924Board.org

[jacobroufa]

Serious inspiration here. I love this build. Thanks for sharing, Britain!
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1980 Porsche 931
1981 Porsche 924 Weissach

[Carrera RSR]

Awesome build
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252

[Rasta Monsta]

Always wanted more power and more mechanical grip in my 924 race car. . .this beast has both, in spades.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[Britain]

Now that our first race of the season has completed, it is time to share the MAJOR changes to the car over the winter break. I wish I could change the title of this thread because this one is awesome!

After our most recent transmission failure in the racecar, we have now sacrificed ~15 G31's boxes for the cause. We could keep going down this route...or we can look for something better. There are several Porsche transaxles out there but they all bring along the Porsche "tax", they are not that much stronger, and they all put the gear stack behind the axle centerline. There are definitely some custom transaxles out there, but at a minimum they are ~$12K to over $25K. For our application, I am going to stuff a good'ol American box in the car...a '04 Z06 transaxle (T56) and torque tube. There are going to be numerous modifications to the chassis to make it fit.

First step...assemble everything to take measurements and weights. The corvette set-up is ~2" longer than the stock set-up which will be addressed by shortening up the torque tube. The total overall weight difference between the two set-ups is right at 80lbs.







Next, we need to make an adaptor between the Corvette torque tube and the Porsche 931 bellhousing. We 3D scanned the Corvette torque tube and 924 bellhousing and after the two parts were in CAD, I was able to design an adapter plate to interface the two and print it on the 3D printer for to verify the pilot bearing depth before it was sent off to the waterjet to get cut.

[Britain]

The test fitment confirmed the adapter thickness of 5/8" and this part was cut on the waterjet.





The next step was to keep cutting till you make it fit. Cut out the center section of the torsion tube and a bit of the rear floor and center tunnel. This is the approximate position to put the output flanges in the same place as the previous transmission. This gave me the appropriate length to shorten the torque tube.





To get the transmission in alignment to the rear wheel centerline we needed to shorten the torque tube and driveshaft by 3" total. We did this on a friend's huge lathe and were able to get the torque tube mounted up, shortened, and I welded it back up.

[Britain]

The driveshaft was sent off to Inland Empire Drivelines in California to be shortened, welded, and balanced...which is critical because this spins at engine speed. We also rebuilt the torque tube with new bearings and installed new couplers on the driveshaft (solid in the rear, rubber in the front)



Before the transmission could be mounted, we need to reinforce the inner swing-arm mount because half of it was cut away to make room for the giant transmission. I fabbed up a 1/8" plate on the waterjet and welded it on the end of the torsion tube...along with some addition plate underneath to tie it together. Now there is plenty of room to slide the transmission in-between.



Also added in two supports for the torsion tube to support the section that was cut out for the transmission.



And finally, a brace to go under the trans and tie it all together.

[Britain]

Due to the location of the fuel tank and other competing factors, we couldn't use the stock mounting locations on the trans. The Corvette transaxle is actually two isolated units...the T-56 transmission and the Diff housing, connected with a spacer/adapter thing. I took this part off the trans and 3D scanned it. I then designed two 1/4" steel plates that capture 4 large bolts on both sides of the trans to make a hanger and machined two pins/cup parts that allows the trans to slide over and then secured with a M10 bolt which isn't actually carrying any weight. Along with the two clocking pins on the trans bellhousing, this allows for fairly easy installation.

[Britain]

Had to redo just about all of the exhaust to clear the bell housing adapter in the front and the transmission in the back. One benefit to this setup will be the ability to pull the trans without having to remove the exhaust.







My plan for the interior of the car was to have the torque tube exposed and seal up the floor underneath.







This is the completed front tunnel with a removable top section. The Corvette torque tube will be sealed where it passes thru the firewall here and then run thru the cabin. This is done in steel for safety and heat management and will be lined with insulation.

[Britain]

We had to cut off the shifter tower from the side of the torque tube and relocate it to the center. This required machining some shifter stands and weld them in place. We then were able to fabricate the shift rod to fit the new placement and length of the torque tube.





And here is the final interior shot with the panel completed.

[Britain]

Couple of more details of the project.

We initially made welded output flanges to adapt the Corvette transmission to Porsche CVs/axles by taking the Corvette output flanges and cut/machined them down to only the splined portion. We then took a pair of Porsche 924/944 and cut off only the flange section and weld everything together.





The above welded output flanges were used to get everything setup on the car...but the last thing we wanted was for one of those to fail and cause the CV to wrap around and throw the axle thru the fuel tank...that would be bad. So, in the background I made a pair of one-piece flanges with the hope we would have them done before the first outing. Started with getting the rough machined, then took them to get splined, then another place for hardening, and the finally to get the them surface ground to run in the bearing/seal surface.



This set-up allowed the use of standard off the shelf Sway-A-Way bars to fit 944 CVs.

[Britain]

To get the gearing close to what we had previously, we swapped the R&P to a 4.10 ratio.





I went thru two iterations of rear sway-bar designs to get what I wanted, but in the end I am very happy with how it turned out. All of the parts of from Tarrett Engineering...944 length rear bar, 911 front arms, 968 pillow blocks, and my own custom drop-links. This should be equivalent to what we had before in terms of spring-rate, but also offer a bit more adjustment.



And here is the final set-up.



Here is the last and final piece of the puzzle...the dual stage cooling pump for the trans/diff. I ordered this kit for Black Friday and it took some begging from the manufacture, a couple of overnight packaged, and a rush order for Summit to get it all here in time. It consists of a billet rear cover, a really cool dual stage pump, a plate that mounts on the pinion nut to drive the pump, some custom adaptors, and a $hit ton of AN fittings.

[Britain]

To handle cooling duties of the Corvette trans, we kept the Porsche 964 oil cooler mounted in the back for the car the previous setup...but with some modifications. The Corvette uses different and separate fluids for the trans/diff and therefore we needed two coolers. Instead of getting two new coolers and having to modify all of the mounting points and associated sheet metal...I decided to make this cooler into two separate passages. A bit of work and several dunks on the water later, we now have a dual cooler setup ready to go in the car.

[Britain]

Here are a couple of videos of the completed car:

First Burn-out:
https://youtu.be/ene_FbGgEQc

Inside the car on the dyno:
https://youtu.be/ciMIV0Br8hA

Outside the car on the dyno:
https://youtu.be/Ab-smi1t1JM

[morghen]

The craziest build out there by far, and the coolest...probably the fastest as well. Thanks for sharing again!
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Supercharger and EFI kits
https://www.the924.com

[madislandguy]

Nice reversal of the usual Corvette bell housing adapted to Porsche torque tube that everybody else does.