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By way of background, the original late model factory solution was to use double-action thermostats which start to open and allow some coolant through the block and head as engine temperature moves past 140 degrees, and they then close off the bypass port in the thermostat housing when the engine temperature reaches 180 degrees. In standard late model bypass systems, engine temperature is free to vary between 140 and 180 degrees, depending primarily on the condition of the cooling jackets, temperature of raw water, and the load imposed on the engine.
Double-action O.E.M. thermostats have become very expensive, and (although still listed in our online catalog) are becoming problematic to keep in stock. We eventually joined others several years ago in offering an aftermarket thermostat that functioned like the O.E.M unit until the source of those thermostats disappeared in the past several months as well.
At that time we concluded that the most cost effective and reliable solution was to incorporate a single-action thermostat into our kits, using a separate method of regulating the bypass loop. The separate method of providing a bypass restriction for single-action thermostats (to finally answer your question) is because they have no ability to regulate the bypass loop as temperature increases past 180 degrees.
On our test stand, a spring loaded check valve providing 2 to 3 psi backpressure maintained normal temperature in a raw water cooled configuration using the single-action thermostat provided in our kits (CSTH_01_510). In freshwater cooled engines it’s frequently necessary to fully close the bypass loop when operating at high power settings in warm climates so a manual ball valve is recommended in those cases as well. In an effort to cover both applications, we currently provide a spring loaded check valve and a small manual ball valve in our by-pass restriction kits (product number CSOT_01_61).
It should be noted that the need to provide a restriction of some kind in the bypass loop is not limited to the single-action concept. Universal themselves realized that unless their bypass type system had a restriction in the bypass loop, many engines would operate hotter than desired. In a technical bulletin from the mid-seventies, they recommended that the 90 degree elbow in the inlet to the thermostat housing be filled with lead and then re-opened by drilling an 1/8” hole in the lead. That technical bulletin was largely ignored in favor of more practical solutions and many Atomic 4s today are already equipped with either a manual ball valve or a spring loaded check valve in the bypass loop.
Don
Double-action O.E.M. thermostats have become very expensive, and (although still listed in our online catalog) are becoming problematic to keep in stock. We eventually joined others several years ago in offering an aftermarket thermostat that functioned like the O.E.M unit until the source of those thermostats disappeared in the past several months as well.
At that time we concluded that the most cost effective and reliable solution was to incorporate a single-action thermostat into our kits, using a separate method of regulating the bypass loop. The separate method of providing a bypass restriction for single-action thermostats (to finally answer your question) is because they have no ability to regulate the bypass loop as temperature increases past 180 degrees.
On our test stand, a spring loaded check valve providing 2 to 3 psi backpressure maintained normal temperature in a raw water cooled configuration using the single-action thermostat provided in our kits (CSTH_01_510). In freshwater cooled engines it’s frequently necessary to fully close the bypass loop when operating at high power settings in warm climates so a manual ball valve is recommended in those cases as well. In an effort to cover both applications, we currently provide a spring loaded check valve and a small manual ball valve in our by-pass restriction kits (product number CSOT_01_61).
It should be noted that the need to provide a restriction of some kind in the bypass loop is not limited to the single-action concept. Universal themselves realized that unless their bypass type system had a restriction in the bypass loop, many engines would operate hotter than desired. In a technical bulletin from the mid-seventies, they recommended that the 90 degree elbow in the inlet to the thermostat housing be filled with lead and then re-opened by drilling an 1/8” hole in the lead. That technical bulletin was largely ignored in favor of more practical solutions and many Atomic 4s today are already equipped with either a manual ball valve or a spring loaded check valve in the bypass loop.
Don
[water flow out the exhaust seems normal]
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