It's been a long time since the T10, one of the racer's best options for a four-speed manual transmission, was plentiful in the junkyards. Today, your best bet is to go through a transmission specialist. That, of course, costs money and is a big reason several budget-oriented classes are now allowing-or even requiring-an automatic transmission.

That's all well and good, especially if everyone is racing the same equipment. But if you are serious about racing a car with a slush box, a new torque converter is one of the easiest, and most beneficial, upgrades you can make.

What Is It?For automatic transmissions, the torque converter performs the same role as a clutch in a manual tranny. But unlike a clutch, which forms a solid, mechanical connection from the crankshaft to the transmission, a torque converter transmits energy from the engine through a fluid coupling. This allows the torque converter to "decouple," or essentially go into neutral below a certain rpm level (set by the manufacturer).

The important thing to understand is that every torque converter has a stall speed, which is the rpm level at which the torque converter locks up, or transmits full power to the transmission. In other words, the converter is like a fully engaged clutch. If the driveline or tranny is locked behind the converter (either by using brakes or engaging first gear and reverse at once in the transmission), and the engine is given wide-open throttle, the engine will only be able to rev up to the converter's stall speed.

Converters are rated to stall within a certain rpm range. Manufacturers cannot tell you that a specific converter will lock up at a certain rpm because there are many variables that affect stall speed. Horsepower, the torque curve, and other factors will affect where a converter will stall in your race car. Normally, engines with loads of torque at low rpm levels will stall at a higher rpm level with the same converter than an engine that hits maximum torque higher in the rpm range.

On The TrackDo not assume that all performance torque converters are alike. Now that you know the rules of stall, you know why. Automatic transmissions are popular among bracket drag racers because of their consistency. Drag racers, however, have the luxury of revving the engine into its powerband at the starting line and keeping it there for the next quarter mile. For this reason, torque converters built for drag racers normally have a very high stall speed.

This won't work for circle track racing because the rpm levels exiting the turns are too low. You need a torque converter built for your needs.

"In a circle track application, when you get on the throttle getting out of a turn, you don't want to have to wait for the converter to slip to its stall speed," explains Scott Miller, head of TCI's oval track technical sales department. "Even if the motor is making peak torque at 5,000 rpm, you don't want the converter to slip up to 5,000 rpm. That's because while that converter is slipping up to five grand, everybody else is passing you. From idle up to the stall rpm of the converter, there is going to be slippage. While that's happening, you are losing power.

"In a drag race application, you are at the line sitting still while you go up through that rpm range. You aren't even moving until you reach your stall speed. In a circle track application, you are constantly moving all the way through your rpm range. Once you leave the line in a drag car, you are only going to vary 1,000 to 1,500 rpm during a gear change. That's all the variance you are going to have from start to finish. In a circle track application, you are on and off the throttle constantly. If the rpm level gets below the stall range of the converter when you are off throttle in the turns, you are slipping the converter and losing power. You are not getting all of the available engine power to the ground."

That's why circle track torque converters are built with a much lower stall speed than drag racing stall converters. The range, according to Miller, is usually between 1,000 and 1,600 rpm-essentially, just enough to allow you to put the car in gear without killing the engine. The ideal condition is for the converter to be above the stall speed everywhere on the track-and that includes yellow-flag restarts. When the car is on the track, you basically want the converter to act as much like a direct-drive unit as possible.

Construction And DesignRule books in most classes requiring automatic transmissions say something along the lines of, "The torque converter must be stock appearing." That's why most TCI converters look almost identical to stock units. But do not confuse yourself into thinking that just because the outsides of both units look alike, that they are the same.

You can even find stock torque converters that stall at the same speeds as a converter built for racing purposes. In that case, the converter will probably be at full lock all the way around the racetrack, but you would still be at a disadvantage to a purpose-built racing converter. "One thing a stock converter will always have is too much slippage," Miller explains. "A converter is always going to slip, even when you are above stall speed. You can't achieve 100 percent fluid coupling. With a stock converter, the average slipping percentage is going to be around 10 percent. Sometimes it's as low as 8 percent; sometimes it's as much as 12 percent. So when you are turning 5,000 rpm, the converter is costing you 500 rpm right there. Even 6 percent slip is high for circle track racing.

"Aftermarket converters, if they are built correctly, can stall at the same speeds, but the lockup percentage will be a lot better. Instead of it locking up at 90 or 92 percent efficiency, it's going to be 95 to 97 percent-sometimes as much as 98. That means at the higher rpm levels, you are putting more power to the ground."

One of the reasons racing converters can achieve a more efficient fluid coupling is because they are built to eliminate as much friction loss as possible. Instead of using thrust washers such as OEM units, TCI converters are built using bearings whereer possible. Furnace brazing also allows the fins in the unit to be more securely attached to the housing and significantly stronger than the stock fins, which are only pressed into place. TCI representatives also say that their machinery and manufacturing processes allow their converters to be held to much tighter tolerances than the stock units, even though they begin with stock cores (in order to slide under the "stock appearing" rule).

The end result is a converter that acts much more like a traditional clutch that moves as much of the engine's torque to the rear wheels as possible. After all, why put so much effort into a high-end racing engine if you aren't going to make the most of it?of it?

Care And Feeding Of Your Torque ConverterCompared to a stock torque converter, heat isn't going to be as much of an issue because it is generated from the slipping, but you still want to take some precautions. As a transmission company, we still recommend a cooler if at all possible. It will make your transmission and everything else last longer. Because the fluid flows through the converter too, it will cool all of the fluid.

Of course, we recommend our RTF transmission fluid, which is a dextron-based fluid with a moly additive. The moly is important because it helps the lubricant bond to the parts better. In layman's terms, it sticks to the hard parts better, which is going to cut down on friction.

A primary mechanical factor to consider when working with a transmission torque converter is the shift linkage. If the linkage isn't engaging fully, it will allow the converter to slip more. We recommend racers check their shift linkage periodically when doing regular maintenance.-Scott Miller

TCI Automotive
151 Industrial Dr.
MS  38603