Hi and welcome to ribnet. They do breath a little at speed from the rocker cover, but then its a 4.2L 4 cyl engine with so each piston is on the large size and presumably some gas gets past the piston rings into the sump and vents out of the rocker cover. The engine design has been around a long time so whilst its not state of art, it can be easily repaired with a set of spanners. Not sure what the box does, mine just vents straight from the rocker cover to the back of the engine box.
The engine should rev to 2600 revs and in standard tuning give 26 knots with a 16.75 x 21" prop. However this is 140hp. If the engine has been tweeked then 165 - 180 hp is reasonably easy to achieve and should give 28 knots. Ian Austin achieved 28 knots like this. 200 hp is possible although a factory rebuild is likely to be needed and will just give 30 knots in light trim.
Standard fuel tank is only 80 litres which is a little small for cruising so consider replacing it with a 170 L tank from Vetus which is a good fit in the same space. We use about 1lpm at 20 knots and 1.5lpm at 24 knots.
There is lots of posts on Pacifics over the years on here so well worth doing a search.
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