Effects Of Elevation And Weather On Performance - Mercury 225 Pro XS User Manual

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:
• Reduces the possibility of detonation.
• Enhances overall reliability and durability of the engine.
OPERATION
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