Re: water flow GPM per HP of an engine?
Forgot to add in the 'slab rust' post. The LAST thing you want to do with such an engine that has the potential to develop 'slab rust' is to use a 'straight' acid to remove any possible scale if it is so fouled. The acid will quickly turn the ferrous deposits into ferric deposits ... and then you have to begin all over again with 'heat soaking' the engine back to 'ferrous'.
Such engines should really only be descaled with inhibited commercial boiler descaler compounds ... such as Ryd-Lyme and once done the engine still should be run hard and long to redevelop the protective black ferrous rust.