What is a stall converter? What does it do? Thanks

 

It's not a "stall converter", as in something that converts stall. It's a "high stall" converter, as in a torque converter with a higher "stall speed" than a factory converter would have.

 

wahts stall speed?

 

It's the maximum speed that the torque converter will allow the engine to reach while the converter's turbine is held stationary. It's similar to the "flash speed", which is the speed that the engine will reach up to in a "flash" when you launch from a standstill. It's sort of like choosing the RPM to dump the clutch at in a manual.

 

The torque converter is the fluid coupling between the motor and the transmission. For an oversimplification, imagine taking 2 fans and pointing them at each other; turn one on; the other will be forced to spin by the air passing through it. A torque converter works on a similar principle, but includes a 3rd set of blades in between the 2 "fans", called the stator (because it stands still). That's how you can have the car sitting still but the engine spinning.

Put your car in gear, stand on the brake pedal as hard as you can, and floor the gas.

The RPM the motor goes to, is the "stall speed".

Since engines make different amounts of power at different RPMs (usually more power at higher RPMs), in order to make the car launch harder, you might use a torque converter with a higher stall speed. Stock TCs usually stall at around 1400 RPM or thereabouts; well below a performance engine's peak output RPM. In fact, increasing the engine's output at higher RPMs, usually results in DECREASED output at lower RPMs; meaning, you go and build this mountain motor out of racing parts, and discover that where you 6-cyl used to could spin the tires at a traffic light, your new race motor won't even do that. It's because it makes EVEN LESS power at those low RPMs than the weenie stock motor. On the other hand, put a torque converter in there that lets the new motor rev up to 3000 RPM or whatever when the car is sitting still, now you've got a combo that can stand the car up on its rear bumper.