Torque converters are the most important and least understood piece of a car. I'll attempt a long winded, bouncing around, gross oversimplification.
First off, take 90% of what you read on the internet and throw it out. Most people regurgitate crap they read somewhere else and have never even driven a car with a non-stock converter. A small number have driven cars with crap performance converters. Few have actually driven a car with the proper converter.
Buying a converter with an advertised stall speed is a joke. That "3500" converter is going to perform differently based on application. The converter basically tries to balance two forces...power applied and resistance. The resistance is the force required to move the car. Weight, aerodynamics, rolling resistance, frictional loss, gearing advantage, ect all factor in on the load the converter feels. A converter will slip more and stall higher if either of these two forces increases. A converter that stalls at 3000rpm in a 3000lb 400hp car may stall at 4000+rpm in a 4000lb 600hp car.
A properly designed converter for your cars power and resistance will act like a stock converter when you drive the car normally. You don't have to rev it to 4000 to get the car moving, and it won't be slipping any more than a stock stall during normal driving. The only time extra slippage occurs is when you apply enough power to overcome the coupling ability of the converter. To help visualize the balancing effect the converter has between force and resistance, lets picture a launch. When you leave off the line at 4000 you don't instantly scream to 7000 and have to shift. The converter allowed the engine to hit 4000, now the resistance (the car) has to catch up. As the car accelerates the resistance lessens and the engine is allowed to apply more power and rev further. Watch some in car videos of properly setup cars launching. You will see the car accelerating way out of proportion with the movement of the tach needle.
You certainly can luck out with an off the shelf converter. It's been stated many times by very experienced guys...buy a good converter! It's the absolute best money you can spend. People will spend hundreds or thousands on wheels, valve covers, ect, and ***** about the price for a quality torque converter.
