[Bushwacker]

Excellent question! Manifold vacuum is directly proportional to cylinder pressure and is one of the most valuable gauges you can have on any 4-stroke engine! I'm amazed that more guys running these engines don't monitor it! The exhaust valve is the weak link in these engines and to avoid burning exhaust valves you should not run continuously at a vacuum lower than about 5"Hg. I run one in my truck and also observe the 5" limit when towing, dropping it out of overdrive whenever it won't hold speed at 5". The higher the vacuum, the longer the exhaust valve will live. This a very sensitive measurement which can also detect sticking valves, misfires, retarded timing, etc. Good to have as an indicator of engine health/performance in addition for setting cruise power.

The rationale for this limit comes from the aircraft engine industry. The piston aircraft engines like Lycoming and Continental were designed with a "max continuous" power rating that was about 75% of full power. These are expensive heavy duty engines with sodium filled exhaust valves, stellite valve seats, and positive valve rotators that very few marine engines have. If you run through the thermodynamic calculations, 75% of max power works out to be about 5"Hg manifold pressure. I was an engineer with Pratt & Whitney for about 35 years and all the guys I worked with that ran inboards or I/O's all ran them to manifold vacuum, and always cruised at 5" or higher. Frank Brown, who is an outstanding local mechanic that used to crew for Carl Moesly when he was racing the SeaCraft 21's in the offshore powerboat races in the early 60's, said Mercruiser also used to recommend 5" as a minimum limit for a continuous (cruise) power setting.