's their top end luxury highway boat so no manuals available. Although, for an auto, this thing can shift really quick.

So you think locking it out is all it takes to make it work? I'm not going to melt any spider gears or brick the trans?

You may find that the transmission is not strong enough to put all of the torque through what was originally the front diff.  I have read that this is a problem with the FFR 818 cars which were intended to be a single-car donor (regular WRX) but apparently need to be retrofitted with an STi transmission.

Converting the C5 S6 (4.2L V8) to a manual is fairly well-documented, but I've never seen anyone do it on the C6 V10 cars.

In reply to codrus (Forum Supporter) :

The Audi transaxle is slightly stronger and better built than the Subaru units.  The FWD vs AWD internal difference are quite small and they generally run FWD more often than not.

The GT40 guys run 2wd audi boxes somewhat successfully in a similar setup as yours.  If you were willing to consider a manual option, the Audi o12 and o1E transaxles might be an alternative for you.  We can get them fairly cheap out here, but shipping might be a hassle. 

if it is something you'd consider i can send you a couple documents with the gear ratio info, etc (no idea how the bolt patterns compare).  I also know a guy in Tacoma that has a spare o12 he might part with.

Stefan (Forum Supporter) said:

The Audi transaxle is slightly stronger and better built than the Subaru units.  The FWD vs AWD internal difference are quite small and they generally run FWD more often than not.

Yes, it's stronger, but it's also behind a V10, so it had better be. :)

Longitudinal-mount Audis do not generally run FWD.  The A3/TT/etc cars do (transverse mount Haldex-based system), but the A4 and up quattro cars are driving all four wheels all the time.

Yeah, what Codrus said. Some of the earlier 4WD Toyotas were designed with diffs strong enough but no stronger and when one of the was disengaged and all the drive when through the other one, they got quickly trashed.

In reply to wspohn :

Luckily for me, the project car will weigh just about exactly half of what the S8 weighs, so I'm not worried about doubling the load at the front output of the trans. Just curious if it's possible to make it do what it needs to do.

codrus (Forum Supporter) said:

Stefan (Forum Supporter) said:

The Audi transaxle is slightly stronger and better built than the Subaru units.  The FWD vs AWD internal difference are quite small and they generally run FWD more often than not.

Yes, it's stronger, but it's also behind a V10, so it had better be. :)

Longitudinal-mount Audis do not generally run FWD.  The A3/TT/etc cars do (transverse mount Haldex-based system), but the A4 and up quattro cars are driving all four wheels all the time.

 

Tons of FWD a4s run the standard longitudinally mounted drivetrains with no rear output. And the newer "ultra" quattro cars decouple the rear driveshaft in certain situations like highway driving to get the best mpg possible.

I imagine it's physically possible to setup the transfercase in the back of the stock trans to be locked and output only to the front like the subaru conversions but I wonder how the computer will deal with not having the rear diff there. I thought I remember a haldex-like computer controlled pump in the diff but I definitely could be wrong.

iansane (Forum Supporter) said:

Tons of FWD a4s run the standard longitudinally mounted drivetrains with no rear output. And the newer "ultra" quattro cars decouple the rear driveshaft in certain situations like highway driving to get the best mpg possible.

I don't see how that's relevant?  Yes, clearly it's possible to engineer a transmission like this to be able to send all of the torque to the front wheels without blowing up and yes, Audi did that with the FWD A4 and the "ultra" thing.  That doesn't say anything about whether or not it will survive with a V10 S6, Audi never sold one of those in front wheel drive!

You're not wrong. I can't imagine how rowdy a fwd v10 would be! Ha!

I guess I don't see how it's not relevant? You said they were generally awd setup. I was just clarifying that a significant portion of cars run fwd bias if not fwd all the time. I was just making the theoretical jump that it might be possible to change/modify the transfercase/box for fwd only output. In a car with half the weight, the 450hp is much less stressful(and more fun!)

iansane (Forum Supporter) said:

You're not wrong. I can't imagine how rowdy a fwd v10 would be! Ha!

I guess I don't see how it's not relevant? You said they were generally awd setup. I was just clarifying that a significant portion of cars run fwd bias if not fwd all the time. I was just making the theoretical jump that it might be possible to change/modify the transfercase/box for fwd only output. In a car with half the weight, the 450hp is much less stressful(and more fun!)

Um, he's making a midengined car out of his leftover V10 powered car.  the FWD talk is about how to convert an AWD longitudinal setup to be a RWD midengined setup.  Basically locking or disabling the center differential.

Yeah. And that's exactly what audi does with all of their factory fwd cars. It's like taking an entire fwd drivetrain and moving it to the back of the car. Obviously audi never made a fwd v10, but their i4 and v6 cars but bellhousing bolt patterns are largely the same. The fwd transmission and awd transmission are for all intents and purposes, the same. I don't think the fwd ones use the CVT trans. The transfercase/box bolted to the back is the difference. Again, presuming here, I figured using the same type of mechanism that audi uses in the factory cars to lock their transferbox might be applied to the s8 box.

iansane (Forum Supporter) said:

Yeah. And that's exactly what audi does with all of their factory fwd cars. It's like taking an entire fwd drivetrain and moving it to the back of the car. Obviously audi never made a fwd v10, but their i4 and v6 cars but bellhousing bolt patterns are largely the same. The fwd transmission and awd transmission are for all intents and purposes, the same. I don't think the fwd ones use the CVT trans. The transfercase/box bolted to the back is the difference. Again, presuming here, I figured using the same type of mechanism that audi uses in the factory cars to lock their transferbox might be applied to the s8 box.

I've never taken one apart, but I suspect Audi makes different guts for the same case that only include a single front diff, rather than including a useless-but-expensive center diff that is then locked.

Aren't there front wheel drive D2 A8s in the US? It might be worth having a look at the differences between their trannies and the quattro equivalents. 

This is the closest to your intended application that I can think of. 

In reply to codrus (Forum Supporter) :

Okay, I think this is where the disconnect is. The transmissions don't change. I mean they do, but not really between awd/fwd. It's the transfercase/box that's different between quattro and non quattro cars. They all have the same "front wheel drive" setup. Some just add a rear output to that fwd setup. Which is very, very odd. They all have a normal longitudinal transmission driving a transfercase with a front driveshaft (That is completely encased in the transmission) to a front diff that is cast into the transmission case. It looks like one unit. The quattro cars have a transfercase with two outputs and nonquattro cars have just the front output. I'm not saying that you can just swap transfercases to get fwd, but I imagine using a similar mechanism you could make a quattro box into a "fwd" box and mount it in the back of a car.

edit; that's not a great perspective. This is better.

Having said that, that should apply to this d3 s8. I think the later d4 cars use a more integral design not so easily modified. I don't mean any of this maliciously or sarcastically. I just want to be on the same page. 

Let us not forget that Audi and Lamborghini do something like this themselves for the GT3 race cars with these engines where 4 wheel drive is prohibited by regulation. 

In reply to iansane (Forum Supporter) :

It is interesting to me that every image I find like your shows the front outputs being placed about 6 inches aft of the mating surface but on my trans the shafts are nearly in-plane with that surface, which is what your image shows too. I think this setup is more rare than the others.

Deric said:

It is interesting to me that every image I find like your shows the front outputs being placed about 6 inches aft of the mating surface but on my trans the shafts are nearly in-plane with that surface, which is what your image shows too. I think this setup is more rare than the others.

Audi has been using this basic config for longitudinal mount quattro since the 80s, but one of the downsides to it is that the need to have the front driveshafts coming out of the side of the transmission pushes the motor pretty far forwards and makes the cars front heavy.  To try to mitigate that they've done some kinda crazy things -- in the diagram above the front diff is actually located between the engine and the flywheel/flexplate.  That change came out about 10-15 years ago, I believe the B8 S5 was the first car to use it.

And yes, I understand what you're saying, iansane, but I don't think it changes my point.  Audi presumably designed the "front" differential in the A4/etc transmissions/transaxles to be capable of taking all of the torque from the 1.8T/2.0T when used in a FWD application because they intended to sell it that way.  They weren't going to sell a 2wd V10 so why would they incur the expense, weight, and packaging downsides of overbuilding the front differential in that transmission?

Anyway, I'm not saying it's impossible, just that it's worth being cautious.  Yes, lighter/etc, but also driving the rear wheels rather than the fronts means weight transfer is going to get back some of that load.  If you build the car like this and it eats "front" differentials, what then?  In the 818 they fixed the problem by substituting the STi transmission instead, but AFAIK there's nothing in Audi's parts bin that solves this problem.

In reply to codrus (Forum Supporter) :

I guess err'ing on the side of caution is a good thing but I can't imagine that diff is too weak. Audi also made fwd 3.2 a6 cars and while significantly less horsepower than a v10, they did weigh similarly. Hell, the d4 a8 came fwd TDI in europe. Again, I don't think the transmission/front diff is different between the fwd/awd cars, I think it's the transfercase at the back of the assembly. I don't know for sure since I was only at the dealer for a couple years and audi doesn't really have any part numbers in the catalog for trans internal parts.

I know it's a totally different ballgame since they usually use the 01e transmission but countless kitcars run "fwd" 01e's in high horsepower applications but I presume lots of those are garage queens.

Doesn't the boxster use an 01e transmission in a midengine setup?  I don't see why it can't be used if it does. 

I was thinking about the Boxster when I opened this. The 2.7 used an Audi trans if I remember right. 
RealOem might be your friend here. You might be able to cross reference some part numbers and find out more about Audi trans internals or even the Boxster trans. 

Ok guys, I emailed Torsen directly to get to the bottom of this and, surprisingly, got an email back from an engineer who works there. 

His explanation is very similar to the points you guys are converging on. He explained that Audi did use the same transmissions between FWD and AWD cars, but the FWD versions had the center diff replaced with a fitting that linked the diff input to the front output shaft. He also said that in my situation, the center diff would need to be permanently locked to make this work. Simply disconnecting the rear prop shaft would let it free spin and no torque would go anywhere. However, locking the rear prop shaft would cause the front drive system to spin at 2x the speed it was designed to spin at. I assume this results in half the torque which is not in my best interest haha.