BOTH Supply &
Exhaust is best, but if only 1 fan,
Exhaust is essential. Install the Exhaust fan High, and the Supply air duct Low.
Air is required in
engine compartments, for two separate purposes:
- engine combustion
- engine room
cooling
- Engine compartment ventilation is NOT intended to vent noxious products of combustion, which should be exhausted through the engine exhaust.
An ideal system would provide fully mechanical ventilation - both supply and exhaust air fans.
As implied by Brett, the supply fan might be sized about 10% lower than the exhaust fan (less combustion air), to provide a slight negative pressure in the engine room, thereby preventing fumes entering the occupied spaces.
If engine compartment ventilation is provided by a single fan - it should be an exhaust (extractor) fa,, located in the upper 1/3 of the compartment.
For every 10° above 77° F, an engine may lose 1% to 1.5% of its
power output. So the engine room must be supplied with cool air, supplied as low down as possible (lower 1/3 of compartment), and exhausted out from the upper (1/3) part of the engine compartment.
Formulas for engine room air required for engine combustion, and for
cooling can be found in most engine
installation manuals. Hot climates may require as much as twice the ventilating (cooling) capacity as do temperate climates.