The amount of water that can flow through the OPEN tstat is depend upon a lot of things, but WHEN it started opening isn't one of them. Not for a fully OPEN tstat.
The only thing I can figure is these tstats are built different so they allow either MORE or LESS water to flow (not sure which would help!) when fully open than stock tstats do. That is the only way I can see them impacting the ability of the system to cool and not just the lower bound on the water temperature.
I don't know if it makes more sense that since more fluid is flowing through through the system that doesn't allow the fluid flowing to heat up as much? But the earlier the tstat is fully open does matter..
Here's what I was reading about them...
The thermostat has two important jobs to perform; to accelerate engine warm-up and to regulate the engine's operating temperature. A quality thermostat ensures excellent fuel economy, reduces engine wear, diminishes emissions and blow-by, improves cold weather drivability, provides adequate heater output, and detours overheating. This is accomplished by blocking the circulation of coolant between the engine and radiator until the engine has reached its predetermined temperature. The thermostat then opens as required in response to changes in coolant temperature to keep the engine's temperature within the desired operating range.(1.)
Thermostats have a “rated” temperature such as 180F or 195F. This is the temperature the thermostat will start to open, give or take 3 degrees.
Usually located within a metal or plastic housing where the upper radiator hose connects to the engine, most of today’s thermostats utilize the "reverse poppet" design, which opens against the flow of the coolant. Thermostats have a wax filled copper housing or cup called a "heat motor" that pushes the thermostat open against spring pressure.
As the engine's coolant warms up, the increase in heat causes the wax to melt and expand. The wax pushes against a piston inside a rubber boot. This forces the piston outward to open the thermostat. Within 3 or 4 degrees F. of the thermostat preset/rated temperature which is usually marked on the thermostat, the thermostat begins to unseat so coolant can start to circulate between the engine and radiator. It continues to open until engine cooling requirements are satisfied. It is fully open about 15-20 degrees above its rated temperature.(2.) If the temperature of the circulating coolant begins to drop, the wax element contracts, allowing spring tension to close the thermostat, thus decreasing coolant flow through the radiator.
On some applications, the thermostat performs an additional function. It closes off a bypass circuit inside the engine when it opens the radiator circuit. The bypass circuit circulates coolant inside the engine so that hot spots can’t form when the radiator circuit is closed.
