Fish,
That's a good question but not an easy one to answer! I guess it boils down to "How often do you want to do valve jobs?!" The exhaust valve is definitely the weak link followed by the exhaust valve seats and heads. Failure mode would be loss of compression and power; worst case scenario might be the loss of head off a valve if you ran it long enough with a leaking seat. Sodium filled valves and Stellite valve seat inserts are the preferred improvements used the Continental/Lycoming aircraft engines.
The Lycoming O-360 used in the Cessna 172 has a TBO of about 2000 hours. A typical duty cycle probably consists of about 2 minutes of WOT operation at takeoff; climb out to cruise altitude would probably be done at 75% power or 5" vacuum. If you figure a typical flight lasts about 2 hours that would give you 1000 take offs/2000 minutes or 33.3 hours of WOT operation prior to overhaul. So a limit of about 2 minutes @ WOT might be a good starting pointAnd remember this is a premium engine WITH sodium filled valves and stellite seats! These engines have the ability to adjust mixture from full rich for take off and climb, to a leaner setting for cruise. They may end up cruising at 5"Hg absolute manifold pressure depending on cruise altitude.
A more practical data point comes from one friend of mine that I made several Bahamas trips with who had a 20 or 21' Wellcraft Nova with a 302 Ford/Mercruiser in it. He worked at Pratt and knew about the 5" vacuum limit, but he was not an engineer, so he didn't really understand the physics behind it. He installed a vacuum gage, and just set the throttle to 5" vacuum, and whatever speed that gave, that's what he ran! He basically ran 5" all the time! After about 300 total engine hours, not only were the exhaust valves burned, the seats were also badly burned and both heads were cracked! And this was on a Ford, which is generally considered to be more durable than the Chevy, since it does not have the siamesed ports and associated hot spots of the Chevy! He ended up pulling the I/O and replaced it with a 225 Merc hanging on the transom.
My other friend that owned the Moesly 21 "Unohu" was quite conservative and cruised and at about 9" vacuum. The boat came new with a 150 hp I-6 Mercruiser, and he did at least a couple of valve jobs on that engine before he learned about the importance of a vacuum gage. He subsequently upgraded to a 165 hp I-6 and then a 260 hp 350 V-8. He cruised at about 9" with the 350 Chevy and got at least 2000 hours out of it. A leaking exhaust manifold that put seawater in a cylinder finally killed it. Pistons shouldn't be a problem unless you lean out a cylinder and over heat the piston.
Don't know if this answers all your questions, but it should be a start. Denny
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