Technical Advice April 2004


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High Stall Torque Converters

Posted On: April 30, 2004

High stall torque converters are very desirable in drag race vehicles because they allow the engine to develop maximum torque (say 5000 rpm), before the vehicle leaves the starting line.  This type of converter is designed to slip more, or have a higher stall speed then the typical 1300-1900 rpm factory converter.  Modifying an engine for increased power will raise the rpm it produces at this new usable horsepower.  When the engine power is increased, it will require matching the proper high-stall torque converter.  This is important, in order to allow the engine to reach its usable horsepower range before the converter reaches its stall speed, and hydraulically loads the engine.   Stall speed is the maximum rpm that an engine can obtain against the hydraulic load of the torque converter with the transmission shaft held stationary.

The best performance is obtained with a converter that has a stall speed slightly higher than the rpm of the engine when it starts to produce its strongest usable power.  Too low of a stall speed will result in a bog, and launches will be slow, since the engine will not be in it's usable power range.  Too high of a stall speed will result in the engine reaching its peak power range below the rpm.  Since the converter can efficiently transfer power, this could result in excessive slippage.

The stall speed is controlled by the converter, engine power, and the ability to hold the transmission input shaft stationary.  When engine power is increased, stall speed will go up.  Likewise, when engine power is decreased, stall speed will go down.  Stall speeds with this type of torque converter are generally rated as what's termed the "Flash Stall Speed" or shock. This is the approximate rpm that the engine will go to when the throttle is quickly moved from the idle position to the wide open throttle with the transmission engaged.

This "Flash Stall Speed" will vary depending upon engine power, rear end ratio, and car weight.  It is very important to match all of the engine components; cam, carburetor, torque converter, rear end gear ratio, etc, to work in the same rpm range for the best results.  When modifying the engine, do not over cam or over carburete the engine.  These are two of the most common mistakes.

Select a rear-end gear ratio that will allow the modified engine and the high stall torque converter to perform efficiently in the rpm range they were built for.  A general recommendation, would be to use gears in the following ranges:

3.10 to 3.50 for 2,400 to 2,800 rpm stall converter,

3.50 to 4.10 for 3,000 to 3,800 rpm stall converter,

4.10 and higher range with higher stall converters.

Keep in mind, for a given stall speed, the turbine shaft is held stationary with either the wheel brakes or the transmission brake until the driver is ready to take off.  The use of a trans-brake allows the driver to make maximum use of a given stall speed.  Stall speed will be limited in cars without a transmission brake.  This is because the gear reduction of the transmission and the rear end would provide enough mechanical advantage to overcome the car's wheel brake system.  When using the wheel brakes, often the best launch technique for a high stall converter is to leave from a fast idle.  This will allow the converter to momentarily flash to a higher stall speed (Flash Stall Speed).  This is better then bringing the engine rpm up against the converter and car brakes and also preloading the suspension.   When using this launch technique, the engine must idle clean and have a stumble free response to throttle movement.


The Transmission Brake

Posted On: April 29, 2004

A transmission brake is usually considered a drag racing "only" automotive after-market device. It operates by simultaneously applying the forward and reverse clutch packs of an automatic transmission while its both in gear, and the torque converter is at or near it's stall speed. At the moment the trans-brake is disengaged the reverse element is released (usually the engine is revving at peak power) and the car will immediately lunge forward from its starting position.

You might wonder what the difference is between holding back a revving car in gear with just the wheel brakes versus that with a trans-brake. In order to understand this difference, more must be explained about the car's suspension, and how it operates during the initial take-off. 

If your car has a rear-axle drive with a solid suspension, it most likely also has what's called "anti-squat". This term defines the function of the rear suspension geometry that prevents the body of the car from "squatting" during it's initial acceleration. In fact, the rear of the car will rise if the anti-squat factor is high enough to overcome the inertial forces encountered during this initial acceleration. Many cars from Detroit incorporate an anti-squat design in order to prevent suspension "bottoming" during initial acceleration. You can find out if your car has this feature by performing a simple stall test and if the rear end rises, it has anti-squat. If your car has an independent rear suspension, such as the Corvette, it will squat because this type suspension has a "pro-squat" type geometry built into it.

How is the trans-brake and the rear suspension related? Keep in mind that for every force in one direction, there is an equal and opposite force in another. The force that is opposite to the raising of the rear will appear as high instant tire loading during initial acceleration. Its instantaneous because this force only occurs at a moment during the initial launch. This at the time between when the car is at rest and when full power is being transferred to the rear suspension.

Since the traction forces of the rear tires usually increase relative to their contact patch force, a car having more anti-squat will always have a higher acceleration and therefore will launch much quicker. If the wheel brakes are used for staging, the rear suspension becomes preloaded and all the anti-squat factor is already used up. When a trans-brake is used for staging, the rear suspension remains at rest and is unloaded. At the moment you flick the switch and release the trans-brake, the immediate transfer of engine torque "shocks" or loads the rear suspension, performing as it would with a stick-shift car. The trans-brake also prevents the high-powered/high stall car from overpowering its brakes on the line. By comparison, the trans-brake release action is much faster than lifting your foot off the brake pedal (or even a clutch pedal for that matter).

The trans-brake is used in conjunction with a specially calibrated full manual valve body. During operation the transmission will function normally, even if you choose not to use the trans-brake. A toggle switch is used to simply turn off its operation. Since the trans-brake is an added feature, the operation of the transmission can not be adversely affected when it is not being used.

The FB trans-brake is available for the following types of high performance transmissions:

GM - TH350 /TH400 / Powerglide

FORD - C-4 / C-6 / 4STB / AO3

MOPAR - TF727