There are many considerations for determining the right stall speed for a converter. An aftermarket stall converter is used for just two things--towing and racing. The most important considerations are the weight of the vehicle, the gearing, the size of the engine, and the power and torque available at a certain RPM. Other important considerations are the grind of the camshaft as to the lift, duration and effective range and the type of induction whether normal aspiration or forced induction. The forced induction comes into play only if it is turbo charged as opposed to super charged because of the lag inherent in the turbo.
A normal stall converter has a stall, depending on vehicle, from 900 to 1,200 RPM. As the stall speed increases, the lockup increases, but with normal driving the vehicle will accelerate with small load fairly normally as long as the stall does not exceed 3,000. When max power is called for, the stall will flash to the stall speed before lockup occurs. For example, if a vehicle is doing 30 mph at 1,500 RPM in second gear, when floored, the RPM will flash to 3,000 before beginning to pull hard. The pros and cons to this are important considerations. When the stall flashes, it will throw the engine into the power band and pull much harder than a conventional converter. The con is that, with the stock converter, top speed in second may be 50 mph at 5,500 RPM, and with the stall converter it will be around 7,200 at the same speed. The shift points will have to be considered as to speed to stay within RPM limits.
A stall converter is a necessity in racing to get to max torque as fast as possible. For hauling, such as getting a heavy boat out of the water, a stall of 2,500 max is recommended. Anything higher would create driving problems while hauling, and will kill the fuel economy. Don't forget that going down the road every time the foot is lifted off the gas, or when the vehicle is cruising and a small increase in speed is desired, the RPM will jump to 2,500. In this case, before it will accelerate and when the speed is reached, the RPM will drop lower while cruising. It is almost like the transmission is slipping until the stall is reached.
The proper use of a stall converter for racing is to stage the car and hold the brake pedal just prior to launching, and flash the engine to the stall before releasing the brake. Do not flash the stall in the first staging lights. As you set of the first set of lights, ease the RPM up close to flash, bump-brake the car into the final staged lights, then immediately flash and hold. Flashing early heats up the converter and the transmission fluid if held too long.
There is a lot of technical jargon involved, but for the sake of getting to the point, here are some professional suggestions.
A vehicle weighing around 2,800 lbs. with a highway gear of no lower than 2.70 and a 4-speed overdrive that does around 2,000 RPM at 60 with no more than 180 bhp, uses a 2,500 RPM stall converter.
The same situation as above, but with a modified engine with 240 bhp or better and an RPM limit of 7,500, uses a 3,500 stall to get into the preferred RPM band quickly. Use a wider, stickier tire.
A vehicle weighing 3,600 to 3,800 lbs. with a highway gear and an overdrive and 240 to 350 bhp and 300 foot-pounds of torque uses a 3,500 stall. This will produce the best ET for the hp rating.
Same weight class, lower gear such as 3.73 to 4.11, horsepower rating of 375 to 450 with 420 plus torque rating, uses a 3,000 street and strip or 3,800 strip stall.
Same weight, low gear 3.73 to 4.11, horsepower rating 500 to 685 with 550 plus torque, uses a 2,500 to 2,800 stall. This will give a much better controllable 60 foot time without just spinning the tires with this much torque. With the right rubber, this can give a 1.2 or better 60 foot.
A 3,000 lbs. car with a modified suspension (4-link rear with 4.11 to 4.56 or lower gear), a modified 200R transmission and a modified engine with overhead cams, or using stud girdles on a conventional cam with a large turbo putting out in excess of 850 horsepower and about the same in torque, uses a 4,000 to 4,500 stall. For a super charger, use 1,500 to 2,200 stall.
