My LT1 wasnt that bad because the heater core wasnt hooked up from original car (95 impala) and also it has probably been emptied or drained somewhere in the past (some green coolant was in it). The water that came out when I flushed the system (removed Knock Sensors) was orange but not as bad as I have seen for other LT1s. If you have a real sludgy LT1 system you might have to get a new radiator. After about 5 gallons of water sitting and running thru my radiator it was pretty clean and I decided to reuse it.