03/05/2016
High Performance Converters: Stall Speed, Core Selection, and More
Joe Allen and Andy Dumas
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The torque converter’s impeller is directly connected to the engine; therefore, the impeller will rotate at the same RPM as the engine. This rotation pulls fluid from the stator and pushes it towards the outside of the impeller. The fluid then leaves the impeller and pushes on the outside of the turbine blades, causing the turbine to rotate. From there, the fluid flows toward the inside of the turbine and flows over the stator. Instead of rotating, the stator is held stationary by the one-way clutch. Fluid flowing over the stator blades is redirected before entering the impeller. Since the stator redirects the fluid, the impeller expends less energy redirecting the fluid. Less energy used by the impeller means there is more torque to transmit to the turbine. The redirection of fluid by the stator gives the torque converter the ability to multiply torque.
As the turbine speed increases and approaches the speed of the impeller, the amount of fluid that the stator redirects decreases. The less the stator redirects the fluid, the lower the torque multiplication. When the stator stops redirecting the fluid, the torque converter no longer multiplies torque. At this point there is no torque multiplication and the stator begins to rotate with the turbine and impeller. In applications without a one-way clutch, the stator would create drag causing heat and slowing down the vehicle speed.
What is Stall Speed?
Stall speed is a term frequently used but it is also commonly misunderstood. Stall speed is the maximum RPM at which the impeller/engine can rotate if the transmission output is constrained. The engine RPM can no longer increase and the engine speed “stalls.” Flash stall is the RPM at which the converter will “flash” right after the transbrake (or brake) is released. Car weight, tire size, suspension setup, etc. all affect flash stall speed, making it very difficult to predict.
Brake stall is the maximum RPM at which the engine (and impeller) can turn when the turbine rotation is held (car is in gear and the transbrake or brakes are holding). Brake stall is not related to the car’s wheel brakes. The two things that determine the brake stall speed are the engine’s peak torque and the blade geometry (or K Factor) of the impeller, stator and turbine blades.
Suspension, tires, weight, gear ratios, etc. do not influence brake stall. Brake stall can easily be calculated and can be summed up in the following formula:
The key point to remember is that stall speed is dependent on engine torque and the converter blade geometry. Change the impeller, stator or internal clearance and the stall speed will change. If the engine torque changes, the stall speed will also change. In other words, the stall speed on a given converter will not be the same when coupled to a tame small block engine as it is when coupled to a high performance big block engine.
How Can I Determine the Stall Speed of My Converter?
Use extreme caution when performing any stall tests. First and foremost, make sure the car is secure and that there is plenty of room in front of the car in case the transbrake (or brakes) fail. Also, fluid temperature in the torque converter rises very rapidly when the converter is held at stall. A 10-second maximum stall time, with several minutes of operation for cool down before conducting another test, is strongly recommended.
Brake stall speed testing is performed by putting the car into gear and holding the drivetrain while bringing the engine to wide-open throttle. Using a transbrake is the easiest and surest method for doing this. Testing the brake stall speed by holding the wheel brakes and running the engine against the locked brakes will usually result in wheel rotation before true stall speed is reached. The engine simply overpowers the ability of the brakes to hold the car. When rotation starts, you are no longer at stall and you cannot check the stall speed using this method.
An alternate method is to check the flash stall. This is done by observing the engine RPM reached right after release of the transbrake (or brakes) at launch. Inconsistencies caused by wheel spin and the short time allowed for RPM observation makes the use of this method questionable. Changes to car weight (and distribution), suspension, tires, tire pressure, track conditions or weather can affect the flash stall speed.
The Vehicle is Running Stronger than Ever. What Happened to the Converter?
Engine output is what really determines the stall speed for a given torque converter. For this reason, the converter you have been using may not be adequate when you increase the performance of your engine. This is particularly true when using a higher performance camshaft, improved heads, carburetion or manifolds. If you increase the available power going into the same converter, the stall speed will be higher. The higher stall speed may be nowhere near the optimum for the new engine combination to the point where the overall performance may not only remain unchanged, but could actually suffer. Remember, you want to have the stall speed matched to the particular engine and vehicle combination. Changes in altitude, temperature and humidity can greatly affect engine torque, which will change the stall speed.
Torque Converter Selection
Picking the correct stall speed and Stall Torque Ratio (or torque multiplication) is crucial in making a car perform the way the customer desires. Selection of the stall speed and STR for the customer’s vehicle should be matched to the engine’s peak torque, torque curve and the vehicle weight. You want a stall speed that will keep an engine in its powerband as much as possible.
Typically, the desired brake stall speed would be 500 to 700 RPM below the engine RPM at peak torque. This will allow the car to accelerate through the powerband. Too low a stall speed and the car will “bog down” at launch. Conversely, too high a stall speed and the engine will “wind out” at the end of the run. The STR should be as high as track conditions will allow. The right STR will allow the car to have it’s best 60 foot times, which translates into lower E.T.s. With an STR too high, the car will be difficult to control and 60 foot times and elapsed times will be inconsistent. On the other hand, for customers who use their car for cruising or daily driving, you do not want to keep the engine rpm too high or cause the car to launch hard at every stoplight.
Even after you have picked the correct stall speed, it is still challenging to build the correct torque converter. The more accurately you can determine the engine’s torque, the closer you can come to getting the stall speed you desire. Customers rarely know the amount of torque their engine produces. In these situations, it is better to conservatively estimate the engine torque than it is to overestimate it. If you overestimate the torque output, the resulting stall speed will be lower than intended and is likely to make the vehicle slow off the line, increasing the E.T.
Does Stall Speed Aff ect Normal Street Driving?
Generally speaking, torque converters with stall speeds up to approximately 3000 RPM do not adversely affect normal driving. The vehicle will begin to roll normally and acceleration will be favorably influenced when higher stall speed converters are used. A very high stall speed converter (above 3000 RPM) would not be satisfactory for street use.
