From Cooling System Modifications
Source:
www.grapeaperacing.com
Cooling System Pressure
The pressure in the block is higher than the radiator pressure; this is because the pump is building pressure due to the thermostat being a restriction. This pressure raises the boiling point of the coolant and reduces the chance of steam pockets, so never run with out a thermostat (or some form of restriction).
The radiator cap will usually hold 15-18 psi. If the radiator holds the system at 15 psi, the boiling point of plain water will be raised to 250° F. The water pump can then make an additional 40-45 psi in the engine and bring that boiling point close to 300° F. So as you can see, pressure is important.
Coolant Flow
Different coolants require different minimum flow rates, but contrary to popular belief, you cannot make the coolant flow too fast. This rumour was started because people removed the thermostat to gain flow, because they had an over heating problem, and it only aggravated the problem. The real reason they ran into problems is that removing the thermostat also removes the restriction that builds pressure in the engine, so they gained flow, but reduced the boiling point of the coolant in the block.
Running a higher flow thermostat
and a higher volume pump to maintain pressure, will give no such problems. If you think about it, making the coolant flow twice as fast will also make it flow though the engine twice as often, so there will be more even temperature across the engine.
There has been, and still is, the rumour that if the coolant flows too fast, it will not have time to pick up heat. That is nonsense. As long as there is coolant contact with a surface, the rate of heat transfer will be the same. Coolant that flows twice as fast also flows through the block twice as often.