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Discussion Starter #1 (Edited)
I am beginning to seriously explore the possibility of building a rear-mid engine project car using an LS based motor and incorporating all wheel drive.

I joined your community because the GTM space frame appears to be the best place to start. I know the frame will require modification to accommodate the AWD. I am not worried about that yet because I have not had any luck finding information about a compatible AWD transaxle.

Hopefully a GTM builder has already adapted AWD to their car or I can at least learn from those that have considered this idea before me.

I will share some photos I have been using to plan the build.

The first photo shows how I believe the engine and transaxle (sourced from a rear engine Porsche if I am not mistaken) is typically placed in the GTM.


This photo shows how I believe the Porsche transaxle is situated in its original application.



I do have some unrelated questions here. How is the transaxle modified to work with the LS and to be positioned with it’s tail facing rearward in the GTM?

I have searched for information regarding OE AWD systems that could be sourced for this build with no real success.

The only factory, rear-mid engine AWD vehicles I can think of are the Lamborghini Murcielago / Gallardo and the Veyron.
The next photo illustrates how I believe AWD is achieved in both the Murcielago and the Veyron. Looks like it has an integrated transfer case sending power to an offset rear diff. via driveshaft running under the motor. Does anyone know if the Gallardo is set up differently?



I doubt using anything from a Lamborghini would be cost effective if even possible.

Next I considered using the AWD transaxle from the Porsche but have not been able to find any conclusive information about how it is laid out.
Obviously out of the question if it is situated how I imagined it below.

 

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Discussion Starter #2 (Edited)
CONTINUED.....

Hopefully someone here can tell me if either of the following two concepts can be achieved.

1. Does a transaxle with an integrated transfer case similar to this one exist?



2. Can a typical front engine AWD system be turned around?
(a) Is it possible to change the power split. The factory setup is sending (for example) 70% of the power from the tail shaft to the rear diff. and 30% through the transfer case to the front. In my case, I need the transfer case to provide 70% to the rear and the tail shaft (now facing forward) to provide 30% to the front.
(b) Also how does reversing the entire drive train effect the direction the wheels will turn in forward gears?



Thanks in advance.
 

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I am not the best one to answer but I will take a crack at it.

As you may or may not know, the Porcshe transaxle is inverted (upside down) and the engine is paced it its current location for CG and WD. The trans will not work in your "rear biased" rendering, as the output shafts would be set too far at an angle (if leaving the body unchanged) the frame structure also narrows the further you go toward the tail and of course the weight would be behind the rear wheels which would not be a good thing in the GTM. In the other rendering the cabin would also need to be modified to the point of incompatibility. The adapter that is used to mate the LS to the Porcshe is made by Kennedy Engineering it consists of a flywheel and custom aluminum adapter plate. Running any shafts under the engine is truly not an option as ground clearance is already an issue.

Lots and lots of uncharted waters lay ahead.
 

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This can be easily accomplished using off-the-shelf GM parts, but is only possible with an FWD automatic transmission like the 4T65e found in the Monte Carlo SS. There are no FWD manual transmissions strong enough in the GM lineup, though you could use a manual transmission from an Acura NSX with an adapter plate. You will need a good limited-slip differential.

You are basically using a FWD transmission in a longitudinal setup, using the axle outputs as your drive for front & rear driveshafts.

This is a proven concept in rock-crawling buggies.

Your front & rear driveshafts will feed appropriate front & rear axles.

Voila, instant AWD.
 

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John, I don't think your idea would work for a street car unless you can find a front and rear differential with 1:1 gearing. Otherwise you will end up with a street car with rock crawler gearing. Top speed of around 40 MPH.

You would be using a front wheel drive transmission that has pretty low gearing to begin with and lowering the gearing further with the differentials.
 

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Discussion Starter #6
the Porcshe transaxle is inverted (upside down)
That helps a lot. So to be clear, rotating the transmission corrects for the direction the axles would rotate had the transmission been simply faced the opposite direction?

The trans will not work in your "rear biased" rendering, as the output shafts would be set too far at an angle (if leaving the body unchanged) the frame structure also narrows the further you go toward the tail
If have no aversion to modifying the frame to accommodate AWD components.
Where I really need help is finding actual OEM drive train components that can be adapted to the rear-mid engine setup that will result in an AWD system.

and of course the weight would be behind the rear wheels which would not be a good thing in the GTM. Running any shafts under the engine is truly not an option as ground clearance is already an issue.
I rephrased my rear biased question intended to address power distribution not weight. I am only considering mid engine placement and running the rear shaft along side the oil pan.

Looking forward to your response
 

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Okay I will try this one too.

The concept of the AWD GTM has been brought up a couple of times, and yes it would be possible to do this. There are a couple of ways that I can think of to accomplish this. You would have to go to a rear engine platform similar to the Porsche 911 and then use a Porsche gearbox and drive train to move the power forward to the front wheels. This would be a fairly easy way of working through this issue, but would have it's own stability issues that would follow. Porsche has spent many lifetimes now trying to prefect the rear engine platform.

The other way to do this would be to find a Crashed Audi R8 and pull the gearbox and engine, and AWD system from that. Don't forget the electronics, too. Either way I think it the project would be rather cost prohibitive. I think that you are going to find that this is a very detailed and rather difficult project to take on with regard to the GTM. There will be any number of issues that will arise, but I would be happy to discuss the options with you in greater detail and try to come up with a solution for you if you would like. I would warn you ahead of time, it is not going to be cheap to do.

If I had to take a rough guess I would say easily $20-$25K, and it could be more.

I am sure that there are other ways of accomplishing this, but these are the ones that I can think of right out of the gate. Come to think of it. I can't really find any easy off the shelf solutions for this, other than getting a Lambo drivetrain or a Audi, either one would be very costly I think.

If you were open to making a rear engine based GTM I think a lot of the issues disappear and this becomes a bit easier to envision.

Erik Johnson
California Motorsports
(928) 855-5755
 

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That helps a lot. So to be clear, rotating the transmission corrects for the direction the axles would rotate had the transmission been simply faced the opposite direction?



If have no aversion to modifying the frame to accommodate AWD components.
Where I really need help is finding actual OEM drive train components that can be adapted to the rear-mid engine setup that will result in an AWD system.



I rephrased my rear biased question intended to address power distribution not weight. I am only considering mid engine placement and running the rear shaft along side the oil pan.

Looking forward to your response
As Eric stated and what I was thinking (R8 transplant) or as a long shot a AWD Toyota Previa All-Trac modified trans. It's motor was mounted mid-ships however it was only rated for 135hp but it was a portly thing. A small V8 or a low tq screamer would be the ticket.

Like this bad boy......

http://www.h1v8.com/page/page/4553428.htm

Pick your poison from 250hp to 455hp and it only weights 200lbs.
 

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The only practical way to do this is to mount the engine with the transmission pointing forward into the center tunnel. Then a transfer case of some sort needs to be used in front of the transmission to direct power to the front differential and backwards to an offset rear differential so that the driveshaft goes along side the engine rather than under it. This has been done for rear wheel drive by at least on Lambo replica builder. He used a marine V-drive to send the power back to the rear differential. If a transfer case was installed in place of the V-drive you could have AWD. The big question would be if all of this could be packaged within the design of the GTM chassis.
 

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Discussion Starter #10
I really appreciate everyone who took the time to offer some reassurance that several theories presented in my OP appear to be correct.

You would have to go to a rear engine platform similar to the Porsche 911 and then use a Porsche gearbox and drive train to move the power forward to the front wheels. This would be a fairly easy way of working through this issue, but would have it's own stability issues that would follow.
I am beginning to seriously explore the possibility of building a rear-mid engine project car using an LS based motor and incorporating all wheel drive.
Next I considered using the AWD transaxle from the Porsche but have not been able to find any conclusive information about how it is laid out.
Obviously out of the question if it is situated how I imagined it below.
The other way to do this would be to find a Crashed Audi R8 and pull the gearbox and engine, and AWD system from that.
Don't forget the electronics, too. Either way I think it the project would be rather cost prohibitive. I think that you are going to find that this is a very detailed and rather difficult project to take on with regard to the GTM.

If I had to take a rough guess I would say easily $20-$25K, and it could be more.
The only factory, rear-mid engine AWD vehicles I can think of are the Lamborghini Murcielago / Gallardo and the Veyron.
I doubt using anything from a Lamborghini would be cost effective if even possible.
I agree the R8 AWD system will be as difficult to find and as expensive to buy.

The only practical way to do this is to mount the engine with the transmission pointing forward into the center tunnel. Then a transfer case of some sort needs to be used in front of the transmission to direct power to the front differential and backwards to an offset rear differential so that the driveshaft goes along side the engine rather than under it.
So your vote is for the design illustrated in concept photo #2 ?



So now with some agreement that my OP was headed in the right direction, I would like to further explore the questions I asked in post #2. Nothing has been stated so far to change my mind that the two concepts illustrated there are my only viable options.

1. Does a transaxle with an integrated transfer case similar to this one exist?




2. Can a typical front engine AWD system be turned around?
(a) Is it possible to change the power split. The factory setup is sending (for example) 70% of the power from the tail shaft to the rear diff. and 30% through the transfer case to the front. In my case, I need the transfer case to provide 70% to the rear and the tail shaft (now facing forward) to provide 30% to the front.
(b) Also how does reversing the entire drive train effect the direction the wheels will turn in forward gears?

 

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So your really talking about a rear engined GTM

Ok, This become fairly easy now. Simply find your self a Porsche 993 C4 any year will be fine, that has a blown engine or flood damage or something like that and transplant the AWD system out of it. General wheel base should be about the same. You can then plug a V8 right against this system, with some work.

This would be the most easy way of doing this.


Erik Johnson
 

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Ok, This become fairly easy now. Simply find your self a Porsche 993 C4 any year will be fine, that has a blown engine or flood damage or something like that and transplant the AWD system out of it. General wheel base should be about the same. You can then plug a V8 right against this system, with some work.

This would be the most easy way of doing this.


Erik Johnson
The only thing I see wrong here is that your recommendation is for a REAR engine setup, if I am understanding correctly. Unless I am mistaken and the 993 has the engine IN FRONT of the rear axle, and then with the transaxle mounted in front of the engine? I think THAT is what the OP is considering here. :)
 

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The only thing I see wrong here is that your recommendation is for a REAR engine setup, if I am understanding correctly. Unless I am mistaken and the 993 has the engine IN FRONT of the rear axle, and then with the transaxle mounted in front of the engine? I think THAT is what the OP is considering here. :)
Also, since we aren't inverting or turning around the Porsche transaxle, won't we be turning it backwards by just putting a V-8 in place of the Porsche engine?
 

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Also, since we aren't inverting or turning around the Porsche transaxle, won't we be turning it backwards by just putting a V-8 in place of the Porsche engine?
I don't know, but I thought Porsche engines turned the same direction as most other engines...clockwise when viewed from the "front"? If they didn't then wouldn't it be a slam dunk to make a rear engined Porsche gearbox into a mid engined LS gearbox?
 

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To the OP, how much $ are you willing to spend in order to make this happen? You aren't looking at thousands, you're looking at tens of thousands. The engine and AWD system alone will be over $10K easy. Then there is the large amount of money you're going to spend on fabrication. In order to stay away from redesigning the body, you're going to be faced with some interesting geometry to make this happen. Either you flip the engine / transaxle around or you come up with a clever way to get a rear-mid engine set up to get power to the front. I don't think they make a U Turn transaxle. So, you'll go with flipping the engine around. It's the only sensible way to do this. But, to do that, you're going to have to fab some half axles if I'm thinking correctly. You're looking at some risky geometry in the rear once you flip things around. There is a lot of diagonal movement under power. There is up / down (Y axis) movement and then the power will be coming from a Z & X axis, as opposed to just a X axis. I don't know what you guys think, but to me, those rear axles will need to be properly designed and made of some very strong material or else they will snap.
 

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Okay, turn a V8 engine around from it normal position in the GTM and use a porsche C4 setup and you can effectively make a AWD rear engined GTM.

This does not help with the OP which was I believe a mid engined AWD, but does still allow for AWD in a GTM.

Erik Johnson
California Motorsports
 

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I guess the main issue would be fitment for the rear wheels. Not a lot of space to wiggle in axles. I don't think that you would be able to do this with the current framing, and stability would probably be a nightmare, but I guess if you wanted to redesign the car completely you could.

Erik Johnson
 

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I want to ask the OP what the puepose of his GTM would be. AWD is cool, but it's not the best for performance. When Lambo asked their test driver, who tested almost every lambo made, to design the best performance Lambo in his opinion, he started with demanding RWD. There is a good reason why there are no AWD Ferarris. The car handles better without it. AWD has its place in cars and is pretty cool, but it depends on what you want to do with it. AWD in a GTM will take a lot of engineering, the whole rear suspension will have to be redesigned.
 

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I don't know. The Subaru makes a pretty impressive point to the contrary that AWD just doesn't make sense. Do you remember the Audis in the late 80s early 90s on the race track? If AWD is allowed, like a turbo, it is pretty tough to beat. Of course, like the turbo, it is more complex and expensive, but when working right, they both are awesome things to witness. Computers have made things a lot more managable these days also.

Should we include handling in the rain or on a "dirty" surface, well, I just don't like my odds in a RWD GTM versus an AWD GTM. Drivers being equal, I think the AWD wins every time!

So I guess what I'm getting at is that I entirely understand the desire to have an AWD GTM, but once I get to looking at actually doing it, it just doesn't make a whole lot of sense. May as well just use the body and build an entirely new car underneath it! Only a FOOL would do that! ;)
 

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Crash, there are examples of good cars under AWD power on the track. The R8 and most Lambos will do pretty good. But these are also cars designed around that platform as well from the get go. I brought up the opinion of the Lambo test driver to make the point that not even the test driver likes it when compared to RWD in a similarly set up vehicle. AWD makes its mark in Subaru's because those cars are often off road and in rallies. Give them loose gravel and they will leave you in their dust.

But for a GTM, you would wreck any handling the car has. It wasn't designed for AWD, so you would have to come up with a frame / chassis that does use AWD correctly. If the OP is great with SolidWorks, have at it. Get the rough dimensions of the frame, suspension, drivetrain, etc. and start modeling. That is the only way IMO that someone could do AWD in a GTM and make it worth the trouble. That's a lot of engineering though. The kind that would give him the right to re-badge the GTM if he wanted to. Would be a fun project, but not a project for the light hearted. When the OE's do an AWD vehicle, they spend months, even years, to get the driving dynamics and the engineering to mesh they way they want it to. And that's before even a protype vehicle has been made, I'm talking about just the design stage.

Porsche uses a rear engine setup, by the way. It keeps things simple and a lot shorter. The best way to tackle this, to me, would be to use Porsche parts. But that gets pretty costly and he would still have to redesign the frame to use Porsche internals for the suspension and drive train. The engine would need to be placed as low as possible and slightly towards the front of the rear wheels to help the handling. The interior might have to be tweaked a bit as well to house part of the transaxle. The way I see it, it's that way or have the engine hanging behind the rear wheels.

Here is how Porsche manages to pull off AWD in the Panamera. This is the layout I'd go with, it gives a lot of possibilities with FFRs. Just think about a Cobra or Coupe with this set up. :evil:
 
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