Cold Air and a Warm Manifold

Some of you might think a heated intake might be contradictory with a cool fresh air supply. It isn't. Cooler air supplied to the carb is better for power because it is denser. Once the cool air picks up fuel from the carb, some of the fuel will vaporize (ie, become gaseous fuel) and some of the fuel will atomize (ie, become very small droplets of liquid fuel). When the air stream makes a sharp turn from going straight down from the carb to horizontal in the plenum, some of the atomized liquid fuel won't make the turn and will puddle on the floor of the plenum. The liquid fuel will then work its way along the bottom of the intake manifold to the cylinders to end up in the crankcase. A hot spot on the plenum floor evaporates the liquid fuel so that the vaporized fuel will mix back into the airstream  heading to the cylinders for combustion. Cleaner oil and longer engine life would also be a benefit from intake manifold heat.

Keeping the heated air supply to the carburetor from the temperature-controlled air cleaner will help with fuel vaporization under cruising conditions. This device utilizes a hot air shroud around the exhaust manifold and a duct to lead warm air into the air cleaner. A thermostat in the air cleaner maintains (or tries to maintain) a constant temperature to the carburetor. Although this type of air cleaner is somewhat more restrictive than a open element air cleaner, your gas mileage will probably be slightly higher with it. This type of air cleaner shuts off the heated air when the engine is accelerating (low manifold vacuum ~ 5" Hg) and this is the time when the hot spot in the intake manifold is especially useful.

People sometimes read information and only pick out the parts that justify their point of view. Getting cold air into an engine should not be an end in itself. The true goal should really be to increase the horsepower of your engine or get better gas mileage or both. Unless you really need to pack your engine with as much cold air as possible, like if you are going for a land speed record or you are running a high revving engine on the drag strip, heat in the intake manifold will help your street-driven engine run better.