Originally Posted by joedoh (Post 6214349)

people who talk about hot air in an engine compartment usually arent really clear on what that means, they have heard some explanations that make sense and then repeat them. What people are usually referring to is the pressure/volume/temperature chart that says that colder air is denser at the same pressure, and since we know engines run better with denser air (been to the mountains and seen a decrease in performance? been in cold weather and seen an increase in performance? but dont confuse barometric pressure for anything said below, that is a system pressure change that has more effect than the temperature change) they think the "colder" air outside the engine compartment will be significantly denser than the air inside the engine compartment. the problem with this theory is the effect of temperature on density vs the effect of pressure change on density.

since the air pressure is exactly the same inside the engine compartment and outside it, the density change has to be based only on temperature difference. it takes a HUGE amount of temperature change to affect density the same as even a very small increase in pressure. we also know this is true because even small amounts of boost (3-5 psi, a 25-50% increase in pressure at sea level) from turbos or superchargers affect the power of an engine dramatically. thats what forced induction does, changes the density of air in the cylinder by increasing the pressure. you get a higher effective compression ratio.

engine compartments seem like they should be extremely hot places, after all when you pop the hood after driving it sure feels hot in there. but with the engine fan turning, and especially once you are moving, the engine compartment air is only slightly hotter than the outside air, and there is no pressure change inside vs outside, so the density change of an intake outside the engine compartment vs inside is very very very small. I would be lying if I said it wasnt any change at all, because it is there, but it is very small. Couple that with the fact that internal cylinder temperatures are very high, and the inefficiency of heat transfer by convection, especially with smooth metal surfaces like a cylinder, means the slightly cooler charge will be affected more by the lump capacitance (storage capability of heat in a large dense chunk of metal) of the hot cylinder components than it could possibly affect the cylinder to cool it off.

that's very technical, think about getting a pan very very hot and pouring water into it, unless you pour in at high gpm, the pan will convert the water to steam instantly, raising the water temperature quickly without lowering the pan temperature at all. Its an even simpler concept to imagine as running your air conditioning in a truck that has been sitting in direct sunlight for hours and hours. It takes a long time, usually longer than your drive home, to cool off those interior components that are very hot, even though air conditioning air is sometimes 90 degrees cooler than the surface temps. That is lump capacitance, and convection, explained simply.

most of the improvement people feel with an intake tube is the restriction reduction and flow improvement. and the sound, that really makes it seem like you are thrashing around at 10/10ths.

cold has its place for intake cooling, forced induction motors benefit greatly from cooling that intake charge, because the pressure change of the turbo or supercharger heats up the air a lot (see a PVT chart!), an intercooler will cool it back down slightly, but an intercooler is not usually used for density increasing reasons, but because really hot air at really high pressures can cause fuel to detonate (see "how a diesel works"), sometimes before it should (see "knock" or "playing a really expensive percussion instrument")


sorry for the book. worry less about cold air and more about improving flow as some guys have said. Keep your headlights mounted where they belong. You will see a small benefit if you slug along in 2mph traffic and have the intake routed outside, because the lower speed wont bring fresh cool air into the engine compartment.