What is a plenum and why does it matter?

The plenum is the small reservoir of air between the regulator and the firing valve.

It holds the metered shot of air at the regulator's setpoint pressure, ready to be released the instant the valve opens. If the plenum is too small for the power level you want, pressure drops sharply during the shot and you lose energy. A general guideline among tuners is roughly 0.5 to 1 cc of plenum volume per FPE of intended power. Plenum volume is one of the main variables in a tune.