BOOST BOTTLES AND CHAMBERS As a nitro engine runs and the crankshaft rotates, the crank valve?the window in the crankshaft underneath the carburetor?opens and closes. When the crank valve is open, air and atomized fuel run through the crank at a pretty high velocity; when the valve closes, however, the airflow is suddenly shut off, and that builds up pressure in the plenum (the area between the carburetor and the crankshaft). It's something like shutting off a water faucet very quickly. The water is flowing fairly fast, then boom!; the valve is shut and there is no place for the rushing water to go. You might hear the pipes clatter because a pressure wave is created by the rushing water's slamming into the closed valve. The same thing happens when the crank valve closes. A boost bottle, or boost chamber, effectively increases the plenum area and supposedly holds the built-up pressure in reserve until the crank valve opens again; then the fuel/air mixture that's trapped in the boost bottle is supposed to re-enter the plenum and, subsequently, the engine. In theory, a boost bottle enhances fuel economy and low-end power by trapping atomized fuel that would otherwise be wasted and then pumping it back into the engine. Opinions as to their effectiveness vary because they may be smaller than what is considered to be an ideal size.