For most people, selecting the right torque converter remains a mystery. Unless you have years of experience in the field, it is near impossible to predict precisely how an individual torque converter will behave in your own car. Faced with a page-long listing of converters, what can you do to narrow down the right one for your street machine? When and why would you need upgraded Torrington bearings and furnace-brazed fins? Will an 11-inch 3,000-stall converter work the same as a 10-inch converter rated at the same stall speed?
Why A Converter?
There are two basic reasons why automatic transmissions need torque converters. First, they allow the engine to decouple from the rest of the drivetrain at slow speeds, eliminating the need for a clutch. Second, the slip that’s built into the converter not only lets the engine rev immediately into its powerband, it also allows this slipping action to multiply the engine’s torque. This explains the abnormally sky-high torque curves often generated by an auto trans car on a chassis dyno. So even though many of our favorite slush boxes only have two or three forward gears, the converter adds some effective gear multiplication to get the car out of the hole faster.
So what is all the talk about stall speed? Stall speed boils down to the rpm at which the converter effectively locks the motor to the driveline and multiplies torque at a one-to-one ratio. Since the guts of a converter constitute a fluid coupling, the two halves of a conventional converter are never directly locked together like a clutch disc to a flywheel unless, of course, it is a modern lockup version with an integral friction clutch. A good converter should produce less than 10 percent slippage after the engine rpm has exceeded the stall speed, otherwise the constant, heat-building slippage will lead to wasted engine output and an early death for the trans.
The Hows And Whys of Stall
Converters are rated to stall within a certain rpm range. That is not because the converter companies don't know their product; it is just that your individual engine characteristics and vehicle specs influence stall speed and make a precise number nearly impossible to predict. Even the term stall speed itself isn't perfectly defined. Here are the three distinctions you're likely to encounter.
True Stall: The rpm the engine cannot exceed when the driveline is locked. The most accurate way to determine true stall is by locking First gear and Reverse with a transbrake and observing engine rpm at wide open throttle (WOT).
Flash Stall: The rpm the engine flashes to when launched from rest at WOT. A converter will often briefly flash to a higher rpm than its true stall speed.
Brake Stall: The rpm the engine cannot exceed with the brakes locked and the driveshaft not spinning. Brake stall isn't usually an accurate measuring tool since the engine often overpowers the wheels before the true stall speed is reached.
When a converter company quotes you a stall speed, verify whether it is flash stall or true stall. Most converter companies quoted us flash stall figures. Many factors determine where the converter will flash stall once it’s installed in your car. Heavy cars with tall (numerically low) gears and large-diameter tires offer more resistance to forward motion, so the converter will stall at a higher rpm than it would in a light car with steep gears and short tires. The easier the motor can accelerate the vehicle, the lower the converter will need to stall to get the car moving.
Of course, the power and torque curves of your motor will have a huge effect on stall speed. Generally speaking, engines that produce more low-end torque will bump the stall speed to a higher rpm. Conversely, the same converter will stall to a lower rpm behind a less torquey, higher-winding engine. Converter companies often designate the former as big-block and the latter small-block. When you buy a typical converter that is rated at 2,000-2,500-rpm stall, that rating is meant to span a variety of motors with different power curves. Some companies like TCI will point out that a 12-inch Saturday Night Special converters typically stall at 1,600-1,800 rpm behind a 325-375 lb-ft “small-block and up to 2,000 rpm behind a 400-450 lb-ft big-block. Sure, they'll stall even higher behind a torquier motor, but they are intended for mild, conservatively cammed motors.
Any converter has size limits on the amount of torque it can safely handle. Although larger-diameter converters have bigger parts, that is not always a good thing. Larger fins mean the fluid can exert more bending forces and result in failure. In stock form, bigger converters also allow more torque multiplication and lower potential stall speed, but the internals can handle only so much torque before parts start breaking. Reducing the diameter of the converter reduces its ability to multiply torque, puts less stress on the fins, and raises the stall speed, so small converters are generally better suited to peaky high-performance engines with higher-winding powerbands.
Other internal tweaks, such as fin angle and stator design, can have enormous effects on the converter's fluid coupling, which changes stall speed and torque multiplication. So it is possible to build a tight 2,600-stall converter or a loose 4,000-stall converter in identical 10-inch housings, just by varying the internal design. Most internal mods are proprietary, but building a converter to achieve the right flash stall while maintaining around-town driveability takes a combination of proper stator design and fin angle in the correctly sized case. While it may be possible to build a 7,000-rpm 11-inch converter, it would be horrendously inefficient starting with an 8-inch converter housing would be a smarter choice.
Get Smart
No matter what your buddy tells you, or what advice you are given by the speed shop counter guy, the converter companies recommend you use a converter suited to your specific application.
Be honest with yourself and decide how you want to use the car. Most of our own cars are 80 percent street driven, and 20 percent track abused. Given this information, the converter tech guy would suggest a converter that retains good street manners. If you tell the tech guy your street machine is 100 percent race car, but you are really intending to drive the thing on the highway, you're only setting yourself up for disappointment.
After you've declared your intent for the car, you also need to tell the tech guy the weight of your vehicle, and the following information about your powertrain combo:
• Displacement
• Compression ratio
• Cam profile (duration at 0.050-inch lift, lobe separation angle)
• Carburetor or injector size
• Transmission model year and gear ratios
• Rearend gear ratio
Off The Shelf, Or Off The Wall?
If you run an oddball combination of parts, don't be surprised if a tech guy recommends a custom-built converter. Although companies supply off-the-shelf converter's that will suit 90 percent of their customers, remember that even these ready-to-ship converters have been designed for common yet specific engine/vehicle applications. But while a mild 350 in a 71 Nova is an easy application to match up, there is probably no converter ready to install behind a twin-turbo 400ci 68 Caprice. You get what you pay for. Why back a $6,000 engine with a cheap converter?”
We've used off-the-shelf converters in a myriad of cars; sometimes they work wonderfully, and sometimes they're less than great. Custom converters built for our specific cars have always been dynamite.
