Operating At High Elevations; Effects Of Elevation And Weather On Performance - Mercury 225 Manual

Operating at High Elevations

Your engine automatically compensates for high elevation
changes. A different pitch propeller may help reduce some
normal performance loss resulting from reduced oxygen in the
air. Consult your dealer.

Effects of Elevation and Weather on Performance

The following conditions lower engine performance and cannot
be compensated by the engine fuel or electronic management
systems:
• Above sea level elevations
• High temperature
• Low barometric pressure
• High humidity
These conditions above reduce air density to the engine, which
in turn lowers the following:
• Boost pressure on supercharged engines
• Horsepower and torque throughout the RPM range
• Peak RPM
• Cranking compression
EXAMPLE: An engine run at an elevation of 8,000 feet will have
over a 30% power loss while a loss of engine power on a hot and
humid day could be as much as 14%. These losses apply to
normally aspirated and supercharged engines.
Compensating for power robbing conditions:
• Switch to lower pitch propeller.
• Change gear ratio.
Some boat performance can be regained by dropping to a lower
pitch propeller, but engine performance will still remain lower. In
some cases, a gear ratio reduction may be more beneficial. To
optimize engine performance, prop the engine to allow it to
operate at or near the top end of the recommended maximum
RPM range at wide‑open throttle with a normal boat load.
Other advantages to propeller or gear ratio changes:
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