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.
When attempting to determine the correct orifice size use a nozzle selection chart with your machine specification of GPM and PSI. You can also look at the side of the nozzles provided with your pressure washer for a number sequence. On MTM QC nozzles, for example, you will see 25030 which translates to "25° spray pattern with a 3.0 orifice."" If you were to see this sequence, you would select the 1.1mm nozzle (3.0 orifice) for your foam cannon as it is a closer match to the required specs of your pressure washer.