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Transmission dynamometers

Being able to measure transmission losses accurately is a very useful capability when it comes to optimising a racecar package, and is particularly important in series where regulations ensure that engine power outputs are closely matched. In such series, very small percentage power gains can have a considerable impact on competitiveness. This means that any reductions in parasitic losses due to improvements in transmission efficiency can be decisive in the hunt for victory.

The usual method of measuring such losses is by using a dedicated transmission dynamometer. In general, these use a drive motor and an absorption device such as a water brake. The motor drives the transmission through the input shaft, and the absorption device is connected to the output. Torque sensors on both the input and output shafts allow the losses through the transmission to be deduced. Much as with an engine dyno, transmissions that have an external oil lubrication system with pumps and coolers can usually be accommodated and, with the correct instrumentation, lubricant temperatures and flow can be monitored. 

One interesting area relating to transmission testing is measuring the operation of torque converters, such as those used in drag cars in classes such as Pro Mod. In such applications, the characteristics of the converter – for example its torque multiplication effect and stall speed – have a significant effect on the way a car launches. To clarify, the stall speed is the rpm at which a torque converter’s impeller (known as the stator) 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 specifying 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. The torque multiplication effect 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.

Therefore, being able to measure such parameters accurately can be very beneficial in informing other areas of car set-up. While some aspects of a converter’s operation can be analysed on a regular transmission dynamometer, for more detailed investigation using a dedicated machine is required. With such a device, input and output torque of a converter can be measured, allowing for the torque multiplication effect throughout the rpm range to be assed, while other important factors such as fluid flow rate and temperature can also be monitored. A key aspect of a torque converter dynamometer is a motor powerful enough to induce stall in converters designed for very high power applications; while electrical or hydraulic power is often used, the author has seen a converter test bed powered by a Big Block Chevy motor!

Such dynamometers are expensive pieces of equipment, so are not found in the average transmission shop. However, some competition torque converter manufacturers do have such capabilities in-house. As a result, they can accurately assess the performance of each converter produced, meaning they can provide customers with much useful data on a particular unit. In turn, this data can help end-users get more consistent car set-ups, something that is of vital importance in drag racing.

Written by Lawrence Butcher

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