The enemy of automatic transmissions is heat. Slippage causes heat. A lock-up converter eliminates the slippage so less heat. It also reduces rpm slightly so a little bit better mileage. Not all early lock-up transmissions were as smart as today's models. Some locked up too soon, or stayed on too late. They are supposed to only lock up when the car speed, engine load, and operating conditions are all optimum and the lock-up won't damage the engine by lugging it. I found that having a switch on the dash to enable the lock-up was ideal since I left it off on the streets, and only used it on the highway. I later gave that T-350-C away and it was used on a round-round race car so it was not hurt by my switch.