You can achieve the same effect with better results by raising the system pressure of the whole system, not just at the block as you would by using a restrictor. The way this is accomplished is by using a radiator cap that has a higher rating. A few pounds should do the trick if the system can hold it without leaking. The limiting factors are the ability of the hoses, pump seals, and an old and weakened radiator to contain the pressure. A higher radiator cap pressure imposes a higher pressure on both the inlet and outlet of the water pump. Since the pressure is applied to both sides of the pump, the flow rate of the pump is not reduced. Using a higher capacity pump alone will not raise the total system pressure because the same radiator cap will vent the excess pressure. Also if the pressure increase is caused by higher flow rate and increased friction alone, the benefit dissipates as you move through the system and more of the friction induced pressure is behind you. By raising the boiling point through increased system pressure, the water has the ability to be at a higher temperature without boiling at the radiator. This would increase the temperature differential and increase the heat transfer.
Although raising the pressure at the engine block would be beneficial, a restrictor orifice or relying on the friction of the thermostat may not be the best method to accomplish this. The thermostat is not purposely designed to create back pressure to prevent cavitation. The pressure is always lower on the downstream side of the orifice or thermostat. This pressure reduction may or may not be sufficient enough to allow the water to boil. Since the restrictor slowed down the flow rate in the block, the water temperature is now hotter coming from the block. The tendency to boil on the downstream side of the thermostat would be greater with higher tempreatures and smaller restrictors that cause a larger difference in pressure across the restrictor orifice . Raising the pressure through the use of a restrictor will in fact reduce the water flow not only in the block but at the radiator.
The restrictor plates in the cooling systems date back to 50’s & 60’s when hot rodders had cooling problems in their quest for more horsepower. They were taking stock components and modifying them or using a variety of parts that were never designed to be run as a unit. With a stock cooling system, you are going to impose greater heat loads on the system as the modified engine’s horsepower increases. So as a result of the higher horsepower loads, more heat is dumped into the cooling system and it its operating that much closer to the boiling point. Couple that with the fact that most performance engines also operate at higher engine rpm’s, and you have a hotter fluid now running thru a pump that was not designed to operate at that temperature or rpm. The result could be the cavitation ya’ll mention. Compounding the problem is they typically used an undersized radiator from a smaller car. In addition, may rods have parts of the engines exposed and do not have ducted airflow through the radiator.
I’ve talked to an old racer that got better cooling in high rpm big blocks by cutting off every other impeller blade. The thought was that the coolant was moving too fast through the radiator. He may not have been reducing flow rate that much because the stock pump was cavitating and was not circulating much anyway. His modification changed the performance of the pump and eliminated the cavitation. Keep in mind that the pump is also spinning much faster while racing than a stock pump normally does and airflow through the radiator. On a related note, the Corvette has a lower flow pump in order to keep it from cavitating at higher rpm’s. A side benefit of the lower flow pump is a few more horsepower at higher rpm’s.
This website has some good info on restrictor plates and high flow pumps.
http://www.stewartcomponents.com/tech_t ... Tips_3.htm
Sorry for the length, but heat transfer and cavitation is a special interest of mine.
Doug
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