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most engines have a ranges of temperatures at which they work the best. Most engines do not like to run "cold". This is why you would normally have a "180 degree thermostat" and not a "100 degree thermostat". The engine needs to retain a proper amount of heat to run efficiently. Pull hot air into the intake helps warm the motor faster on cold days.
And if you weren't aware, many, many cars have an antifreeze pipe running through the throttle body or throttle plate to keep the TB warm on cold days. I've been told this is because throttle bodies would "Freeze". But thats more likely an old wise mans tale. Its there to warm that throttle up and keep the temps up during cold starts. Convection occurs really when there is large difference in temperature. So on very cold days, the hot coolant in the TB transfers to the air much better than it would on warm days. If you don't believe me, I've got a test for you to run with your morning coffee. :-p
This relates to why there are Smog pumps / Air Pumps / EGR Valves. While at full temp they recycle some exhaust gas, but in cold conditions they are "fully open" and recycle large quantities of unburnt gas.
It is true that cold air is more dense, and will give you more potential power. But you need to think of the overall systems at work. There is a reason you have mutliple sensors on the engine including Mass airflow sensors (MAF) / MAP ( Air Pressure) / Speed Densisty Sensors and air temp intake sensors, combined with your readings from the other bits like a knock sensor, oxygen sensors, transmissions readings with automatics, The computer in your car uses tables of numbers to determine what is the best Air/Fuel ratio to maintain, if it should shift, how much the emissions system should recycle, etc. Its basically what makes a modern car that much more complex.
But basically the idea is to get the car up the temp sooner, and on very cold days help to keep it running warmer. If you're new to tracking your gas mileage you will soon see those numbers drop off come the fall as the days get cooler.
I see that you live in Texas, so depending where you are, you may have never try to start a car at below freezing. Here in Pennsyltucky we aren't the coldest state, but we do get periods of time where the temperature will not break freezing.
Even that being said, there is still a fuel economy (FE) difference between 45 degree weather and 95 degree weather. That variance will be ~10-15%. Its much worse if you're driving short distances.
Also, in the cold we need to run the defroster, which takes an additional 10-11% off my Subaru's FE. So if its really cold, and I'm defrosting the windows, I loose 15% FE due to cold: 25MPG-> 21.25 MPG (this is accurate I get ~20-22 in winter) And then if I need to run the defroster it pulls an additional 10% off FE: 21.25 MPG -> 19.125 MPG. And this is about spot on with my worst tank last winter. Right after graduating school, and not working, I only drove short trips around town in the cold. The worst tank I had was ~18.7MPG.
And thats why I would like to build a hot box for the Subaru's intake!
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