there's so much confusion on internet forums over how a thermostat works, that i've decided to do a blog article, strictly dedicated to it. been seeing a trend of people asking for help on diagnosing overheating issues, and theres always a handful of people that for some reason believe that a lower temp thermostat will make a car overheat. this is absolute nonsense, and ill circle back around to that in a couple minutes. take note, if you've read most my other articles, then you will notice, that i'm going to be repeating myself a lot on this one. so far, i've tried to avoid posting articles like this about generic information that isnt specific, or centered around the nailhead engine family in general. hopefully this is one of the few exceptions to the rule. please check the "diagnosing overheating issues" article for much more information, which is located many articles down the main page.
a thermostat does two things. it insures that the coolant temp does not drop below the specified temp once it reaches it, and it adds a restrictive small hole to drastically slow the speed of water. despite popular belief, YES you can pump water too quick, which will drastically hurt cooling. a thermostat MUST be equipped on every street engine. if you leave it out, the engine will run too cool, until temp "catches up" and then it will over heat, as there's no provided restriction via small hole in thermostat, to slow the speed of water down.. it takes X amount of time for water to absorb heat from the block and X amount of time for air to dissipate heat from the radiator. the factory spent countless time and money engineering the system. it worked when it was new and it works just the same 60+ years later,. if it flows too fast, then it wont have time to do either. if the engine never reaches operating temp, whether thermostat is stuck open, or someone used a racing application restricting washer to take its place, then the engine will receive premature wear over time. your engine is designed to run at that specified temp when metals have expanded,. late model modern cars are designed to run even hotter, as we've found hotter running engines are much more efficient all around.
take note that i stated that it insures that the temp does not drop below the thermostat specified temp once it reaches it. it does NOT insure that it stays at that specified temp. many people for some reason believe the thermostat pulses in alternating cycles or something. even been told "a 160 thermostat doesn't hold the water in the radiator long enough, compared to a 180." huh???? all you have to do is put a thermostat on the stove in a pot of hot water with a thermometer to visually grasp what it does. bring the temp up above the rated temp, and then let it cool off again. watch what it does,. think of it like the heater in your living room. if you set the thermostat/heater in your house to 75 degrees, for instance... does it guarantee that is stays 75 in your house once it reaches it? of course not. it very well might go higher if its a hot day. BUT, if the temp in your living room tries to drop below 75, then the heater will kick back on to maintain that selected temp minimum.
until the engine reaches the specified thermostat temp, the water circulates through the engine, from water pump, to back of block through jacket, before going up the the cylinder head in the rear on both sides, and then coming forward to the thermostat housiug. from that point, the water get pulled down through the bypass and continues the cycle. this bypass circulation helps to keep the system from developing hot spots in specific areas, and aids in keeping the system from building extra pressure. the hottest point the water gets in the cooling system, once the thermostat opens, is right before it reaches the thermostat. the temp difference from the back of the head to the front of the head is typically 15 degrees hotter in the front, so if you have a 59 and earlier car with temp guage reading from the back of the head, its not going to be as accurate. once the water reaches that specified temp, the thermostat begins to open allowing water to now flow through the radiator, exiting the engine from the upper hose. from that point, the thermostat now stays open UNLESS the temp tries to drop below it. the lower the temp thermostat, the sooner the cooling system activates, and the cooler the engine can/will potentially run. different engines run cooler than others. (for more about that, please read the other cooling system article ive written.) on perticular custom applications, consisting of many variables, a 180 thermostat might encourage the engine to run 195 all the time, for instance, which might get much higher than that on a hot day, past whats "comfortable." ... and then after putting a 160 thermostat in it, the engine runs 180, 24/7. fresh street engines also run hotter when they are brand new via extra friction from break in, so i always run 160 thermostats in them to compensate for that. operating temp on nailheads is ideally between 180 and 190. if your car gets 200+ once in a while, its not too concerning unless it always that hot. the factory idiot light didnt even come on until 235 degrees. i always run a 160 thermostat UNLESS the engines runs too cool and never reaches that operating temp. typically that's only an issue with the 322/264 and occasionally the 364, where after break in, i've actually had to put a 180 in it to get operating temp to come up higher.
pictured below are both a 160 thermostat and a 180 thermostat. take note they are essentially identical, aside from their specified heat range in which they are designed to open.
i'll also add, that water cools better than coolant. its a double edged sword however, because coolant raises the boiling point. running 50/50 green is what you want for a balance of corrosion protection and efficient cooling. stay away from dex cool, which has piss poor corrosion inhibiting elements.. run as high of a radiator cap pressure rating, as the radiator will allow. the higher the pressure rating, the higher the boiling point becomes.