Sprinkler valve modification
This refers to modifying a sprinkler valve to actuate via a pilot valve with a higher flow than the solenoid—typically a blowgun, although larger valves (1.5-2" and up) require something with more flow to preform well. The larger vent decreases opening time, and does not require batteries like the stock solenoid. A step by step tutorial of the process is available at Spudfiles
The faster and higher flow the pilot valve, the faster the actuated valve will open. This means that performance can be increased by using a QEV, which offers the additional benefit of eliminating the human variable. This has been shown to yield muzzle velocities as much as 2 times as consistent.
Another modification that can be made is enlarging the pilot leak hole. If the sprinkler valve is opened it contains a large diameter rubber disk (called the diaphragm). About 1/2-1" in somewhere around the diaphragm (in a often used Orbit Watermaster sprinkler valve) will be a small plastic piece poked through the diaphragm. If this piece is removed, the pilot area above the diaphragm will be pressurized instantly while the cannon is being filled, and so the valve will be more sensitive and will (in some instances) seat better.