Easy Overdrive

The password for cool '90s street performance is...overdrive. Late-model automatic and manual overdrive transmissions are quickly becoming the sweethearts of the street set. Not that long ago, a fast street car meant compromises, including deep gears, high-rpm highway cruising, and poor mileage. But the high-tech '80s and '90s have brought us more than just electronic fuel injection. They also delivered durable automatic overdrive transmissions.

While the TH700-R4 has garnered most of the attention as the performance automatic overdrive of choice, companies such as TCI and Art Carr consider the TH200-4R every bit the 700's equal in terms of torque capacity and durability. There are several advantages to the TH200-4R: It has a better First-gear ratio, it's generally the least expensive overdrive automatic, and best of all, it's virtually a bolt-in replacement for Powerglide/TH350 transmissions.

With all these TH200-4R benefits, we thought it deserved a closer look. What this story will concentrate on is what it takes to install a TH200-4R in a Chevelle, Camaro, or Nova.

Bolt It In

One of the biggest advantages of owning a Chevy is the Bow Tie brigade's amazing parts interchangeability. This is no more evident than with the TH200-4R. This transmission is the same basic overall length as the Powerglide or Turbo 350 automatics, uses standard flexplates, and uses the same 27-spline slip-yoke as a 'Glide or a TH350. The only thing that isn't the same in terms of just bolting the transmission in the car is the rear transmission mount. The trans-mount location is roughly 6 inches farther rearward than for a TH350 using the same-style trans mount.

There are a couple of ways to accomplish locating the rear crossmember. One solution is to use a Turbo 400 crossmember, which comes very close to bolting up. One trick that seems to work is to take a typical Powerglide/TH350 crossmember, cut the mount off, and reweld it on the trailing side of the crossmember. If you would rather buy a new one, Classic Industries offers these crossmembers new. An inexpensive alternative for Chevelle swappers is to merely relocate the stock crossmember the required distance rearward and drill new holes in the frame.

Once the trans is in place and the driveshaft is installed, there are still a few details to overcome. Cooler lines might need to be changed, but the fitting location is similar to the TH350's. The shifter is also easy. If you're using the stock Camaro or Chevelle floor shifter, The Shiftworks sells a kit to convert a stock '67-'69 floor shifter to operate either a three- or four-speed automatic. These kits are also available through Classic Industries and Original Parts Group. Of course, aftermarket four-speed shifters are available through B&M and Hurst in several different styles.

The 200-4R and 700-R4 overdrives use something called a throttle valve (TV) cable. This is important because while a Turbo 350 or a 'Glide can be operated without the kickdown cable, the overdrive transmissions must have the TV cable properly connected and adjusted. Vacuum modulator valves use engine vacuum as an indicator of load. As engine load increases and vacuum drops, the modulator valve increases pressure to the transmission. The TV cable does this job, basing load on throttle position, which is why adjusting the TV cable is so important. If the cable is not adjusted properly, a combination of high load with low trans pressure will destroy clutches in an extremely short period of time.

Converter Talk

Once you've decided to bolt in a 200-4R, you have a bunch of options as to the level of modifications to the transmission. Even if you elect to keep the transmission stock, a basic requirement for a swap into an early Chevelle, Camaro, or Nova is to use TCI's Universal Lockup Wiring Kit. This kit retrofits both 200-4R and 700-R4 automatics to engage the lockup converter in Fourth gear while disengaging the lockup under acceleration of downshifting. The kit also provides a manual override option. Of course, this is only necessary if you intend to use a lockup torque converter.

Art Carr's approach to both overdrive automatics is to use tight, 8- and 10-inch nonlockup converters for high-performance applications. TCI also offers high-stall, nonlockup converters for both overdrives. These may be the best choice for quick times down the dragstrip, but for a car that is driven primarily on the street, TCI recommends using a lockup converter. In fact, TCI offers a Super Streetfighter lockup series converter with 2,600-4,000 stall that can lock up under cruise conditions. This converter is intended for hot street cars with a big cam and deep gears. For a milder street driver, TCI offers converters with tighter stall speeds as well.

Gear Ratios

The real reason for choosing a 200-4R is for the overdrive. The "It's All in the Ratios" chart lists all the different ratios for the various Chevy automatic transmissions from the 'Glide to the TH700-R4. As the chart illustrates, the TH200-4R uses a 2.74:1 First-gear ratio which is deeper than a TH350's ratio but not as deep as the TH700-R4's 3.06:1 ratio. While the TH200-4R doesn't fit into the "more is better" theory of most street tire-burners, our experience is that the TH200-4R's engine rpm drop between First and Second is less severe, which helps the car accelerate after the gear change. There are many enthusiasts who don't understand this, but if you think in terms of total rpm drop between First- to Second-, and Second- to Third-gear changes, it will make sense. This is why the TH350s and TH400s work so well since the ratios are closer together.

More importantly, the overdrive ratios between the 200-4R (0.67) and the 700-R4 (0.70) are virtually identical. Using a 3.55 rear gear as an example, the TH200-4R in Overdrive will take that 3.55 gear down to an effective 2.38:1! While that not only reduces engine speed, it also drastically improves engine wear since the engine is now turning 33 percent fewer revolutions to travel the same distance. That's why overdrive transmissions are so cool.

In Overdrive

If finding a used trans is difficult, fresh performance transmissions are available from both TCI and Art Carr. The good news is that since the 200-4R is still rather overlooked, you might get a great deal on one. But don't be intimidated by the swap, it really is easy overdrive.