If I want to flush the radiator on a 1993 Mustang LX, 8 cyl. engine, how much coolant do I need? No manual with car

Check your owners Manual for the proper type and amount ,But the best thing to flush a radiator with is a 1/2 cup powder dishwasher detergent ,just dump it in the radiator after removing the cap of course and close it back up ,drive the car for a few hours then pull the plug ,the dishwashing detergent doesn't foam up but its properties will remove things you wouldn't believe ,after dumping run water thru it while the plug is removed for about 5 minutes ,then replace plug and refill as ford recommends with motorcraft coolant and mix as directed by Ford Tech or owners Manual ..