How Elevation And Climate Affect Performance - Mercury D4.2L Manual

CD476

How Elevation And Climate Affect Performance

NOTE: Engines equipped with EDI (D-Tronic engines) reduce the effects of changes in elevation and climate
by automatically adjusting fuel flow for weather conditions and elevation. EDI engines however, do not compen-
sate for increased loading or hull conditions.
Generally, elevation has a very noticeable effect on the wide-open-throttle power of an engine. Since air (contain-
ing oxygen) gets thinner as elevation increases, the engine begins to starve for air. Humidity, barometric pres-
sure and temperature do have a noticeable effect on the density of air. Heat and humidity thin the air. This condi-
tion can become particularly annoying when an engine is propped out on a cool, dry day in spring and later, on
a hot, sultry day in August, doesn't have its old zip.
Although some performance can be regained by dropping to a lower-pitch propeller, the basic problem still exists.
In some cases, a gear-ratio change to more reduction is possible and very beneficial.
Summer conditions of high temperature, low barometric pressure and high humidity all combine to reduce the
engine power. This, in turn, is reflected in decreased boat speeds, as much as 2 or 3 miles per hour in some
cases. Nothing will regain this speed for the boater, but the coming of cool, dry weather.
In pointing out the practical consequences of weather effects, an engine -- running on a hot, humid summer day
-- may encounter a loss of as much as 14% of the horsepower it would produce on a dry brisk spring or fall day.
With the drop in available horsepower, this propeller will, in effect, become too large. Consequently, the engine
operates at less than its recommended RPM. This will result in further loss of horsepower at the propeller with
another decrease in boat speed. This secondary loss, however, can be somewhat regained by switching to a
lower-pitch propeller that allows the engine to again run at recommended RPM.
For boaters to realize optimum engine performance under changing weather conditions, it is essential that the
engine be propped 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.
Not only does this allow the engine to develop full power, but equally important is the fact that the engine also
will be operating in an RPM range that discourages damaging detonation. This, of course, enhances overall reli-
ability and durability of the engine.
24