Breathing Easy
Principally, the engine needs to be able to
breathe freely. If the exhaust line is too small
in diameter, has too many bends, or is
clogged, pressure called back pressurebuilds
up and the engine has to use some of its
power simply to expel the spent combustion
gases—a real waste. After all, for the engine
to run, it absolutely must make room for
the next charge of fresh air being sucked into
the cylinders.
Back-Pressure Guidelines
As a rough guide, exhaust back pressure
shouldn’t exceed 2. 5 inches (63 mm) of mer-
cury (Hg) for naturally aspirated engines and
should be less than 2. 0 inches (40 mm) Hg for
turbocharged machines.
Specific engines have specific require-
ments, so consult the installation manual.
The exhaust riser should be fitted with a tap
for reading back pressure with a manometer
(Figure 7-2; see also the accompany sidebar),
and back pressure should be checked during
major overhauls.
Generally, a wet exhaust system has about
twice the back pressure as a dry system of the
same diameter. To keep back pressure at ac-
ceptable levels, this means that the wet exhaust
diameter should be 40 percent greater than the
dry exhaust diameter for the same engine.Wet Exhaust Diameter ≈ 1. 4 × Dry Exhaust
Diameter (rule of thumb)Chapter 7: Wet Exhaust Systems
Figure 7-2. Taking
manometer read-
ings (Courtesy
Westerbeke
Corporation)