Peak temperature in the combustion chamber is in excess of 5000 F. Aluminum melts at 1220 F, Iron at 1990-2300 F. Therefore, the obvious primary function of the cooling system is the prevention of component damage. However, spark ignition (SI) engines experience pre-ignition and subsequent detonation at temperatures much lower than those resulting in component failure. Poor cooling system performance results in component damage in SI engines but, this damage is a result of pre-ignition/detonation. Not the temperature alone. This secondary function of controlling pre-ignition/detonation is actually the most important in the SI engine. Coolant temperatures are not an accurate indicator of metal temperatures. The coolant's maximum temperature is it's pressure corrected boiling point. The metal can be several hundred degrees hotter than the adjacent coolant. Temperatures of critical areas must be determined by checking the metal at a controlled distance from the combustion chamber surface. This eliminates discrepancies caused by the variances in metal thicknesses.
Some have advocated replacing the mechanical pump with an electric pump. The Mazda 13B rotary requires about three HP to drive the mechanical pump at 6000 RPM and provide the coolant flow necessary to keep the metal temperature near the combustion chamber within bounds. The metal can be several hundred degrees hotter than the adjacent coolant. The flow rate required is on the order of 30 to 40 gallons per minute.
Higher coolant flow will ALWAYS result in higher heat transfer. Coolant cannot absorb heat after it reaches it's pressure corrected boiling point. Furthermore, coolant absorbs heat at a progressively slower rate as it approaches this point.
In the last 100 years there have been no breakthroughs in impeller shapes that would allow a significant improvement in water pump efficiency. On the contrary impeller shapes have been simplified to lower the cost. Here are some photographs of a second generation and third generation Mazda 13B water pump impellers.
Therefore it would also require about three HP from an electric motor driving a coolant pump to provide sufficient water flow. There are 746 watts in one HP so three HP is a minimum of 2,238 watts. Since the electrical system voltage is 12 volts and watts is equal to voltage times current it would require about 186 amps out of the alternator just to provide the required coolant flow. None of the after market car electric coolant pumps available claim that magnitude of current drain so obviously they cannot pump enough coolant to keep the metal temperature within bounds in a rotary engine generating 150 HP continuously. This is not just an opinion. It is based on the laws of physics. If one wants to reduce the water flow rate in the engine and take one's chances one must install thermal couples near the combustion chamber to monitor the metal temperature.
Electric water pumps work great in cars as 99% of the time the car engine is generating less than 40 HP. Here are some numbers. A car frontal area is about 25 square feet average. Height times width times 80% for rounded corners. Drag factor is C sub d and is .3 on average. Therefor the aero drag is .3 X 25 square feet times the dynamic pressure at 60 MPH which is six pounds per square foot. .3 X 25 X 6 = 45 pounds of aero drag. 60 MPH is 88 feet per second. HP is therefore 88 X times 45 or 3960 pound feet per second. Divide that by 550 to get HP in terms familiar to the public. 3960 / 550 = 7.2 HP. Double that for tire rolling drag and you get 15 HP. Assume the drive train is 80% efficient so we are looking at 19 HP out of the engine.
Here is another way of calculating it. Assume for the moment it is 20 HP and the BSFC is .5 giving the engine the benefit of the doubt. That means the engine is burning 10 pounds of fuel per hour. Fuel weighs 6.25 pounds per gallon so we have burned 1.6 gallons of fuel to go 60 Miles or 37 MPG.
One can change these key variable numbers anyway you want.
One can change the speed which will have the largest affect on the HP and MPG.
One can change the frontal area. Height times width times 80%.
One can change the BSFC in the range of .4 to .6. .4 being real good.
One can change the proportion of rolling drag to aero drag. 40% to 60%.
One can change the aero drag factor. A Prius is .26 and a Ford Model A is .5.
The bottom line is no matter how one manipulates the numbers one will still come out with a number of less than 40 HP. This is also the reason 300 to 400 HP car piston engines are very light duty indeed and do not make good aircraft engines.