Q: What is the safe water temperature range for a 604?

A: Ideally you want to see water temps between 180-210 degrees. But it's not uncommon to see temps between 220-230 degrees during certain times of the year, or under certain racing conditions. Many cooling systems, depending on the water pressure your system will handle, will start "pushing water" around that 220-230 degree temperature range. Keep in mind that when you "push water", that water is then replaced by steam in the cooling system. That steam then forms "steam pockets" inside of the engine, which will eventually lead to engine failure (i.e. blown head gaskets, warped cylinder heads, melted pistons from detonation, etc.). So essentially, you can in fact run water temps in excess of 220-230 degree's, provided you're not pushing water out of the system. Pushing water out of the cooling system is what causes temperature related engine failures. 

Common causes for high water temperatures include: No fan shroud, inefficient fan shroud, water pump belt slippage, too much restriction or lack of flow in the cooling system, dirty radiators, lack of fresh air to the radiator (i.e. your nose is sealed to the ground and no fresh air can come under the car to be pulled through the radiator), and engine related issues such as ignition timing and detonation.