MTM Hydro Foam Cannon orifices are meant to allow foam cannons to be used on a variety of machines with varying PSI and GPM. These orifices (a.k.a nozzles) will more accurately accommodate a machine requiring a larger or smaller orifice which is estimated in the ranges of flow exceeding 4 gallons per minute or below 2 gallons per minute. The purpose is not to increase or decrease pressure but rather to match as closely as possible, to the recommended Orifice size for your machine. All MTM Hydro foam cannon models come with a 1.25mm (3.5 sized orifice) installed.