So, one of my favorite replacement parts I got for the transmission is the TCI-376020 Constant Pressure Valve Body. I can not rave enough about this kit. It comes with way more than just the valve body. It includes a Torque Converter vac lockup kit, servos for firm shifting, wiring for the Torque Converter lockup and a bunch of other little stuff. The best part is that it comes with extra small parts that give you the option to rebuild it as: Heavy Duty (Firm), Street (Extra Firm) or Street Plus (Competition). If you want to go back and change the shifting at a later date, just change out a few parts. Really a good bonus.
Also, as a side note in researching I found out the the firmer (within reason) the shifts are the longer the clutches last. The shorter transition time for a firm clutch means less slip, less heat and less friction material falling to the wayside.
"In order for manufacturers to get a smooth shift for the general public, they have to make the transmission slip into each gear slowly. They have studied and tested and compromised to get the balance between slipping, smooth shifts and a long lasting transmission. They have done fairly well to get both. As you start increasing HP the slipping smooth shifts will eat the transmission clutches in short order. This is one reason why people install shift kits. It allows the shifts to become firmer with less slip. This will last longer with the added HP.
As HP keeps going up more mods are needed to keep the clutches alive. Added hydralic pressure, added clutchs with better heat resisance, high temp bands, manual valve bodies to speed up shift time. At this point the trans will require more maintainance.
As a general rule, a firmer shift will allow the clutches to live longer. On the other hand if the transmission "bangs" into gear on a road driven street car, the u-joints will need more maintainance. So firm is good and banging is bad."
http://www.fordmuscleforums.com/c4-t...nsmission.html