A private post asked me for more of an explanation, so I am adding it here in case others are interested. I will also add it to the submitted Tech Tip and the draft posted on Photobucket. Feel free to correct if I have stated anything in error.

Matt
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Coolant will flow from the water pump through the block to the thermostat housing. Without a bypass, it just stalls there until the thermostat opens. The thermostat acts like a flood gate. The exception is that the coolant will flow through the heater feed line in the lower housing if the heat control is turned toward hot. If not, the heater valve also acts like a flood gate. 

By adding the bypass to the lower housing, the coolant will flow back to the water pump and through the engine block in a constant loop even when the thermostat is closed. In the modified lower housing, it flows through the 3/8" nipple inserted through the bottom of the housing. It 'bypasses' the thermostat, hence the name. Since coolant is always in circulation, it eliminates hot spots and steam pockets in the water jacket caused by relatively stagnant coolant waiting to heat up.

When the coolant in the bypass loop gets warm enough, the thermostat will open. The coolant is circulating past the thermostat through the lower housing as part of the bypass loop. Without the bypass, the coolant surrounding the thermostat is more stagnant and relies more on convection for enough hot coolant to reach it to cause it to open. The circulation of the bypass causes the thermostat to be more efficient because it operates based on the temperature of the coolant circulating through the system, not just the temperature of the relatively stagnant coolant against the thermostat. In a non-bypass system, coolant in the engine block is much hotter than coolant adjacent to the thermostat in a running engine with a closed thermostat.

When the thermostat opens, the coolant in the loop has reached the thermostat temperature and now needs the radiator to cool it. The flow of the cooling circuit needs to change. Two things need to happen. First, the path of the of the coolant now needs to be past the thermostat (flood gate opens) to the radiator for cooling back to the engine via the water pump. Second, the path through the bypass circuit needs to be closed so that the radiator can do its job. This is accomplished by the disk at the bottom of the bypass thermostat sealing off the fitting that was added to the lower thermostat housing.

In a situation in which the thermostat is partially open, a hybrid loop needs to happen. Some of the coolant needs to flow to the radiator and some can continue through the bypass. That is accomplished by having both gates partially open. As the thermostat opens part way, the flow to the bypass becomes restricted as the base disk of the thermostat moves toward the new fitting. The thermostat acts like a mixing valve. This normalizes the flow better than with just a cycled open/closed thermostat in a non-bypass system.

The heater is mentioned in the first paragraph because the heater circuit can also act as a bypass. The heater control must be on hot or the heater valve will be close off the bypass loop. Moving the heater valve toward cold acts the same way as having the base disk on the bypass thermostat close off some or all of the flow to the bypass as the thermostat opens. The heater loop will function as a bypass regardless if the thermostat is open or closed. The heater core will also act to lower the temperature of the coolant. The bypass system does this all automatically. Plus, do you really want the heater on full blast in the middle of summer?

Did you ever hear that if your temperature gauge is getting too hot that you should turn your heater and fan on full blast? The reason is twofold. One is to get some coolant circulating just in case the thermostat is stuck. The second is to get the additional cooling effect of the heater core. It is not an old wive’s tale…it is true.


Last edited by Maybellene; 05/03/2016 3:33 PM.