Automatic Transmissions
Lock up torque converters

Most late model automatic transmissions come with lock-up torque converters. The theory here is that even when driving down the highway on level ground at a steady speed, a conventional converter will have a couple hundred RPM of slippage. This energy gets turned into heat, and is a waste of fuel. 

The problem got even worse when overdrives were built into automatics, as now they wanted to go down the high way at around 2000 RPM and the slippage became impractical. Factory engineers designed a hydraulic clutch system that would automatically “lock” the converter under these conditions. Works great improving highway gas mileage and reducing heating problems in these overdrive automatic trannies.

Some high performance trans builders use non-lock up converters on trannies that would normally have them. While this might be a good idea in a full race application, we recommend use of a lock up converter for highway use, for heat and fuel consumption reasons.