Rear transaxle conversion

[Spartarus]

I'm trying to kill a few birds with one stone here, but I'm less than 100% sure and want advice from the community.

Attached are images of an unbolted (upside down) FRS rear subframe and a Porsche Boxster S transaxle. I am wondering... It looks to me that cutting out the center box beam section where the diff bushings sit and welding on tubular mounts to fit the attach points of the porsche gearbox is feasible. It would require a torque tube to connect the transmission/transaxle to the engine.

This would allow the stock transmission, driveshaft, differential, and axles all to be replaced all at once (more on this below).

An older Boxster S or Cayman transmission can be had for about $3,000.00, and can handle significantly more torque than our stock transmission.

It would extend a short way past the subframe in the rear, but removing the stock muffler will leave a cavity with plenty of space.

Now, I have done plenty of welding and mechanical work but I have zero experience with torque tube cars. (I hope there's a Corvette guru around..) I know DSS and others can fabricate one... As I understand, the flywheel and clutch can stay put, and the tube will need custom bellhousings on both ends, the front accommodating the starter. Correct me if I'm wrong. The shifter will also need to come out and be replaced with a cable shifter...And I know almost nothing about cable shifters.

Objectives and benefit:

Gets rid of the fragile manual transmission in the stock car. (Also axles and driveshaft, although I haven't seen a lot of those breaking).
Improve balance (Working on exact numbers, but my car is on a barge for the next 8 weeks.)
Compatible with any eventual engine swap...
I'm betting on overall weight reduction, but I'll hold my tongue until I have numbers.

Concerns:

I've overlooked something and this will not work.
Blah blah blah polar moment of inertia...whatever.
Subframe or engine movement breaking the tube.
Starting a senseless argument about weight distribution (please god no)

More information needed, I'm working on it:
Porsche transmission weight, model Number, gear ratios.
Stock diff weight (have stock trans [115lb] and muffler [26lb] weight) Correct if wrong.
Any important sensors I haven't thought of? F*** the gear indicator, I don't care.

[Spartarus]

Here is the underbody illustrating the diff, subframe, and muffler placement.

I'll elaborate a little more on why I care about static balance, and why i'm shooting to get closer to 50/50. As you know, weight doesn't transfer nearly as well on slick surfaces, so static balance matters more for maximizing traction. I learned this lesson over and over and over in Alaska, in many different cars. I am moving out of Alaska, but I'd still like to build a car that's happy doing rally, dirt track, and ice racing. I know AWD is much better for those conditions, but it isn't nearly as FUN.

...And isn't that why this little car exists?

[Turdinator]

It seems like an expensive way to go about things but I can see some sdvantages to it too. In for results

EDIT: How would the trans compare height wise to the diff we have?

[Dipstik-sportech]

Stock diff weight is 95ish lbs. Won't the fuel tank be in the way of the bell housing for the transaxle?

[Crazy Drew]

Interesting idea and I'm curious if it pans out. TBH I'm not sure there's really that much to be gained by converting to a transaxle. The stock trans is light, I know it's a bit "fragile" but it does well in 99.9% of applications, as does the diff. I'm also a few beers in for the night so please forgive me but is removing 30-40lbs from near the center of rotation on the car really worth the expense?

As Dipstick said I'm sure the fuel tank will be in the way, as well as the trunk floor.

My main question (playing devils advocate here, I really want to see this done but need to be a realist) is, who is going to make the torque tube? You're talking about a pretty serious piece of equipment to manufacture by the time you consider the alignment and strength and stiffness concerns between engine and transaxle.

[cdrazic93]

Ohhhhh I like this thread.

[D K]

Another dreamer with a project that will never see the light of day

[KoolBRZ]

Am I the only one to notice that he'll have 1 forward speed and 5 in reverse? This transaxle is meant to be in front of the engine, not behind it. Unless it's from a front-engine Porsche, like the 944, or 928, in which case you'd already have a torque tube and custom bellhousings from the donor car.

[Dipstik-sportech]

The boxster/cayman are mid engined cars, the transaxle is behind the engine so it would be fine.

[Koa]

Originally Posted by KoolBRZ

Am I the only one to notice that he'll have 1 forward speed and 5 in reverse? This transaxle is meant to be in front of the engine, not behind it. Unless it's from a front-engine Porsche, like the 944, or 928, in which case you'd already have a torque tube and custom bellhousings from the donor car.

you're ridiculous koolsquad

[gramicci101]

Where would the clutch be? The bellhousing sticks up a lot, and that would interfere with the chassis. If the clutch is still on the back of the engine, you'll need a separate housing just for it, then a driveshaft feeding back to the transaxle. Which would need a lot of fabrication to knock the bellhousing down a bit.

[wheelhaus]

I'll be impressed if this actually makes it into modification steps, and blown away if it's made to work reliably. I'd love to see this pan out for the sole reason that it's unique and ambitious.

Couple questions... What's the purpose of a torque tube, why not use the driveshaft? Where would the clutch go, by the engine of trans? I guess it would be easier at the engine, but I'm not sue how well the driveshaft would balance at 7400 rpm. What speed does it spin on a stock drivetrain at 150mph?

[KoolBRZ]

If you bought a wrecked '85 on up 928 s4 or gts, you would not only have the transaxle, but the bellhousings, torque tube, clutch, and a light-weight aluminum V8 as well! That would make an FR-S/BRZ chassis really fly. It would sound great also.

[gtpvette]

Originally Posted by KoolBRZ

If you bought a wrecked '85 on up 928 s4 or gts, you would not only have the transaxle, but the bellhousings, torque tube, clutch, and a light-weight aluminum V8 as well! That would make an FR-S/BRZ chassis really fly. It would sound great also.

I'd say this or a C5 or C6 transaxle would work better. These boxes are made to be remote from the motor.