Quote:
Originally Posted by cdavisdb
Anybody know if you can install a vacuum guage on a big outboard, and what readings would be normal?
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This is a good question, indicating Connor and Blue Heron have come to understand the importance of a vacuum gage on a 4-stroke engine from their personal experience and maybe from some PM’s/e-mails I've sent them. Perhaps some background info on the subject would be of interest to other folks running 4-stroke motors (Inboard, I/O and OB versions).
The piston aircraft engines (Continental, Lycoming, etc.) were designed to cruise at about 75% power, typically called the “Maximum Continuous Power Rating”. The Takeoff Power rating is 100% power and typically only used for a short time during takeoff and initial climb out. Similar ratings are used on other industrial engines as well. Since the exhaust valve is the hottest and most limiting part in any 4-stroke engine, these heavy duty engines are built with some very special (= expensive!) parts to address that weak link. They use sodium-filled exhaust valves, positive valve rotators, and Stellite (a very hard high temperature Cobalt alloy) valve seats to increase the life of those parts.
Thermodynamic calculations show that an intake manifold vacuum reading of 5 inches of mercury (" Hg) works out to be about 75% of max power on any naturally aspirated Otto cycle engine. (100% power = 0” Hg reading on a vacuum gage or an absolute pressure of 29.97 “ Hg/14.7 psi). Doesn’t matter whether it’s a 2 of 4 cycle engine; the calculations are the same. Since aircraft engines are built to give a reasonable service life at this 75% power setting, using the corresponding 5” Hg manifold vacuum level is a logical starting point for trying to establish the “Max Continuous/Cruise” power setting on our marine engines, and I understand this is the factory recommendation for a cruise power setting on MerCruiser engines. Since they don’t have all the exhaust valve goodies of the aircraft engines however, you’d get more valve life if you cruised at a higher vacuum level. Frank Brown, who was an ace mechanic that used to crew for Carl Moesly on the race boats, told his customers their engines would last longer if they cruised at about 7” instead of 5" Hg! I’m really amazed that this info is not widely published by the factories, especially on the 4-stroke outboards. I hate to think what it’d cost to do a valve job on those fancy 4-valve cylinder heads they use! If I was running a 4-stroke outboard, I’d definitely have a vacuum gage on it and would observe a cruise limit of about 7” Hg!
Since 2 strokes don’t have any exhaust valves to burn out, there is no benefit to using a vacuum gage on them. The limiting 2-stroke parameter seems to be piston temperature, since BRP went to a higher temperature piston alloy on the E-Tec motors. Even if it doesn't melt or come apart however, a piston that gets too hot, either from detonation or too lean a mixture, can expand enough to start scuffing the bore. You’d need a knock sensor and an exhaust gas temperature or oxygen sensor to prevent that. I believe the 300 E-Tec does have knock sensors but haven't heard about any O2 sensors on them. I think the Yami HPDI O2 sensors are a good idea. Does anyone know if they pop a warning flag if the EMM cannot correct a lean condition, like from a plugged injector or fuel filter? Denny