High technology trickles down from cutting-edge industries, eventually making its way into the performance aftermarket, and helps to demystify the “black arts” of car crafting. Even stock EFI computers are no longer hallowed ground; there’s enough information on the Internet to reprogram ’em any way you like. Nevertheless, some black arts have never been totally understood by the masses.
For most people, ourselves included, selecting the right torque converter remains a mystery. Unless you’re bolstered with years of experience in the field, it’s 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? Throwing darts is a bad decision-making process; our advice is simple: call the manufacturer. In fact, we called five torque converter manufacturers, grilled their tech guys on this very subject, and asked each of them to recommend a converter for a typical car. Before making that call yourself, this article should help narrow down your converter choices so you don’t have to spend hours on the phone like we did.
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’s 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’s a modern lockup version with an integral friction clutch (see sidebar). 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’s not because the converter companies don’t know their product; it’s 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’s 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’s rated at 2,000-2,500-rpm stall, that rating is meant to span a variety of motors with different power curves. Scott Miller of TCI points out his company’s 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’re intended for mild, conservatively cammed motors.
Miller explains that any converter’s size limits the amount of torque it can safely handle. Although larger-diameter converters have bigger parts, that’s 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’s 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’d 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’re given by the speed shop counter guy, the converter companies recommend you use a converter suited to your specific application. Even TCI and B&M, which sell almost exclusively through off-the-shelf dealer networks, suggest you contact them directly for a recommendation before purchasing. “Choosing the right converter is like a visit to the doctor,” says Jim Hughes of Hughes Performance. “The more information you can give him, the better he’s able to correctly diagnose the problem.”
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’re 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 like TCI and B&M believe an off-the-shelf converter 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’s probably no converter ready to install behind a twin-turbo 400ci ’68 Caprice.
JW and Hughes sell fewer off- the-shelf converters because their clientele is comprised of a greater percentage of racers. John Winters, President of JW, told us that although his off-the-shelf converters work well for certain vehicles, he believes many gearheads shy away from custom units to save money. Art Carr builds every one of its converters to order, and technical sales guy Steve Lancaster doesn’t usually recommend anything less than a mid-level Heavy Duty Super Torque converter. Steve’s philosophy is, “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. We’re not sure the stars of last month’s “Street Car Shootout” would have run the times they did were it not for their custom 9-inch converters we spec’d from Art Carr, although the $900 list price hurt us a little.
