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Summary:
The reasoning behind installing an overdrive transmission is running a low rear gearing to get off the line, but still retain manageable RPMs at freeway cruising speeds. This seems like the best of both worlds, but this article will examine some reasons why an overdrive transmission is not always be the best solution.

Traditionally GM overdrive transmissions such as the 700-r4 and the 200-4r were viewed as anemic pieces for gas sipping transportation appliances. The last several years have revealed that these transmissions can be modified to handle significantly more power then in base configurations. A well-built and properly adjusted overdrive transmission should be able to handle around 450 ft/lbs of torque. Is that enough for the engine combinations that people run in their hot rods? In many cases the answer is no and I for one, have been through enough “built” 700-r4s for a lifetime.

So what then is the solution?
The solution is the tried and true TH-400, more specifically the variable-pitch version of the TH-400.

The variable-pitch, or switch-pitch TH-400 is nothing new as it has been around since 1965. Originally found in Buick, Olds, and Cadillac the switch-pitch offers two stall speeds, a low stall and a high stall. Applying a positive 12-volt signal to the appropriate terminal on the transmission case makes the selection of low or high stall. The high stall gives you the advantages of torque multiplication to get off the line with a taller rearend gear and the low stall decreases slippage for cruising or on the big end of the track.

The variable-pitch stator is not the only advantage that the TH-400 holds over its more modern overdrive counterparts. A TH-400 will generally cost less then an overdrive and it can be built to handle some serious horsepower numbers. The TH-700 and TH-200 also require a T.V. cable to be installed and adjusted correctly or serious damage and shifting problems will result, and getting it correct is often harder then it seems at first glance. Another advantage of the TH-400 is a small spread between gears. First lets put some gearing numbers out there for reference:

Gear Ratio Comparisons:

It is clear that the two overdrive transmissions in the comparison chart have lower first gear ratios as well as have the advantage of overdrive, but let us look at the numbers that are hidden between these ratios.

Between Gear Gap Comparisons:

Notice the large gaps between gear ratios. The TH-700 has a particularly nasty gap between 1st and 2nd. In fact if you combine the 1st-2nd and 2nd-3rd gaps on the TH-400 you get roughly the same gap as the 1st-2nd on the 700-r4. The result of this is that your first gear in the TH-700 may launch you like a rocket, but the huge gap is going to lug your engine and drop RPMs, hurting performance. The TH-200 has a more favorable gear separation then the TH-700, but the TH-400 still has a slight edge.

Now to address the flaw in my argument that I am sure many of you are thinking about. The TH-400 does not have an overdrive gear and if you don’t run an overdrive then how can you possibly expect to have a respectable freeway cruising RPM?

The solution to this is easy, run a taller rearend gear. I argue that the many hotrods rarely, if ever, see the drag strip and spend a lot more of their time on the road. So why then do we put in a low rearend gear that makes freeway driving unpleasant and limits out terminal velocity to 115 miles per hour. An overdrive transmission will make up for our low rear gearing, but this introduces several problems.

1. Overdrive was never intended to be an acceleration gear, which it is being forced to become if you are running a deep rearend.
2. High drive shaft speeds

Running a 4.11 gear with a TH-700 sounds like a good idea until you consider that with a 275/40R17 tire at 100 mph your drive shaft is spinning at somewhere in the neighborhood of 5383 RPM for comparison a TH-400 with the same tire and a 3.08 rearend gearing will turn 4034 RPM. The same comparison at 150 mph yields 8075 for the overdrive versus 6051 for the TH-400. Spinning the drive shaft that fast takes horsepower and can have other problems like magnifying the effects of driveline alignment problems, drive shaft imbalance, and driveline component wear.

With the right setup, a switch-pitch TH-400 provides a viable alternative to the modern overdrive.

Controlling the converter:
There is a multitude of possible methods for controlling when and how the converter changes from high to low stall. Use a delay box to have the converter go to low stall after you leave the starting line. An RPM switch could have the converter lock up once your car is into its power band. Wiring a relay to the brake switch would allow you to put the converter in high stall for stoplights in a car with a radical camshaft. A simple switch can be used to allow full driver control of the system. For most those who want to have the most control of their driving experience this probably the best option. [article on wiring a switch pitch transmission]

Where do I get my hands on a switch pitch TH-400?
I purchased the unit in my 1966 Malibu from PAE Enterprises in Texas. They offer both Chevy and BOP cases for a resonable price. PAE also sells conversion kits so those of you that already have a TH-400 are in luck. PAE’s contact information can be found in the sources section of this article.

Sources:
How to Work with and Modify the Turbo Hydra-Matic 400 Transmission
by Ron Sessions
ISBN: 0-87938-267-8

P.A.E Enterprises, Inc.
4401 Turf Rd. Bldg E
El Paso, TX. 79938
915.855.6009
915.857.4727 -> Tech Line 8-10am and 4-5pm MST M-F
www.paeenterprises.com