Building a hot motor and dropping it into your muscle car isn’t the only way to add some punch. Upgrading the stock 3-speed slushbox to a 4-speed overdrive will add more than just higher top-end speed; the closer ratio lower gears, will add quicker acceleration and the lock-up feature of the torque convertor will increase fuel economy.
The GM 2004R and 700R4 overdrive transmissions are well known for their ability to handle reasonable amounts of torque and horsepower. The 700R4 is the most common and easiest to find, but the 2004R is actually stronger. While a 700R4 can be swapped into just about anything running a Chevy motor, the same cannot be said for a Buick, Olds or Pontiac. For these cars, particularly the A-bodied musclecars like the Skylark\GS, Cutlass\442, and LeMans\GTO models, the 2004R will bolt up. The BOP (Buick-Olds-Pontiac) bellhousing bolt pattern differs from the Chevy pattern, noted by the valley between the two upper mounts for BOP, where as the Chevy has a peak. 2004R cores are available in either BOP or dual-pattern mounts, allowing the tranny to mount to all Chevy motors as well as the BOP brands. The 2004r is commonly found in intermediate and fullsize cars from 1981 through 1990.
For our 1971 Buick GS convertible, we found a dual-pattern 2004R and rebuilt it using a Trans-go reprogrammer kit, Eagle red-oxide performance clutches, and a Sonnax 4th-gear billet servo. The transmission was built with the intent on putting as much as 500 horsepower in front of it. To transfer the power from the flexplate to the tranny is a TCI Breakaway lock-up convertor with an approximate 2500-rpm stall.
The install is pretty simple, in the GM A-body, the factory crossmember simply needs to be moved to the TH-400 location (about 4 inches back from the TH-350 location), the driveshaft should even be the right length. There are, however, a few items that make the operation of the transmission a little easier. The TV cable is perhaps the single most important aspect of an overdrive transmission. On both the 700R4 and 2004R, the TV (Throttle-Valve) cable takes the place of both the kickdown cable and vacuum modulator. The TV cable operates directly off of the carburetor and controls a solenoid inside the transmission. This solenoid controls when, and to what gear, the transmission downshifts under partial and full throttle. If this cable is not set up correctly, the tranny will never shift right, leaving you the not-so-good feeling of making a mistake buying an overdrive. The overdrive should shift from 4th to 3rd, 2nd and 1st, 3rd to 2nd and 1st and 2nd to 1st under specific conditions and should NEVER shift back and forth between 4th and 3rd.
The problem with these cables comes from the swap itself. When you swap an overdrive to another vehicle, chances are that you are not using the same motor with the correct carburetor or throttle body. When using any set up other than the stock original unit for the transmission, the TV cable geometry is altered. To correct this, we used a kit from Bowtie Overdrives. This very unique unit comes complete with the carburetor adapter for your carb, a mounting plate correctly placing the cable (which we needed desperately on our Buick 350), and the cable itself. The set up process is simple, but does require a pressure gauge be installed in the transmission to complete the set up.
The other issue with an overdrive transmission is the lock-up feature. Swapping the 2004R into the GS required some wiring. There are two ways to accomplish this task; wire the A-pin (top left pin on the 4-pin plug) to ignition hot, leaving the convertor locked up all the time or installing the $109 TCI lock-up kit like we did. The lock-up kit installs in about 30 minutes, and keeps the convertor from locking up until 4th gear, but allows you to add a switch to lock it up manually if needed, such as towing applications (NEVER tow with the tranny in overdrive, it is a sure-fire way to destroy the tranny in short order).
With the tranny buttoned up and all the peripherals installed, the tranny slid right into place. Check out the pics for the details.