Just lool up in the top left corner and click on search and type in "torque converter". This will bring up tons of info on the subject. Your question has been addressed many times. In a nutshell, a torque converter is the "middle man" between your engine and automatic tranny. (In a manual trans, the driver controls torque conversion.) When you press the gas, your engine comes to life and start revving up into its powerband (RPMs 1000, 2000, 3000, etc.) Then all that power is sent through the drivetrain 2 the rear end and the torque turns your rear wheel(s). Once you start adding performance mods 2 your engine or if you add headers, you may lose some of the low end torque. To compensate, a torque converter is placed between to "stall" the transfer of power from the engine until it reaches a certain RPM. Check the different postings about what size 2 use because it all depends on many factors like engine, gears, vehicle weight, etc. Hope this helps.