Trans-Fusion

Want To Install An Automatic Overdrive? We Have What You Need.

Mike Petralia Sep 1, 1999 0 Comment(s)

Step By Step

Overall case length can make the difference between an easy swap and eternal damnation. The cases pictured left to right are TH400, 200-R4, TH350, and 700-R4 equipped with the police car tailshaft housing. Notice that the TH400, 200-4R, and TH350 are all equal in length and that the mounting bolt holes in the TH400 and 200-4R line up.

This shows the mounting bolt hole locations that are distinct to the TH350 and 700-R4. A TH350’s crossmember must be moved approximately 1 inch rearward when installing a 700-R4. Installing a 700-R4 equipped with the police car tailshaft housing shown makes it almost a direct bolt-in for a TH400-equipped car.

In the foreground is the Corvette tailshaft housing, which can make installation of a 700-R4 a nightmare. If your transmission core came from a Corvette and is equipped with this type of tailshaft housing, discard it and find the police car unit that is on top of the trans in this photo.

The shape of the oil pan is always a quick and easy way to identify a trans. The 200-4R (left) has its own unique shape that is somewhat similar to a TH400, while the 700-R4’s pan is completely square, making it look similar to a TH350.

There has been a recent influx of aftermarket performance electronics upgrades geared toward improving the shifting and lockup qualities of the 700-R4 transmission. JET Performance Products offers this 700-R4 Fourth Gear Lockup Kit to allow installation of a lockup converter in non-computer-equipped cars.

When viewed from the top, the 200-4R (left) and 700-R4 look quite different.

The 700-R4 input drum (left) is much larger and made from thicker material than the 200-4R’s. This is one of the areas where the 700-R4 excels.

The two sprag units shown side-by-side clearly illustrate their size differences between them. The larger 700-R4 sprag (left) is stronger than the 200-4R’s.

The main weakness in a 700-R4 is its Third-to-Fourth clutch pack. The factory unit can hold only six discs, while the Altco Performance clutch pack shown can hold up to nine discs.

Servos are another spot for improvement. Pictured are the three servos GM built for the 700-R4. On the left is the Corvette servo, which, despite its small appearance, is much stronger than its bigger brothers.

Under or over? Long or short? How would you like your trans? GM has been building automatic overdrive transmissions for almost two decades, and you’ve probably already thought about putting one in your classic musclecar. Some say that the early overdrives—the 200-4R and 700-R4—are weak and can’t survive behind a performance engine. We’ve found that’s not the case, and while both of these transmissions do exhibit minor weaknesses, they are capable performers and can live comfortably behind most street engines, making as much as 500 hp.

We researched the differences between the two and will outline them here so you can choose the one that best suits your needs.

Identification Please

GM began installing overdrive automatics in the early ’80s, and, much to the dismay of the hard- core racing public, the first overdrives were pitifully weak. GM has since improved on the four-speed automatic overdrive’s durability—so much that it has made three-speed automatics obsolete. Both the 700-R4 and 200-4R were available in many different GM passenger cars and trucks until the mid-’90s, and it’s easy to distinguish between the two at a glance. If you’re searching for that perfect overdrive core, the pan is the quickest way to differentiate them (see photos). If you need another identifier, all 700s feature a removable tail housing, while the 200-4Rs are a complete cast unit.

Installing The 700-R4

While many stories have been written about installing a 700-R4, most fail to mention that there are both easy and difficult ways to make modifications. There are also alternate methods available to eliminate some of these moves entirely.

Installing a 700-R4 involves moving or modifying the factory transmission crossmember. Fortunately, several different tailshaft housings were made for the 700-R4, and the one to look for if swapping the 700-R4 into a previously TH400-equipped car was stock on mid-’80s police cars (Caprices) and the early Impala SS. This tailhousing features mounting bolt holes that make bolting in the 700-R4 much easier, requiring only slight elongation of the factory transmission mounting holes in the stock crossmember. The driveshaft must also be shortened by approximately 3-¾ inches. A new driveshaft yoke may be required if you’re replacing a TH400 transmission, but the 200-4R, 700-R4, and TH350 all share a common yoke, so replacing it won’t be necessary if you’re swapping out a TH350. But when replacing a TH350 trans, you must either relocate the frame’s crossmember mounting holes to the rear approximately 1 inch or, if your car ever came originally equipped with a TH400, switch to its factory TH400 crossmember and elongate the holes.

A TH350 tailshaft housing can be installed on the 700-R4, which would shorten its overall case length from 30¾ inches to 29½ inches. However, you would still need to move or change the factory crossmember for this installation, as the TH350 tailshaft housing does not feature any mounting holes so the original mounting holes in the 700-R4’s case must be used. The 700-R4’s output shaft would need to be shortened accordingly and a different speedometer gear would also be required.

Corvette 700-R4s use an entirely different case with no cast-in mounting bolt holes and a unique tailshaft housing. For this reason, the Corvette transmissions should be avoided. Its U-shaped mounting bracket uses two long bolts to secure the back of the trans, and, while it is possible to fabricate a crossmember to work with this tailshaft housing, it’s better to replace it with a more common passenger or police car housing.

Installing The 200-4R

Installing a 200-4R is an easy swap because its overall case length is nearly identical to TH350s and TH400s, meaning a shortened driveshaft is not necessary. The driveshaft yoke will not require changing when swapping a 200-4R for a TH350, but it will need to be changed if you’re swapping out a TH400. Fortunately for TH400 equipped cars, the 200-4R’s mounting bolt holes are in exactly the same position as the TH400s, making it a direct bolt-in to the factory crossmember. The 200-4R is lighter and a bit more compact than the 700-R4, so it’s a clear choice for tight confines under a small musclecar.

Strengths And Weaknesses

If you are searching for a 700-R4 transmission core to rebuild and install in your car, the best cores to look for are from ’88-’93. The first 700-R4s produced in 1982 were equipped with a weak 26-element sprag, an essential item that directly affects transmission operation; all 700-R4s were upgraded to a stronger 28-element sprag in 1988.

The main weak spot in all 700-R4s—even those built after 1988—has always been the factory Third-to-Fourth clutch pack. The factory pack contains only six clutch discs but, fortunately for us high-power hot rodders, a company called Altco offers an upgraded clutch pack that can go up to nine discs. Altco Products are available at most major transmission shops. By increasing the number of discs, slippage during shifts is decreased, creating less heat and extending the life of the clutches and trans. Due to the 700-R4’s heavier internal mass, it weighs more overall than the smaller 200-4R and takes more horsepower to turn.

The 200-4R has long been thought of as the performance underdog, although it’s not entirely deserving of such a title. All 200-4Rs feature a smaller input shaft and planetary gears than the 700-R4. The weak link in a stock 200-4R is definitely the small factory input drum. Both ACT Performance Products and TCI offer improved input drums that increase the transmission’s power-handling potential. While most 200-4R transmissions feature a dual bolt pattern bellhousing and will bolt behind any GM V-8, as well as some V-6s, there is one particular 200-4R that features a single POB (Pontiac, Olds, Buick) bolt pattern that won’t work behind a Chevy. They are easy to spot because they have only six bolt holes, while the dual-pattern bellhousings have 12. It’s relatively easy to swap in a 200-4R, and due to the fact that they have been overlooked as a performance transmission and cost less because of that, the 200-4R wins the sleeper’s choice award for automatic overdrives.

Lockup Converters

Let us make one thing perfectly clear: If this is a street car, run a lockup converter. The lockup mechanism is a simple device used to physically lock the crankshaft to the transmission’s output shaft in Fourth gear. By doing so, you improve economy and extend the life of the transmission. There are circumstances, however, when you’d want the converter to unlock to allow for slippage in Fourth gear. Several methods can be employed to wire a switch for the lockup converter, all of which involve connecting a 12-volt ignition-keyed source to the wire on the transmission. A switch, either vacuum or mechanically operated, turns the lockup feature on and off. While there isn’t enough room in this story to go into much lockup converter operation detail, just know you should run one in any street car.

When swapping in an overdrive automatic, a new torque converter will be necessary. The 700-R4 and 200-4R both use a different input shaft, so their torque converters are not interchangeable. The input shaft diameters of the TH350 and TH400 transmissions are different from the 700-R4 and 200-4R as well, meaning their converters will not fit either. Converter manufacturers such as Precision Industries offer stock and high-stall lockup and nonlockup–style converters for street or race engines using automatic overdrive transmissions. Either way, installing an overdrive transmission in your early musclecar will benefit your daily commute as well as your weekend play. Swaps such as these may not be considered budget by most people, but after it’s done, your cost-to-drive budget will decrease and your fun-factor will probably double.

COMMENTS

TO TOP