When an engine is cold, it runs better with a rich mixture (more fuel, less air). Years ago, the choke pinched off the airflow to the cylinder (hence the name “choke”) and was operated manually. Later it was operated automatically (often turning off too soon and causing the car to stall a few minutes after starting). In modern cars, there is no choke, just the computer controlling the fuel and air mixture on a moment-by-moment basis.
I’m not sure that it is better to put premium gas in your car during the winter. If you car operates properly on regular during the summer, then it should also operate properly on regular during the winter. Actually, summer gasoline and winter gasoline are slightly different. When it’s cold outside, gasoline doesn’t evaporate as easily so it needs to be reformulated to make it more volatile. During the winter, the gasoline manufacturers add more small molecules to the mixture so that it turns into a gas more easily. But they try to retain the same resistance to ignition in each of their gasoline grades. In any case, gasoline doesn’t freeze at normal temperatures, so that’s not a problem. Only water that condenses in your gas tank will freeze and can plug your gas line.
A muffler’s job is to control the flow of exhaust from the cylinders to the outside air, so that the abrupt fluctuations in pressure created by the opening cylinders are smoothed away by the time the exhaust leaves the car’s tail pipe. The pressure fluctuations create sound and, by smoothing them away, the muffler quiets the engine. But the easiest ways to smooth away the pressure fluctuations also impede the flow of exhaust from the cylinders. The result is that some exhaust is trapped in the cylinders and interferes with the operation of the engine. The car’s gas mileage drops. A good muffler smoothes out exhaust pressure without impeding its flow and without reducing gas mileage.
According to David Ingham, a nitrous kit is a system that injects nitrous oxide into the air intake. This technique was developed during WWII as a way to obtain short bursts of extra power from gasoline engines. Keith Spillman points out that the nitrous oxide is injected as a dense liquid so that it greatly increases the number of oxygen atoms inside the cylinder at the moment the fuel ignites. Since nitrous oxide breaks down into nitrogen and oxygen at high temperatures, it supports combustion and allows more fuel to burn during each engine cycle. The engine thus produces more power. The liquid nitrous oxide also provides an “intercooling” effect when it evaporates—it cools the gases in the cylinder prior to compression so that there is less possibility of knocking.