Re: water flow GPM per HP of an engine?
The old Faryman and Universal atomic A4 EXHAUST manifolds are notorious for 'slab rust' which partly or 'occasionally' block the water flow. Only way to check for 'slab rust' is to remove the manifold go inside with a stiff wire and break up all the 'slabs' while the manifold is inverted.
Only way to prevent slab rust is to never ever allow the manifold to drain ... and you must run the hell of the engine so that the stratified cast iron develops an internal thick deposition of 'protective' Black/Blue 'ferrous' rust. If the passage is drained, Red 'ferric' rust will form and if it forms in the 'stratifications' (laps) in the casting the red rust will continue to push-the-casting-apart .... as 'slabs'.
When 'storing' any engine with a cast iron exhaust manifold or cast iron block the block/manifold should be filled with 50:50 anti-freeze with RUST INHIBITERS to retard the 'growth' of red/ferric rust. The US Navy hardly ever 'shuts down' an engine that is made of cast iron ... as soon as the temp. drops the ferrous rust begins to change to destructive red/ferric rust.