Torque converter stall speed is the maximum amount of engine RPM that can be achieved in an automatic transmission-equipped vehicle while the transmission is in a forward operating range without generating any driveshaft motion.
All advertised stall speed ratings of all torque converters are nominal ratings regardless of the manufacturer or converter design. These advertised ratings are not indicative of the actual stall characteristics of a torque converter. Advertised stall speed ratings are published simply as a general guideline to help in the selection of the correct torque converter for a customer’s application. A torque converter will provide a range of stall speeds based on the amount of load that is placed on the torque converter. Many factors influence torque converter stall speed including; output torque of the engine, vehicle weight, rear axle ratio, rear tire height, torque converter diameter, impeller fin angle design, stator design, impeller-to-turbine clearance, stator-to-impeller clearance, stator-to-turbine clearance, and brake bias.
When considering torque output of the engine, you must not only consider peak torque but average torque, and must consider all components and design characteristics of an engine that will influence torque production. Several key factors to consider in engine design that will have a dramatic impact on torque production and the overall shape of the torque curve include but are not limited to; stroke length, static and dynamic compression ratios, camshaft specifications, cylinder head intake runner volume, intake manifold design, header primary tube size and length, etc. Something as seemingly simple as an incorrectly tuned idle and/or part throttle circuit in a carburetor or incorrectly tuned ignition timing can have a dramatic negative impact on the stall speed characteristics of a torque converter.
Advertised stall speed ratings of Hughes Performance torque converters are generalized nominal ratings based on use with a properly tuned big block street or mild street/strip engine in a properly matched overall vehicle combination. Small block applications will typically vary 300 – 500 RPM lower in maximum observed stall speed as compared to advertised stall speed. This is especially evident in small displacement engines with short stroke length such as a 289 or 302 Ford V8 engine, as well as small block engines with low static compression ratio (generally less than 9.5:1), large cylinder head intake runner volume (generally 215cc and larger), large camshaft profiles (generally 250 degrees @ .050” and larger), and/or camshaft profiles with a tight lobe separation angle (generally 107 degrees and tighter). Small block engines featuring one or more of these design characteristics may generate stall speeds even lower than the general 300 – 500 RPM rule mentioned previously.
There are three common methods to gauge the stall speed characteristics of a torque converter. Footbrake stall is the maximum amount of engine RPM that can be achieved in a forward operating range with the brakes fully applied to prevent the vehicle from moving forward. Footbrake stall is not an accurate method of determining the true maximum stall speed of a torque converter. Furthermore, advertised stall speed ratings are not based on footbrake stall speed. Maximum static stall is the maximum amount of engine RPM that can be achieved in a forward operating range without generating any driveshaft motion. Maximum static stall can only be verified in a transbrake-equipped vehicle. Flash stall is the amount of engine RPM (or flash) that is observed upon initial acceleration under load. The easiest method for checking flash stall is to drive the vehicle at low speed in second or third range and immediately transition to wide open throttle. The RPM level that the engine immediately accelerates to is the flash stall. Flash stall speed is one of the most useful ways to truly gauge the stall speed characteristics of a torque converter and how those characteristics will influence the acceleration potential of a vehicle.
All street and street/strip torque converters manufactured by Hughes Performance are built with the highest quality components and are intended for performance use. In fact, Hughes Performance street and street/strip torque converters have many of the same internal features that you will find in many custom built race-specific torque converters. However, Hughes Performance street and street/strip torque converters are not designed for use in any transbrake-equipped application. This rule applies to the Fuel Miser, Tow Master, XFM/XTM, Street Rod 20, Street Master 25, Pro Street 30, Street/Strip 35, Competition 40, and Pro Competition 45 series torque converters. Special consideration must also be given when using these converters in applications that utilize a power adder. Hughes Performance offers several heavy duty upgrades on our street and street/strip converters to make these units compatible with mild power adders such as nitrous oxide, supercharging, and turbocharging. A custom built Hughes Performance Pro Series torque converter is required for use with any transbrake application as well as any application using multiple power adders. A Pro Series torque converter is also generally a good choice for any application exceeding 650 flywheel horsepower.