Originally Posted by 64Joker
Hypothetically, if the thermostat does not open until 185 degrees, and the water pump works continuously prior to the thermostat opening,what prevents the water in the radiator to flow out and empty into the engine prior to the engine warming up?
The bypass routes coolant around the thermostat but not through the radiator. This allows the coolant to circulate around and around inside the block and heads warming the coolant while leveling the internal metal temperatures much faster than if it always went through the radiator. It also prevents dead heading the pump.
Everybody's engine does this in some manner of design. Chevrolet's system routes from the return passages at the front of the intake to the heater core if not equipped with air-conditioning and through a 3 way valve that shunts coolant around the heater core when the AC is on so it doesn't have to cool the heated heater core. The return is from either the heater core for non AC models or the three way valve if it has AC to the suction side of the pump. The return is classically into a fitting on the top of the pump that feeds to the impeller inlet but there some other arrangements where the return hose feeds into the pump inlet side of the radiator tank or into a tee of the pump inlet hose. Everybody uses some configuration of a system similar to this. Some isolate the heater circuit from the bypass running the bypass hose directly from the intake return sourced behind the thermostat into the pump, Ford and Chrysler are examples of this arrangement.
The purpose besides providing faster cabin heat on cold days is to prevent hot spots from forming inside the engine and eliminate pump cavitation before general circulation starts with opening the thermostat. Hot spots mostly form around the exhaust seats and spark plugs; the metal immediately around these parts can become so hot they boil the coolant while everything else is cold. When this happens the hot metal tries to expand with the increasing temperature but is trapped by the colder metal surrounding the hot area, this can and does lead to cracking the casting. Keeping a circulation prevents the formation of local hot spots while the general warming of the circulating coolant tends to level the temperature throughout the castings so hot spots aren't trapped when surrounded by the cooler metal further away from the concentrated heat source.
In terms of cavitation; when the pump is dead headed, it beats the coolant into froth. Bubbles do not offer any cooling to hot engine parts, they only block flow and encourage further localized overheating. Additionally, the froth within the impeller beats on it shock loading the pump shaft, its bearings and seal all of which reduces the life span of these components.
There are those who argue that the constant flow of hot coolant into the mainstream after the thermostat opens contributes to overheating. The engineers that designed this system took that into account. Actually the LT1/4, the LS engine series and most other current production engines not to mention those of heavy trucks and equipment have for many years if not decades used a two part thermostat to feed hot coolant from the engine into cold coolant from the radiator to develop a tightly controlled temperature within the engine for best power and maximum efficiency. The modern Mustang V6 couldn't get to 300 horsepower and 30 miles to the gallon with many tricks this being one of them. I would suggest that overheating problems are better solved with not under diving the pump on street engines, and using a clean and properly sized radiator along with adequate fans and shroud to pull air over the entire core surface.
For straight up competition engines the thermostat is often eliminated as it can become a potential failure item that will take you out of the race. So a simple restrictor is often used to manage the full power operating temperature, which usually proves to be more than necessary cooling at slower speeds. This also eliminates the external plumbing of the bypass system without incurring the problems of no circulation as would happen with no bypass and having a thermostat that is closed when the engine is cold. The restrictor will cause the engine to be slow in initially warming up and the temp will tend over a wide range between idle and screaming around the track. This is less of a problem for racers as they are in the engine for maintenance a lot and the big time racers are using after-market castings which whether iron or aluminum are much more robust than is OEM production hardware.