th400 convertor question

Marcel,
There are 3 parts to a torque converter, impeller, turbine, and stator. The impeller is connected to the engine output (crank), the turbine drives the transmission input shaft. The stator is between the impeller, and turbine. It redirects fluid within the converter to increase converter efficiency. Without the stator, a torque converter would simply be a fluid coupling. The stator enables the torque converter to multiply available engine torque. When the impeller is turning faster than the turbine, the torque converter is multiplying torque. Think of it like a 10 speed bicycle. When you want to go up a hill, you shift into 1st gear. Your legs move very fast, the bicycle moves very slow, but you can pull yourself up the hill easier, because the gear reduction makes more torque at the rear wheel, than if you were in a higher gear. When the engine is under high load, the the two halves of the converter will move at different speeds, and there will be an accompanying torque multiplication (and heat buildup)

Stall speed is defined as the maximum RPM a given engine can turn the impeller, when the turbine is held stationary. Engine torque has alot to do with this. An engine with more torque will stall a given converter higher, than an engine with less torque. A torque converter may stall at 3000 RPM behind a big block, but only 2500 RPM behind a small block. It is therefore impossible to specify a stall speed for a converter. Torque converter manufacturers can make a good guess if you supply them with enough information about your engine, and any modifications you have made (cam, exhaust, induction, head flow, etc.) Car weight, rear axle gearing also affect stall speed because they load the engine more or less.

Getting back to the stator. The stator has a one way clutch, which keeps it from rotating, as long as the impeller and turbine are turning at different speeds. As soon as the impeller and turbine approach the same speed, the stator starts to turn (freewheel), and it no longer redirects fluid within the converter. Torque is transmitted one to one, and there is no torque multiplication. There are 2 types of stator failure. Constantly stuck, or constantly freewheeling. A constantly freewheeling stator will make a car accelerate very poorly up to about 30-40 MPH, then it will have good pulling power at higher speeds. A constantly stuck (locked up) stator will make a car accelerate good to a point, but top speed will be limited to 40-65 MPH, the engine will run hot, and gas mileage will be terrible.

Of course your transmission could be worn and slipping, and it might be rebuild time. Stock stall speeds for most cars is 1400-1700 RPM If you put the car in drive, and hold the brakes down hard while increasing throttle opening, the engine should start to push hard against the brakes well before the rated stall speed of the converter (between 1100-1500 RPM depending on converter stall)

Racers use high stall converters because they basically slip more, and allow the engine to flash up to a higher RPM before the car actually moves. This is necessary because these engines(with big cams) begin to make power at a much higher RPM than a stock engine would. A racer wants to leave the line at peak torque(if he has good traction), and the high stall converter lets him do that. A high stall converter may be no fun on the street. It will build alot of heat in the fluid, and will slip more once the car is moving. Gas mileage will suffer, and the car will feel sluggish in normal slow speed driving. A converter with a stall around 2000 RPM is a good compromise for a stock or lightly modified motor. Hope that explains a few things.

 

Stall speed depends on what cam you are running. The cam description usually includes a converter recommendation.