Diesels run unthrottled intakes, manifold vacuum is going to be a bit less than atmospheric but shouldn't be way less. In a turbocharged diesel the manifold is pressurized after all. How did you arrive at the 4% figure? If an engine is using 0.4 pints/hour/hp (a good rule of thumb) then 60 hp requires 3 gallons or 18 lbs of fuel an hour (rough figures). At 1:14 fuel/air that is about 3400 cu ft/hr or 1 cu ft/sec consumption. I ran these calculations when I built my boat because the engine room is pretty much sealed other than one snorkeled air intake. I worried it would get way too hot in there. But there is enough air pulled in by the diesel intake to keep things pretty cool. It heats up on shut down since the air pump stops.