A torque converter is a fluid coupling device that sits between the engine and the transmission in automatic transmission vehicles. It allows the engine to continue running even when the vehicle is stationary, and it provides torque multiplication to help launch the vehicle from a stop.
Essentially, it is a rating of the converter’s performance. The higher the stall speed, the faster your vehicle can accelerate and launch out of a dead stop. Most stock converters have a stall speed of 1500-2500 RPM, but aftermarket converters can double and occasionally triple that amount.
Here is how the torque converter torque-speed relationship works:
Fluid Coupling
The converter consists of an impeller, a turbine, and a stator, all submerged in transmission fluid. The impeller is connected to the engine crankshaft, while the turbine is connected to the transmission input shaft.
Torque Transfer
When the engine is running, it spins the impeller, which in turn moves the transmission fluid. The moving fluid transfers torque to the turbine, causing it to rotate.
Torque Multiplication
At low speeds, such as when the vehicle is starting from a stop, the turbine experiences more torque than the impeller due to the design of the torque converter. This torque multiplication helps the vehicle accelerate more effectively.
Stall Speed
The stall speed is the maximum engine speed at which the converter can hold the vehicle stationary before the turbine starts to rotate. Below the stall speed, the torque converter provides maximum torque multiplication.
Lock-up Clutch
In many modern torque converters, there is a lock-up clutch that mechanically connects the impeller to the turbine once the vehicle reaches a certain speed. This eliminates the slip between the impeller and turbine and improves fuel efficiency by creating a direct mechanical connection.
Torque Converter Characteristics
The converter’s characteristics are influenced by factors such as the design of the impeller, turbine, and stator, as well as the viscosity and temperature of the transmission fluid. These factors determine how efficiently torque is transferred between the engine and the transmission across a range of speeds.
Torque Converter Stall Speed
Torque converter stall speed refers to the maximum engine speed at which the torque converter can hold the vehicle stationary before the turbine starts to rotate. In simpler terms, it is the engine speed at which the torque converter begins to transfer power to the transmission.
When you start your car from a standstill, the engine’s power is transferred to the transmission through the torque converter. At low speeds, the torque converter provides torque multiplication, helping the vehicle to accelerate more effectively. The stall speed is the maximum rotational speed of the engine while the vehicle remains stationary, and the torque converter is fully engaged.
Stall speed is influenced by several factors including the design of the torque converter, the characteristics of the engine, and the vehicle’s weight. It’s an important consideration, especially in performance applications where achieving optimal launch and acceleration is critical.
In high-performance applications, torque converters are often designed with stall speeds tailored to the engine’s power characteristics and the vehicle’s intended use. For example, in drag racing, a higher stall speed might be desirable to allow the engine to operate closer to its peak power band during launch. Conversely, in everyday driving situations, a lower stall speed helps provide better fuel efficiency and smoother acceleration from a stop.
Finally
In summary, the torque converter allows for smooth power transfer between the engine and transmission while providing torque multiplication at low speeds to aid in acceleration. Its design and operation enable the vehicle to operate effectively across a wide range of driving conditions.
When you are selecting the right stall speed for your car, be sure to match the engine’s peak torque, torque curve and vehicle weight.
- Peak Torque – The maximum torque your engine can exert at a certain RPM.
- Torque Curve – The place where the engine has the maximum amount of power at a certain RPM.
- Vehicle Weight – The weight of your vehicle has a big impact on the measurement, and don’t forget to factor in aftermarket modifications.
