There have been concerns expressed lately, both here and on other sites, regarding increased engine wear associated with changing to a lower temperature thermostat than OEM specifications.
I copied this information off of Underdog Racing Developement's website. Even though it addresses the 3.4 V6 engine, I think the same theory should apply to any engine.
A LINK TO THE WEB PAGE FOLLOWS TEXT
What Thermostat do I use with my 3.4 L V6? Can I buy one from URD?
UNDERDOG RACING DEVELOPMENT recommends that you use a Stant 170 degree thermostat or a stock 180 degree thermostat in your 3.4 L 5VZ-FE. The Stant part is #14007.
Although we supply a 170 degree thermostat with our Fuel Kits, we are not selling the thermostat by itself. They are readily available from NAPA and Advanced Auto Parts. We donít want to overcharge you for an item that is easily picked up by you for less.
When engine coolant temperature is lowered in the supercharged 3.4 liter 5VZ, power marginally improves. You likely wonít feel much power gain in the seat of your pants.
There is some misinformation out there as to why a cooler thermostat increases engine performance. Some people think a cooler thermostat makes the engine run richer. It does not. It heats the air entering the engine less creating a denser fuel/air charge thereby developing more power. Cooler induction air is less prone to induce spark knock so the ECU can run more aggressive ignition timing resulting in greater power.
UNDERDOG RACING DEVELOPMENT experimented with two lower temperature thermostats available for the 3.4 V6 engine; the pricey TRD 160 degree thermostat, and the Stant 170 degree thermostat.
We have three minor concerns about the 160 degree thermostat. They are: Engine wear (very minor concern), fuel economy, and cold weather driving. Hot engine coolant enables the heater to keep occupants warm. How much power gain is worth freezing your bottom? NONE should be the correct answer for most of us. We have tested both units. The 170 degree thermostat keeps us nice and toasty. The 160 degree thermostat never gets hot enough in very cold weather.
Modern gasoline requires higher engine temperatures to burn efficiently and provides the best fuel economy. The correct water temperature is required for cylinders to achieve a minimum specific temperature in order to allow a fully homogenized air/fuel mixture to combust efficiently. The 170 degree thermostat reduces actual coolant temps from just over 190 degrees on our test vehicle to 180 degrees. Lowering coolant temperature in the engine any further may cause poorer fuel economy and reduced engine life through additional wear. There are better ways to make extra horsepower in our opinion.
In summary, UNDERDOG RACING DEVELOPMENT believes the Stant 170 degree thermostat increases engine performance, and at the same time it does not negatively impact fuel economy or potential accelerated engine wear. It is a cheap upgrade. Heater performance is also minimally affected. This is the thermostat we have decided to run in our own trucks. You can find it for around $13 at your local auto parts store.