It is also important to take into acount the volume of the output hose from the muffler to the top of the loop. The volume of the muffler should be large enough to accomodate water equal in volume to the volume of that hose segment since that water drains back into the muffler when you shut the engine down. Having the muffler located so its top is 12" or more below the injection point provides an extra safety margin.