Torque Converter stall is a commonly used term but it is also commonly misunderstood. Stall is the speed at which the converter will hold back or limit the engine speed if transmission output is prohibited. By not allowing further gain, the increase in engine RPM "stalls."
The key point to remember is that stall speed is a balancing act. Stall speed is always a balance between the engines ability to produce power and the converters ability to hold it back. A change to either side will alter the balance and change the resulting stall speed.
The speed at which stall occurs with a given converter is a function of engine peak torque. It is clear that the stall speed on a given converter will not be the same when coupled to a tame small block as it is when coupled to a high performance big block.
Why is proper stall speed important?
( Proper selection of stall speed will make for quicker launch, better 60' time, and a better overall ET )
Selection of the right stall speed for your vehicle should be matched to the engines peak torque; engines torque curve, and vehicle weight. In general, the desired stall speed would be 500-700RPM below the engine RPM at peak torque. The speed allows a margin for application of the torque reserve on takeoff.
When selecting stall speed without having an accurate and precise engine peak torque rating, it is better to conservatively estimate the engine torque than it is to over estimate it. If you over estimate the torque output, the resulting stall speed will be lower than intended and is likely to make the vehicle slow off the line, increasing your ET.
How can I determine the stall speed of my converter?
( Stall speed is most accurately measured if the vehicle is equipped with a Transbrake in order to lock the drive train )
Testing 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. This method, often called brake stall, is not as accurate or consistent as observing engine RPM with transmission output completely prohibited.
An alternative method of measurement is to launch at wide-open throttle and observe the engine RPM reached at launch. This is commonly referred to as flash stall. Here again, inconsistencies caused by wheel spin and the short time allowed for RPM observation makes use of this method questionable.
( Use extreme caution when performing stall tests. Fluid temperature in the converter will rise rapidly when the converter is forced to hold back engine power. A 10 second maximum stall time with several minutes of operation for cool down before conducting another test, are strongly recommended )
Does stall speed effect normal street driving?
Generally speaking, normal driving is not adversely effected by converters with stall speeds up to approximately 3000 RPM. The vehicle will begin to roll normally and acceleration will be favorably influenced when higher stall speed converters are used. A very high stall converter ( above 3000 RPM ) would not be satisfactory for street use.
Is stall speed the only consideration in selecting a converter?
While stall speed is very important it is by no means the only consideration when selecting a converter. Torque multiplication at launch and high-end efficiency are equally important. Selection of components and assembly tolerance choices will not only affect stall speed, but will influence converter efficiency. A higher stall speed might be obtained at the expense of looseness at low speeds and loss of performance at higher speeds after launch. You want a converter that produces the right stall without sacrificing performance down the street or down the strip.
The vehicle is running stronger than ever, what happened to the converter?
Engine output is what really determines stall speed for a given converter. For this reason the converter you have been using may not be adequate when you improve the performance of your engine. This is particularly true when using an improved camshaft. Improving heads, carburetion, installing turbos or manifolds will also affect stall speed.
If you increase the available power put into the same converter, the stall speed will be higher. That new higher stall may be nowhere near the optimum stall speed for the new engine configuration, to the point where the overall performance may not only remain unchanged but could actually suffer. Remember that you want to have stall speed matched to the particular engine and vehicle combination.