engine compartments must provide sufficient volume of air for the engines(s) to breath and must vent enough hot plus fumes. So its double duty. The bigger cubes of an engine will always require more incoming air and vent more hot air / fumes.

Pressure differential of outside engine compartment versus inside engine compartment should not exceed 2 in (51mm) of water measured with a manometer at max air flow rate.

2" of negative PSI, correct BUP?

i totally lost track of this thread, and started searching for it after i saw this thread

http://www.offshoreonly.com/forums/g...rving-air.html

you guys are awesome i appreciate the input

6 yeah, i'm talking about the harwood scoop on the hatch that faces forward, there is no relief vent as you call it, that's exactly what i'm talking about
the only vent is on the transom that the bilge blower is connected to. and i'm looking for a way to get air out, or (traveling thru) the engine compartment. enough to feed the engine and keep the engine room cool

post 21 bup this is exactly what im talking about but i don't understand the 2 number or how to achieve it

i like those naca ducts and think 2 of them on the engine hatch facing backward would let air flow thru the engine room, problem is almost every video i see turbulent air/ water mixture on the transom so the issue of placement is another factor.