Stall Speed?

[bigtoy]

What exactly is stall speed when talking about auto transmissions?

 

[MSaine]

Automatic transmissions use a torque converter between the flywheel and trans input shaft, no mystery here. The torque converter itself converts the motion from the flywheel to the trans input shaft. A very simple analogy is to take two electric box fans and place them an inch or two apart and turn one of them on. The air flow from the drive fan will blow on the blades of the driven fan and cause it to turn. The converter uses trans fluid under pressure, (in addition to some other components which re-direct fluid to produce higher effeciencies) to acheive similar results.



Again this a very basic idea of what's happening in the converter. OK so the stall speed is the rpm point at which the converter is reaching it's maximum effeciency, i. e. the least differencial in rpm between flywheel and input shaft. Please note that in no case will a torque converter impart all the motion to the input shaft unless there is a clutch or other mechanical device to directly couple. Some slippage will always occur, unless you install a mechanical locking mechanism in the converter to mechanically lock the converter. Hence the term Lock-up, it has a mechanical clutch inside the converter. Sometimes people will say "my converter locks up at such-and-such rpms", but if there's no clutch in the converter it's not actually locking up, it's just reaching it's point of max effeciency.



Typically the stall speed is specified as Brake Stall Speed, (the rpms reached when the throttle is applied and the brakes locked) Brake stall speed is not very reliable because in essence you are attaining the speed at which the tires spin.

OR Flash Stall Speed (the stall speed if you mat the gas and the converter flashes to it's max stall).



Stall speed is determined not only by the converter design, but also applied engine torque, effective gearing, vehicle weight, available traction, and to some degree trans fluid temp. The converter doesn't cause your engine to stall up, your engine causes the converter to stall up. More input torque on the same converter and it will stall higher. For example, a 10" converter built for approx 1500 rpm stall behind a small block gas motor with 200 ft/lb @ 1500 rpm, would stall to the moon behind a roots blown big block.



To sum up, the stall speed is the highest rpm the engine will turn before the input shaft on the trans reaches maximum speed in relation to crankshaft speed.



Hope this helps a little.

 

[jcarey]

MSaine, thanks. Great explanation. I've been wondering myself.



Jim

 

[bigtoy]

MSaine, thanks for the post, that clears it up for me!