Originally introduced in 1948 alongside Buick’s two-speed Dynaflow transmission, the torque converter originally helped make transmissions fully automatic, eliminating the clutch necessary for manual transmissions. Many roadcars are still fitted with torque converters, but in motorsport they survive solely in the domain of drag racing.
What makes torque converters unique is the transfer of power from the engine to the transmission, with fluid being the only connecting factor, allowing the engine to idle without the threat of stalling. Along with preventing engine stall, the converter helps improve performance by nearly doubling the torque of the engine, offering a higher degree of acceleration - a key factor for a quarter-mile racer.
One of the most important issues with a torque converter in racing applications is the stall speed. This is the rpm at which a given torque converter (impeller) has to spin for it to overcome a given amount of load and begin moving its turbine element (the component that transfers drive to the gearbox). This is a vital consideration when speccing a converter for racing use, as the stall speed is the rpm at which the converter needs to be spinning to create enough fluid force to overcome vehicle inertia at wide-open throttle.
Clearly the inertia is governed by vehicle weight, gear ratios, the rolling resistance of the tyre, suspension set-up and chassis stiffness. All of this has to be factored in when specifying a converter for a particular car, as the stall speed will in essence govern the launch rpm of a vehicle. If the stall speed is too high, the engine will produce too much power and the driver will smoke the tyres; too low and he/she will bog down off the line. The units used in classes like Pro Mod are adjustable at the trackside for stall speed, allowing the crew chief to fine-tune the launch dynamics of the car based on track and environmental conditions. Whereas a standard-style converter is welded together as a sealed unit, these race-specific components can be fully dismantled in order to change the characteristics of the stator and turbine.
The torque multiplication through the stator occurs when the converter is in ’stall mode’ and during initial vehicle acceleration. As the vehicle accelerates, the torque multiplication decreases until it reaches a ratio of 1:1 with the crankshaft torque. The design of the stator will influence the torque multiplication characteristics of a converter, with a typical figure being in the region of 2.5:1. The more drastic the change in fluid path caused by the stator from its ‘natural’ return path, the higher the torque multiplication ratio a given converter will have. Torque multiplication does not occur with a manual transmission clutch and pressure plate, hence the need for heavy flywheels, very high numerical gear ratios, and high launch rpm.
This is where the relationship between converter stall speed, engine output and chassis and tyres becomes very important. If an engine producing 200 lb-ft of torque at 3000 rpm is matched with a converter with a torque multiplication of 2.5:1 and a stall speed of 3000 rpm, 500 lb-ft will be available at launch. However, if that engine produces 300 lb-ft at 4000 rpm, a converter with a higher stall speed would give a bigger ‘hit’ of torque at launch.
Whether this is beneficial depends on if the chassis and tyre set-up can handle the extra torque without breaking traction. If not, and the torque overcomes the chassis, a converter with a conversion factor of 2:1 and a stall speed of 4000 rpm could be a better option. Although the initial torque hit will be lower, the disadvantage will be outweighed by a clean start and a resulting lower elapsed time. Clearly the different permutations of this scenario are almost endless, but it gives an insight into the complexity of using a torque converter to best effect.
Written by Lawrence Butcher