I'm writing this to help others understand how to size a pressure washer pumps. I recently purchased this pump and realized very quickly that the sellers had no clue on how to support technical questions. The specifications listed for this pump was its sold by Erie Tools, Triplex Pressure Washer Pump for Cat General AR, 1 inch hollow shaft rates at 4 GPM, 3000 PSI and a 13 HP engine required.What if you have a 9 HP Gas Engine with a 1 inch shaft will it work? If you contact the sellers they have no clue and will tell you it will not. This is not accurate but does require a little math to figure what will work and at what pressure.So let's assume you have a 13 HP gas engine. If I use the following formula GPM x PSI / 1450 x 1.5 and plug in the numbers on this pump 4 GPM x 3000 PSI / 1450 x 1.5 = 12.4 HP. That will give us a little overhead relief on the gas engine at max capacity.So now let's assume you have a 9 HP gas engine. If I use the same formula and change the nozzle to 3 GPM x 3000 PSI / 1450 x 1.5 = 9.3 HP. Since this is over our 9 HP gas engine we will also need to de-rate the PSI. So what if we use 3 GPM x 2800 PSI / 1450 x 1.5 = 8. 6 HP.To make this work you will need to purchase 3 GPM nozzles and a quality pressure washer gauge. Adjust the unloader on the pump to the lowest pressure, connect water hose, pressure gauge, pressure hose, turn on water, bleed air from pump and crank engine. Once the engine and pump have warmed up a little set engine to highest speed pull the trigger on your wand, "do not point at anyone". Slowly increase the pressure by adjusting the unloaded until the gauge reads 2800 PSI.If you ever use a different nozzle size you will need to readjust the unloader to match the HP. It's not rocket science or magic but math...