Thread: P38 coolant woes

In terms of fluid resistance having no thermostat is not much different to a thermostat in place and fully open, especially when considering the resistance along the entire fluid path of the block, hoses and radiator which is what largely determines the flow rate for a given pump speed.

The actual flow rate of water through the block is determined by engine/water pump speed and not thermostat position, the job of the thermostat is to essentially bypass the radiator when cold and progressively direct more water through the radiator and less through the bypass pipe as it heats up.

The removal of the thermostat (or it being fully open) won't cause any hot spots in the block, it just makes the cooling system remove heat as fast as it can, this means a cooler engine at all times and although less than ideal in the winter it really helps when working it hard in the summer.

In most places this isn't a good option but in Brisbane the winter is warm enough for the engine to run at about 70C and the summer is hot enough for it to run at about 90C. On an engine prone to overheating and self destruction this is a good thing.

Originally Posted by benji

Mmm, I cant really see that happening, but the restriction is important else in really hot weather water will flow too fast through the radiator and not have time too cool down.

The thermostat isn't there to restrict flow, although it does a little, it's a temperature sensitive valve to provide a gradual change in flow between the radiator bypass pipe at low coolant temperature and the radiator itself at high temperature.

The radiator is a heat exchanger, on the outside cool air moves over the vanes and becomes heated and inside hot water passes through the tubes and becomes cooled. When the engine is working hard the important thing is the amount of heat removed and not the temperature difference between water inlet and outlet. In a stable system (temperature gauge not moving) the heat removed by the radiator is essentially equal to the heat generated by the engine.

Now, for any given flow rate of cool air through the radiator the amount of heat removed overall is directly proportional the difference in temperature between the cool air and the average water temperature throughout the radiator. More heat is actually removed at the top of the radiator where the water is hotter than at the bottom.

If the water travels slower it cools more in the radiator but then also takes longer as it passes through the block so it is hotter when it re-enters the radiator at the top. For faster flowing water there is less temperature drop through the radiator and also less increase as it passes through the block. In both cases the average water temperature in the radiator will be the same.

Removing the thermostat (and bypass pipe) will always make the radiator cooling system run as efficiently as it can, this is great on hot days for an engine prone to overheating but the downside is that on cool days the engine won't reach its most efficient operating temperature. As I live in a warm climate it works well for me, for others it may not