Thermostats are just little valves that are held in place by a spring, a needle, and some other little magic pieces.
All it does is just control when the coolant should start to flow through the engine, and at what temperature it will fully be flowing.
Eg, if you have a 160 and a 180 degree thermostat, he 160 will just start opening up about the 160 mark, whereas the 180 will maybe will just start to open around the 180 mark.
anything over 160ish mark, the coolant will be running fully through the engine as the thermostat will be wide open.
the 180 thermostat won't start doing this action untill it hits 180+ degrees.
The manufacturer chooses the thermostat temperature to be "safe" as well as emission friendly.
it's like how our engines are tuned to be able to handle fluctuations in fuel quality. It isn't necessarily the best and most efficient tune, but it's the safest for the engine.
so yes, you have the potential to get better 1/4 mile times with a colder thermostat, but your car will take longer to warm up, your emissions will be worse, and if you choose a thermostat that is too cold, you will start to build excessive carbon in the engine, which with our DI engines, we already have to worry about that enough as it is on our intakes. This will also fowl spark plugs, and sensors, so it's only a matter of time for a check engine light if you chose the wrong thermostat.
if you should decide you want to change your thermostat, you will see best results if you add a higher quality coolant, or one more suited to the temps you plan to run.
for those of you sitting with a JB3 at home and wondering what map 7 feels like, but can't get 93 or higher octane where you are, a different thermostat might help with knock and detonation, but i would caution against it as i don't think anyone has tested this yet.
Unfortunately, i don't know how the BMW cooling system is setup, but i believe it's an electronic water pump? so it's controlled by the ECU, so any tampering with a thermostat might not really do anything, as it may use something completely different inside to help control flow. I'd have to do more research on that.
I'm sure there's stuff i missed, it's getting late, but i hope that helps answer some questions. If anything doesn't seem clear, just point it out and i will try to clear it up.
One more thing to point out as i don't think it sounded very clear while i was writing all that... the thermostat number is the indicated "cracking" point where it just starts to open, an engine will tend to run about 10-20 degrees higher then what is the indicated cracking point.
***I edited a couple things since the original post, kinda correcting myself as i go...
and i'm going to bed before i confuse more people
