[Tommy]

Originally Posted by bobcowan

You need either a thermostat, or a restriction plate. If you leave those out, the water moves too quickly through the system and doesn't transfer heat. The water temperature will look good, but the heads will overheat. And that's a bad thing.

This is a very common opinion, but incorrect based on my understanding of fluid dynamics and thermodynamics. Here is a more authoritative source: Flow Kooler