installing a corvette c5 6-speed transaxle in a porsche ... · pdf fileinstalling a corvette...

928 Motorsports Transmission Swap Copyright 2014, 928 Motorsports, LLC All Rights Reserved

Installing a Corvette C5 6-speed transaxle in a Porsche 928 using the 928 Motorsports kit

© Copyright 2014 928 Motorsports, LLC. All rights reserved.

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Toll-Free Tech Hot Line:

877-FOR-928M 877-367-9286

Please do not copy this manual and give copies to your friends. Our ability to bring you this kit at this price relies on our customers coming to us for our knowledge and experience. Much of this information is hard fought and the product of multiple trials and errors. Please do not give any section of this manual to your friends, but rather, encourage them to contact 928 Motorsports, LLC for their own kit. Please understand that THIS IS AN ADVANCED KIT. Installation of this kit will require some modifications to the car, and the skill to carry out those modifications. Call us for Tech Support early and often as needed. If, after reading these instructions you feel you are over your head, consider having it installed by a professional mechanic or having us install it for you. We recommend that you sit down with this installation guide and read it from end to end be-fore beginning. Then you will have a better idea of what tools you will need and what other service or parts you may want to replace at the same time.

NOTE: “Left” and “Right” are used in this manual frequently. Left and Right are always as seen from the driver’s seat, as you sit in the car to operate it.

© Copyright 2014 928 Motorsports, LLC. All rights reserved

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Optional Items There are a number of options that you will have during this installation. • Transmission Mounts Our kit can be ordered with either polyurethane mounts or solid aluminum mounts.

Your kit has been shipped with the mounts you requested. Just let us know if it changes. • Driveshaft Couplers: Your driveshaft was assembled with the stock Corvette driveshaft couplers that

came on the driveshaft you provided us. If they were obviously bad or broken, we would have contacted you about it already. However, there are upgraded solid driveshaft couplers available from us for high HP applications—contact us if you want to discuss this option now before installing the torque tube.

• Shifter and shift knob: We do not provide a shifter. You can use the stock Corvette shifter and shift knob,

or you can use any of a number of after-market shifter designed for the C5 Corvette. We have access to a good short-throw shifter here, contact us if you would like to discuss it.

Fitment Notes • Exhaust: Some 928 exhaust systems will require modification to clear the rear of the new differential if

they currently cross over from the RS to the LS behind the differential. Expect this. • Fluids: Your transaxle and differential were likely shipped dry to you from the junkyard. Do check them

to see if they have fluid or not before installing. It is much easier to put the fluid into the trans and dif-ferential while it is on the floor than after it is installed.

• Speedometer: If the Corvette transmission is going in to a 1991 or later 928, your speedometer (and

odometer) will continue to work fine. However, if your 928 was built before 1991, then you will need a GPS pulse generator to make the speedometer work again with this transmission. Contact us for details if you need.


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Phase 1: Removal of the Stock Drivetrain Disconnect the battery at the battery. Remove the air filter box (upper and lower halves) completely. On cars equipped with belt-driven fans, you need to remove the upper fan shroud. This will allow the engine to rock down at the rear without breaking the fan shroud or the fan. Remove the entire exhaust, disconnecting at the exhaust manifolds or the headers (if so equipped). The exhaust manifolds can stay. Remove all the exhaust heat shields, and the mounts in the center of the torque tube tunnel that they bolt to. Some like to photograph these parts before they remove them. That can make re-assembly easier later. Remove the clutch slave cylinder from the lower bell housing. Remove the lower bell housing. What to do with the starter: on 1982-and-older models, the starter is mounted directly to the lower bell housing, and can remain attached to the lower bell housing. Just swing the lower bell housing to the side and the starter goes with it. On 1983-95 models, the starter is mounted to a steel bracket. The starter can stay there, but you may want to loosen the mounting bolts temporarily to make working in the bell housing easier. Note that the 928 bell housing stays on the motor. Clutch: if your 928 has a good clutch and you do not want to do service to it at this time, leave it alone. It is not necessary to remove the clutch to install this kit. If, however, you want to take this opportunity to upgrade or replace your clutch, remove the clutch now. Clutch Sliding Sleeve: remove the Allen-socket bolts from the sliding clutch sleeve and slide the sleeve backwards onto the driveshaft. This will disconnect the driveshaft from the engine. Remove the shifter knob, boot and the shifter. Remove the rear tires, and remove the nut from the center of the rear hubs while an associ-ate holds the hub still by pressing on the brake. Be prepared—the stock torque on this nut is 322 ft lbs! Then, while your associate continues to hold the brake pedal firm, break torque and loosen (but do not remove yet) each of the axle-to-transmission flange bolts (6 per side). Disconnect the hand brake cable inside the car at the handle, and pull the cable out from un-der the car. The hand brake cables stay attached to the rear crossmember at this time.


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Unbolt the rear calipers. Hang the rear calipers up with bent wire so they do not hang from the flexible brake lines. The flexible brake lines do no have to be disconnected.

Disconnect the rear sway bar by unbolting the drop link from the lower trailing arm. Mark the location of the rear toe eccentrics with a paint pen or Sharpie. Then remove the rear toe eccentric bolts (1978-1982 cars). On 1983-1995 cars the toe link is mounted into an aluminum sub-frame, which you can easily remove with three fasteners on each side that hold the toe bracket to the body. (See picture, page 29) Unplug the reverse switch wiring harness, and unplug the wheel sensors if the 928 is equipped with ABS. If the car is 1990 or newer, it should be equipped with Porsche Limited Slip Differ-ential. Disconnect the hydraulic line for the PSD at the coupler between the hard line and the flexible line. From inside the rear hatch, remove the 3 nuts per side that hold the shock tops to the body.

Unbolt the torque tube from the bell housing (4 bolts inside the rear of the bell housing). The bell housing stays on the engine. Unbolt the rear crossmember from the car. (2 bolts in each fender well and 1 bolt on each side of the transaxle that hold the crossmember to the body). Do not unbolt the transaxle from the cross-member.

Lower the entire rear suspension, transmission, and torque tube out from the car as a single unit as shown here.


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Phase 2: Assembly of the New Transaxle and Crossmember Your C5 ZO6 transaxle needs to be fitted with our special axle mounting flanges. Install the custom axle flanges provided to your differential now.


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Attach the 928MS shifter linkage to the shift rail coming out of the front of your transaxle now. There is a small compression pin that gets tapped through the hole in the shifter rail to hold it in place. Then slide the torque tube onto your transaxle, with the shift linkage positioned out through the hole provided in the top of the rear bell housing for it. As the bell housing gets close to the transmission, slide the forward end of the shifter rail onto the shift box at the same time.

Now bolt the bell housing to the transaxle. You may or may not have been given the fasteners by your junk-yard when you bought the used parts. They are M10 x 1.5 tp x 40mm long, and you need 8 of them.

If you do not find flanged-headed fasteners that size, use regular metric bolts and place washers un-der the heads. Torque to 37 lb/ft.


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Place the shift linkage bolt into the end of the shifter link-age at the shifter box now and be prepared to tighten it.

Mount the shifter atop of the shifter box and place the transmission in neutral. Adjust the shifter stalk so that it is nearly straight up and centered side-to-side, then tighten the shift linkage bolt. Torque to 22 lb/ft. Once the shifter is adjusted (and there is not a lot of range provided by Chevrolet for adjustment) then remove the shifter so it is not in the way when we lift the torque tube into the car.


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Place your new transaxle and torque tube up on jack-stands as shown in this picture. Two jack-stands are lo-cated under the axle flanges, and one under the torque tube.

The stock Tremec T56 transaxle has to be fitted with 4 transmission mounting tabs to attach to our rear cross-member. Each of the 4 mounting tabs is named, and located as shown in this picture. Note we are looking down at your new transaxle from the top.


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Mounting tab ZO6 LTM1 (REFER TO PICTURE, Page 9) 1 Custom aluminum bracket 1 M10 x 50mm bolt 1 M7 x 35mm bolt 1 M7 nut

Remove the stock M10 transaxle fas-tener shown here and place aluminum mount LTM1 in location and attach it with the longer one provided. Do not drill the hole for the lower M7 bolt at this time. We will come back to that.


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Mounting tab ZO6 LTM2 (REFER TO PICTURE, Page 9) 1 Custom aluminum bracket 1 M10 x 50mm bolt 1 M7 x 35mm bolt 1 M7 nut Several spacers Remove the stock M10 transaxle fastener shown here and place aluminum mount LTM2 in location and attach it with the longer one provided. The spacers gets placed between the mount and the casting in the recess as shown. Stack spacers until you get the right amount to fill the recess in your casting. Do not drill the hole for the lower M7 bolt at this time. We will come back to that.


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Finishing Mounting tabs ZO6 LTM1 and LTM2 1 1/2” X 3.5” long flanged head bolt 1 1/2” flanged Nylock nut Slide the large bolt through the two mounts you have now installed, and adjust the mounts up and down until the bolt is horizon-tal. Then mark the transmission for drilling by us-ing the second hole in each mount as a guide. Make sure the hole you have marked to drill is centered in the casting tab before drilling.

Then drill the transmission for the 7mm bolts provided, install the bolts and fasten with nuts on the other side as shown.


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Mounting tab ZO6 RTM1 (REFER TO PICTURE, Page 9) 1 Custom aluminum bracket 1 M10 x 50mm bolt 1 M7 x 35mm bolt 1 M7 nut Remove the stock M10 transaxle fastener shown here and place aluminum mount RTM1 in location and attach it with the longer one provided. Do not drill the hole for the lower M7 bolt at this time. We will come back to that.


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Mounting tab ZO6 RTM2 and 3 (REFER TO PICTURE, Page 9) 1 Custom aluminum bracket 1 Custom steel twisted mount Several spacers Remove the stock M10 NUT shown here, place the spacers provided over the stud, and then aluminum mount RTM1 in location and attach it by re-using the nut you removed. Stack spacers until you get the right amount to fill the recess in your casting.

Also remove one M6 bolt from the differential flange where shown and install mount RTM3 underneath it and then re-install the fastener.


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Finishing Mounting tabs ZO6 RTM1, RTM2, and RTM3 1 1/2” X 3.5” long flanged head bolt 1 1/2” flanged Nylock nut Slide the large bolt through the two mounts you have now installed, AND ALSO THROUGH RTM3 twisted bracket as shown. Adjust the mounts up and down until the bolt is horizontal. Then mark the transmission for drilling by us-ing the second hole in RTM1 as a guide. Make sure the hole you have marked to drill is cen-tered in the casting tab before drilling.

Drill the transmission for the 7mm bolt pro-vided, install the bolt and fasten with the nut on the other side as shown.


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When viewed from the top, your transaxle should now look like this:

Putting your rear cross-member together Your custom rear cross-member was shipped in 3 sections. Using the four M14 x 35mm bolts provided, bolt it together like the pictures on this and the next page. It can only put together one way, so do not worry that you could get it wrong. You can’t.


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We recommend that you leave the four large bolts a little loose at this time. This will help with installation. After the crossmem-ber is in the car, you can tighten them up all the way.


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Test-fit the cross-member to the transaxle

Please test-fit the rear cross-member to your transaxle now. This is to confirm the fitment and further familiarize you with the location of everything before you put it in the car.

Make any adjustments to the location of the mounts now. Then you may put a final tighten on the fasteners you have installed in the mounting tabs on the previous pages.

Torque the M10 bolts the hold the aluminum mounting tabs to the trans to 37 lb/ft. Torque the M7 fasteners with nuts to 12 lb/ft Torque the M14 fasteners into the rear cross-member to 80 lb/ft.


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The Skip Shift Solenoid and the Reverse Lockout There are two solenoids on the 6-speed that you may want to understand now before you install the transaxle in your car. On the top of the transmission is a large square box with a solenoid attached—see item A in this picture. This is the reverse lock-out solenoid. It has to stay and you cannot remove it.

But, you can choose to wire it or not. The transmission has a default detent in it so that you cannot accidently grab reverse when you are shifting to 5th gear. You can simply leave the reverse lockout unwired, and you will have to “muscle it” over the detent and into re-verse when you want reverse. This is what I do, and it isn't too bad. It causes no harm to the transmission to do this. Or, If you want to electrically remove the detent when

reaching for reverse, most customers wire the solenoid to their brake light system so stepping on the brake pedal activates the solenoid, and they can get reverse without effort. On the side of the transmission (at location B in the picture) is the skip-shift solenoid. The de-fault position for this solenoid is non-invasive. That is: right now you can get all gears and none are locked out. That is a good thing. You can leave it as-is and unwired and install the transaxle. It isn’t in the way. Racers wishing to save a little weight may remove the skip-shift solenoid and replace it with a threaded plug.

Fluids Your used Corvette transaxle probably was drained before it was shipped to you. Check it now—it is a lot easier to install the fluid now then it will be after the trans is in the car. The spec for the transmission fluid is Dexron III and it needs 4.1 qts. The spec for the differential fluid is GL5 spec 75W90, and you need 2 qts with limited slip modifier.


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Transfer everything from old cross-member to new cross-member Refer to the pictures on the following pages and transfer everything that is on your old 928 rear crossmember and suspension to the new rear crossmember and suspension. NO NEED TO DISASSEMBLE YOUR SUSPENSION. If you look at these pictures, you will see we simply disconnected the upper “banana” link at 15 and 20 in this schematic and the lower Camber eccentric at 31 and 33. Then the suspension comes off as a complete unit. You may cut the hand brake tubing mounts to free the hand brake system from the rear cross-member as shown on page 23.


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The stock transaxle and rear suspension as it came out from the car (this one happens to be an automatic, but the attachment points are the same.

Here we have disconnected the suspension from the stock cross-member at the upper banana link and the camber eccentric bolts. The old transaxle and torque tube are set to the side…

...and the new transaxle and crossmember are lined up to be dropped onto the rear sus-pension.


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With a man on each end of the new cross-member, pick up the assembly and drop it into position on the old suspension.

Close-up of one side of the suspension con-nected to the new cross-member.

Put your new axles in place and snug up all the fasteners. Later, when the brakes are re-connected, we will use the brakes to hold the rotors still and tighten the axles and the axle flange nut.


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Thank you for your interest in our 928 Transaxle Conversion kit. This is the first half of the complete installation instructions that we provide. You will receive the complete installation instructions when we ship the kit to you. Here are some extra pictures of the finished installation for your reference.

(Optional exhaust system shown)


installing a corvette c5 6-speed transaxle in a porsche ... · pdf fileinstalling a corvette...

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