What is stall speed and how does it actually affect your car? This article does assume that you have some basic knowledge on how torque converters work.
Let us begin by showing how the stall speed actually works. Even on light loads, any vehicle that has an automatic transmission will begin moving as soon as you release your foot off the brake. The stall speed then comes into play during the load condition. Whenever we talk about stall speed, we are talking about the RPM of your engine. If your vehicle isn’t moving by time that the impeller hits the stall speed, it will either begin to move or the RPM won’t increase. Basically, the stall speed is the RPM that the torque converter sends power to the transmission.
The torque converter stall speed is roughly equal to the clutch engagement for a manual transmission. If you are driving a manual around town, you would give the car a bit of gas and ease off the clutch to start out smooth. Under most conditions the torque converter will begin sending power to the transmission at really low RPMs.
Performance & Racing Stall Speeds
If you need a lot of power to start out with a heavy load or to make a fast getaway. You will rev the engine to where it has the most power before you let up on the clutch. It is similar to these circumstances that stall speed is important. The torque converter will let the engine build RPM without having to turn the output shaft or turbine until the stall speed has been reached.
Unlike a gas engine, a diesel will develop a lot of torque at a low RPM – which is enough to get a heavily loaded truck rolling just by easing off the clutch without touching the gas pedal. Dynamometer tests have shown that a Ford’s 7.3-liter Power Stroke engine hit its peak torque at 1600 RPM and will start to drop off at 1850 RPM while the Dodge 5.9 Liter Cummins engine peaks at 1400 RPM and will start to drop off at 1800 RPM. While the RPM keeps rising, the torque will decrease.
Stall Speed Too Low
How does this translate to a torque converter with a low stall speed? Both the Dodge and the Ford torque converters will stall between 2000-2500 RPM – so, with a heavy load, the converter will not begin to turn the back wheels until beyond the torque peak.
Stall Speed Too High
In this particular case, the stall speed will be too high and it will be impossible to get the engines full power to the back wheels. In order to access this potential power of the engine, the stall speed needs to be lowered.
Lower Stall Speeds & Temperature
Lowering the stall speed will have another advantage as it can reduce the temperature inside the transmission. If you look inside of a torque converter with a high stall speed during a heavy load, you will notice that the impeller within the torque converter is going to be spinning quickly. You will also notice that the turbine will be slowly spinning or it won’t spin it all. The impeller creates motion energy that gets transformed into heat energy, which is passed to the transmission fluid. The higher stall speedsare, the higher that the temperature will be. Heat is the nemesis of the transmission.The overall goal is to keep your transmission fluid cool. When you have a lower stall speed, less time passes before the motion energy is converted into energy to drive the turbine, so the transmission will run cooler and thus allows it live longer.
Tuned Torque Converters
What most people do not know is that your torque converter is tunable. The stall speed is determined by different factors, which include the distance between the turbine and impeller as well as the design of the stator. When you properly modify the internal components of the converter, it is quite possible to alter the stall speed and create yourself a torque converter that is turned just for that engine.
Easy Installation & Improved Performance
Billet Torque Converters are designed to stall at a speed that will match you power system – at the right point where the SUV or truck diesel engine will put out maximum pulling power. Because the new torque converters hook up much more quickly, they will run much cooler even under a high load. You won’t need to use a dynamometer to see the results. Your truck will accelerate faster, the fuel economy will go up, the transmission temperature will go down, and your transmission will live so much longer. It is seriously a match made in Heaven.