GENERAL SERVICE MANUAL NUMBER 23
Page 7A-2 90-861326--1 MARCH 1999
Exhaust System Requirements
IMPORTANT: It is the responsibility of the boat manufacturer or installing dealer to
properly locate the engine and install the exhaust system. Improper installation may
allow water to enter the exhaust manifolds and combustion chambers and severely
damage the engine. Damage caused by water in the engine will not be covered by
MerCruiser Limited Warranty, unless this damage is the result of defective part(s).
Determining If Exhaust Elbow Risers Are Required
To determine if exhaust elbow risers are required, take measurements (a) and (b), with boat
at rest in the water and maximum load aboard. Subtract (b) from (a). If (a) minus (b) is less
than specified in chart, select appropriate size exhaust elbow riser kit and exhaust extension
kit that will correctly position exhaust elbow.
72700
a
b
Exhaust Elbow Measurement On All Engines
a-From Waterline To Top Of Transom
b-From Highest Point On Exhaust Elbow To Top Of Transom
Model
(a) Minus (b) Must Be at Least
All 13 In. (330 mm)
MCM (Sterndrive) Engines With Thru-Transom Exhaust
When designing and installing exhaust system, the following must be observed:
Minimum Hose Size Inside Diameter (I.D.)
Model Single Outlet Dual Outlet
7.4L MPI 5 in. (127 mm) 4 in. (102 mm)
454 & 502 Mag – 4 in. (102 mm)
1. Exhaust flanges must be of proper size to accommodate 4 in. (102 mm) I.D. exhaust
hoses. They must also be equipped with internal water shutters, and use an exhaust
flapper over each outlet.
2. The exhaust hoses and pipes must not be higher than exhaust elbows at any point.
3. The exhaust outlet must be located so that a minimum of 1/2 in. (13 mm) drop per foot
(305 mm) downward pitch exists in the exhaust hose from the engine exhaust elbow to
the outlet. (This is an American Boat & Yacht Council recommendation.) The drop must
be constant so that a low spot does not exist at any point in the exhaust hose.
4. Exhaust outlet must be slightly above the waterline with boat at rest in the water and full
load aboard.
5. Back pressure must not exceed 4 in. (102 mm) of mercury when measured with a
mercury manometer to exhaust elbow outlets.
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