Lock-up 904 ?

The rpm drop is dependent on how close your cruise ropm is to the stall speed of your converter. But it is not an OD gear it just locks up the torque convertor. The reason they were used was with the high gears (2.45) that were in mid 70's cars coupled with the 55mph speed limit it put the convertor in or near stall speed which caused slippage and heat. The cure was to lock up the convertor. Then when the OD trans hit the market the the rear end gears could be lower but with the OD gear the rpm and stall speed was still a problem that's why we still have lock up convertors today. If your cruise rpm is above your stall speed the lock up convertor will not effect your rpm very much if any.