I am no expert here... anybody know where vacuum pressure should be on a 454?
Got a friend with a custom 28' cruiser with a 330 h.p. Mercruiser 454, and a TS outdrive that throws a large prop. He doesn't want to take the engine over 2600 r.p.m. and is super cautious about vacuum pressure being low. Long ago the boat was known to do 4000 - 4400 r.p.m. at about 40 m.ph., but someone said low vacuum pressure was a bad sign, and when he revs, the vacuum pressure falls, so he won't push the r's. So the low rpm and big prop lugs the engine and outdrive, I think to the detriment of the outdrive. The boats gone through prop hubs pre-maturely, and it may be too much lug on the prop/drive.

Sorry, I meant a 454 with a TR outdrive (not a TRS).
Any help much appreciated!

minslee2

I think your friend is confused. Low vacuum AT IDLE could indicate a problem. It is supposed to drop as you throttle up the engine. Too big a prop, resulting in only 2600 rpm max, will prematurely destroy the engine.
See attached:
http://www.marinemechanic.com/site/page55.html

Vacum at idle should be somewhere between 17 and 21 inches of mercury. Vacum on a loaded engine at WOT should be approximately 0 (zero) as the pressure under the carburetor throttle plates nears atmospheric pressure. Low vacum at idle can indicate a worn camshaft, worn(stretched) timing chain,
bad valve sealing,worn rings or just mal adjusted ignition timing. A rapidly fluctuating reading can indicate a burnt valve or broken valve spring. Check it at idle and if it ok, tell him to forget about it. Reading 0 under throttle is ok.

Vacuum drops as you open the carb butterflies and allow atmospheric pressure to equalize. Completely normal. Fuel consumption tends to go up as manifold vacuum goes down.