The Hi Performance valve called J is used with turbo Detroit Diesel engines with horse power equal or greater than shown below.
4-71TI (191 to 310 HP)
6-71TI (271 to 485 HP)
6V71TA (261 to 310 HP)
8V71TA (340 to 525 HP)
12V71TA (511 to 870 HP)
If your engine runs on lower horse power, you may use the U valve.