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High Altitude and Mercruiser 6.2L 300HP + Bravo III Drive
caseymeraz
Member Posts: 4 ✭
Hey all, I'm new to boat ownership and I'm looking to purchase a 320 Rinker with the mercruiser 6.2L 300HP + Bravo III Drives.
I didn't realize that there needs to be some work done to make it ready for high altitude.
Does anyone know the best approach here to make the boat high altitude ready?
I didn't realize that there needs to be some work done to make it ready for high altitude.
Does anyone know the best approach here to make the boat high altitude ready?
Comments
What altitude will you be boating at?
“The rule of thumb, for us, for fuel-injected engines, is approximately a three-percent power loss per thousand feet,” said Robert Grantham, product director for MerCruiser in Stillwater, Okla. “That’s typically what you’ll find on a normally aspirated four-stroke engine.”
https://www.boattrader.com/resources/high-altitude-boating-where-did-the-power-go/
From Wikipedia 😁
A naturally aspirated engine, sometimes known as a normally aspirated engine, is an internal combustion engine in which oxygen intake depends solely
on atmospheric pressure and does not rely on forced induction through a turbocharger or a supercharger.
PC BYC, Holland, MI
I did test drive the boat on Lake Havasu, which is only like 400 ft, and it would jump on plane INSTANTLY.
Keeping a spare nearly 7k bravo leg plus props for those trips doesn't.
I'd watch the rpm gauge and see how she does. The first go around. Talk to hill 4x4 and they may be able to get you a prop that will work great at your local frequently used lakes and be acceptable when you head further up.
efi is operated by a computer- but unless the engine is a closed loop cooled and o2 sensor monitored- it doesn't do didly for altitude. it could with JUST an o2 monitor, but the issue with the open (raw) water cooled is the temperature would never reach the trigger point for the computer to close loop and trim fuel + advance/retard spark. for that reason, and IF you wanted to compare these engines with a car at altitude, it would need BOTH a monitored exhaust (o2 sensor) AND a steady operating temperature above 178* (where most engines close the loop and start operating off of sensor data alone).
you COULD approach a performance shop that specializes in mercruiser computers and see if they can tune for altitude... they could do it by re-writing the base tables (which is what these engines operate off of- there is no open/close loop without the o2 sensor monitoring) and providing for likelihood of predetonation by lack of o2 coupled with advancing spark... they'd retard the spark a few degrees and simply adjust the fuel trim. done.
because o2 is less altitude, EVERY engine is less powerful. yeah, you can tune for optimum power- but unless you're injecting o2 you're not getting the same as at sea level. it doesn't matter how your engine is tuned or aspirated, apples to apples- the precise same engine tuned optimum for altitude will NOT produce the same power at altitude.. ain't a thing that can be done about it.
gear ratio at altitude- they used to run the 1.62 as opposed to the 1.5 at higher altitude... which makes zero sense to me as many here have offered... JUST like a vehicle is impacted by the overall diameter of the tires, a boat's prop is huge with 'overall drive ratio'... at WOT with 34" tires a auto may be able to turn 6kRPM or may not.. same with a boat trying to turn 4800/6100 (depending on your engine) RPMs way overpropped.. it won't happen. put the smaller tires/prop on? it may turn TOO easy... it's WAY easier to fine tune the overall gear ratio with tires or prop than with gears (be it a ring and pinion in an outdrive or one in a differential). my experience, if you're interested- is the difference in 1.62 and 1.5 is a little over 2 pitches... that 'little over' can be covered by overall diameter of the prop.. so.. at sea level you may turn a 14.25" 23p prop at optimum RPM... where at 10k' altitude you'll be well served with a 14" 21p... maybe even better a 13.75" 21p...
now if you REALLY want to play with that thing? slap a closed loop (fresh water cooled) cooling system on that thing, and slap a plate between your riser and elbow (they actually make these!!) for your o2 sensor (it has to be done prior to the raw water slipstream into the exhaust)... THEN.... slap a computer from a vehicle on there instead of the mercruiser module... you'll get the most out of that engine in that configuration.... still not as good as you would at sea level, but better than you could otherwise.