I feel I have to jump in here because this has been oft-repeated by several people. Plus, another person above is claiming a lower temp thermostat will make the engine run hotter.
I don't see a first-order scientific explanation to how more coolant velocity could give less cooling. That doesn't mean it isn't true, but if true it would be due to some "second order effect" like sucking air into the system or maybe redirecting a jet flow inside the head in certain engines (like the Coanda effect). Otherwise, higher fluid velocity gives higher convective heat transfer and a higher boiling coefficient.
Who am I to comment? A mechanical engineer with graduate degrees who has published many papers in heat transfer.
