What do stall converters do? Do they just set the engine rpm while running idle?

A torque converter is a fluid coupling. Like 2 fans mounted opposed to each other,when one (attached motor side) spins it forces fluid into the other causing it to spin. A stall converter will allow the engine to reach a certian rpm before moving enough fluid to cause the other to spin or move the vehicle. Stall is measured in rpm, however many factors effect the true engaugement speed.


They are used to allow a engine to reach an rpm that is closer to a point where peak torque is reached, typically 50 to 150 rpm's before peak torque.

They are also what allows an auto transmission car to come to a stop, in gear, and not stall the engine.

Vehicle weight and horsepower will affect the actual stall speed compared to what it is advertised for.  If the car is heavier the convertor will slip more to an rpm above the advertised rpm.
If the car is high horsepower, it will slip more to an rpm above the advertised rpm.

The best idea is to call the manufacturer and provide them with as many relevant variables as possible.

So say we have 2 identical cars. One has a 2800  torque converter and the other one has a 1500 torque converter. Will the car with the 2800 have faster take offs?

Quoted: So say we have 2 identical cars. One has a 2800  torque converter and the other one has a 1500 torque converter. Will the car with the 2800 have faster take offs?

Yes, along with lower fuel mileage, and a lower top speed.