Assuming that the blower (Supply Air) and sucker (Exhaust) fans are roughly equivalent, and you have separate combustion-air ducting, you have a conventional balanced system.
If the fans provide combustion-air in addition to ventilation, you have a negatively pressurized system.
I prefer a slightly negative air pressure in the engine compartment (relative to the living space); which will ensure that pollutants don’t migrate from engine rm. to
cabin (living space).
See Caterpillar’s excellent paper on Engine Room Ventilation:
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http://www.gregorypoole.com/products...ENTILATION.pdf
See also, some earlier discussions:
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http://www.cruisersforum.com/forums/...-it-37561.html
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http://www.cruisersforum.com/forums/...ort-23264.html
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http://www.cruisersforum.com/forums/...tion-7269.html