Engines have been around for a long time -- longer than some of the sciences that explain how they actually work. In the early days, figuring out the size, shape and design of things like intake manifold runners was mostly a matter of trial and error. Modern understanding of things like fluid dynamics and acoustics have solved some of the puzzles of yore, and allowed us to start tuning engines to be the best that they can be.
Engines with more than one cylinder need a way of distributing air -- and fuel, in the case of carbureted engines -- from a single inlet to the various cylinders. Typically, air will enter through a single hole and go through a throttle valve plate that can open and close to control the amount of air that goes in. On a carbureted or throttle-body-injected engine, the fuel goes in right before the throttle plate, so the plate passes both air and fuel. After the valve, the air -- and sometimes fuel -- goes into a central holding chamber called the "plenum," which acts as kind of a reservoir.
A series of tubes or channels in the manifold distribute the air-fuel mixture from the plenum chamber to the individual cylinders; these are the "manifold runners." Carbureted engines need very straight runners, because the fuel droplets in the air don't like to go around corners. The length, inside diameter, volume and shape of the runners are absolutely critical in terms of power output, and where in the rpm range the engine makes that power.
Air pressure going into the engine through the runners bounces back off the head's intake valves when they close like a spring. The pressure waves shoot back up to the end of the runner, and come back down; each of these bounce cycles is known as a "harmonic." If you open the intake valve at just the right time, you can catch the pressure wave on the way back down; the pressure waves shoves air through the valve, much like a supercharger.
But this supercharging effect only works in a certain rpm range, and the pressure waves can bounce back and forth two or three times before the valve opens again. The length of the runner determines how long the harmonic takes to get back to the intake valve. For this reason, long runners will "supercharge" the engine at low rpm, and very short runners will do it at high rpm. The height, width and volume of the runner determine how much air can go through, which dictates top-end horsepower.
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