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Professor Wizard -> RE: What is a Torque Converter? (2/24/2006 10:03:51 AM)
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Lets see if the Professor can help.... Stall Speed is simple, yet confusing...
Stall is the speed at which the converter will hold the engine speed and not allow any further gain in RPM. (i.e., the engine "stalls"). That is to say, if you go WOT with your wheels locked, the engine would not go any further in RPM then the stall speed at which point it would either stall/die or just lug out at that RPM, unti you release the brakes. The Stall speed rating on your converter is the RPM at which the power transfer is most efficient. Below the Stall Speed - lots of slip - at and above the stall speed - near lock transfer.
The simple answer - you want a converter with a stall speed just below your “Sweet Spot” of Horsepower output RPM for maximum performance, however that wouldn't work very well for everyday driving as your engine would overrev every time you want to take off..
The more complicated answer involves as much the engine then the Torque Converter. The speed at which stall occurs with a given converter is a function of engine peak torque. The stall speed on a given converter will not be the same coupled to a tame small block engine when compared to a big block with all of the muscle features added.
When comparing stall speeds it is important to account for the engine that drives it. True converter stall can best be determined when a Transbrake is used. Testing for stall value by locking the wheel brakes generally does not produce a true stall value because the engine power can often cause wheel turn by overpowering the brakes, but it will give you a general idea.
Selection of the right stall speed for your vehicle should be matched to the engine peak torque, engine torque curve shape and vehicle weight. In general, the stall speed selected for your converter would be 500 to 700 rpm below the peak torque. This speed allows the margin for application of the torque reserve on takeoff. When selecting stall speed without having prior experience to go by, it is better to conservatively estimate the engine torque speed than it is to over estimate it. If you over estimate the torque output you will have a converter with a stall speed too low, making your car slow off the line and have slow ET. A properly selected stall speed will give you better launch and better ET. Within the converter, stall speed is balanced off against inefficiency after launch. Getting desired stall at the expense of performance after launch is just as costly as improper stall speed to begin. The optimum converter has careful selection and design of changes to the impeller, turbine and stator.
Stall speed should be matched to engine performance, the car weight, tire size and gear ratio. Proper selection of stall spec will make for quicker launch, better 60 ft. time and better ET. It is very important that these parameters be specified when ordering a converter to assure satisfaction.
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. Stall speed can be attained in many ways that cripple the converter in other ways. Stall speed can 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.
Stall speed is also affected by the torque converters efficiency ratios. Some brands are very efficient, and will stall the engine earlier in the RPM curve, while others are not as efficient, so there is always slip requiring a lower stall speed to get the HP to the wheels. Finding the correct stall speed will require a bit of experimenting depending on make/model of converter as well as engine performance statistics.
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