Originally posted by 69Boss302:
At lower temps the fuel burns more efficient. Greater efficiency yields more power. Keep in mind that going to cold, say 140 or 150 deg. t-stat can actually hurt power. Ever notice that on cool days the car runs better and pulls harder? That's why.
You are mistaken here. (well except a colder thermostat usually hurts power by itself)
A lower temperature engine will burn fuel less efficiently.
On a colder day the reason your car feels stronger is "colder, denser air"
The car is computer controlled. Therefore it will always try to get up to the operating temperature preprogrammed in the PCM (180 degrees for the Duratec BTW ~ 192 degrees for the Zetec)
Before it reaches operating temperature it's in open loop (limp) mode. So it's possible a colder thermostat will keep the car from reaching operating temps in colder weather. (that's very bad)
Also the longer it takes to warm up the more wear your engine sustains.
The reason the hypertech calls for a lower temp thermostat is the fact it programs the the PCM to accept it and also the lower temp engine can accept more timing advance. This used in conjuction with the cooler operating temps is where the power comes from. However in the summer you are still at the mercy of your cooling system and said advance can still cause detonation.
A thermostat WILL NOT make your car any cooler. It just opens at a set temp to allow the engine to warm up to said temperature sooner & help keep it there in colder weather.
It also acts as a tool to slow down the coolant flow through your engine so it has more time to draw the heat out of it. This is also why running no thermostat can actually make your car run hotter.
(think of a 2-stroke on a 'damp' morning!)