I personally like the 550 engine ,440 pump combo it really brings the boat to life if you have the correct pitch impeller and some mods such as pipes,higher compression and a steeper pitch on the impeller.For a conversion boat with a 650-750 engine don't even think about the 440 pump it dosen't work that well with low rpm engines,the main reason People used to swap is that they were turning 10,000 rpm on the 440 superstock and 550 mod engines,the 550 pump was extremely inefficient at those rpms whereas the 440 was basically an rpm driven pump.The basic difference is the 440 pump is essentially just a straight tube,the water comes in straight and is compressed through the reduction nozzle in the back,you are only limited by rpms,the faster you spin it the more thrust you produce.The 550 pump is quite different the water comes in through an angled inlet and is compressed and leaves through an angled exit,it is a more restrictive pump design but it does produces more thrust at a lower rpm,it is more suitable to lower rpm engines because the bottleneck at the intake limits the amount of water it can flow.The pump in the 650-750 is basically just a bigger updated version of the 440 style pump,it does have its own shaft and bearing setup in the pump though.
