I believe the intake pipe on a four stroke gets cold because of the alcohol in the fuel. It evaporates some on it's way up, and evaporation is a cooling process. Not sure because I really haven't studied it (although I have observed it a lot). However, speeding up airflow will not cool the air. It produces more friction between the molecules, which would actually act to warm the air up further. Velocity stacks are useful for a couple of reasons. First, they allow less of the fuel to be wasted by being blown out of the carb. You can attach an air filter to one, and even further increase this advantage. Secondly, they help keep the prop blast from sucking fuel out of the carb through venturri effect. They can also help promote more positive airflow into the carb..........