Mercury 8.1S Horizon, Product Applications Manual

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Exhaust System
90-863758060 AUGUST 2006 Page 6A-25
All exhaust systems have some restriction to flow or exhaust back pressure. The power
output of an engine is directly related to the amount of exhaust that can flow out of the
exhaust system. For example, large displacement engines will generally lose
approximately 10 horsepower for the first 7 kPa (1 psi) of exhaust back pressure. For every
additional 7 kPa (1 psi) of back pressure, an engine may lose approximately 5 horsepower.
Back pressure at WOT must fall within the range shown below.
Models Exhaust Back Pressure
All gasoline powered engines
1 psi (7 kPa) Minimum
2 psi (14 kPa) Optimal
11 psi (76 kPa) Maximum
IMPORTANT: Some exhaust back pressure is required to prevent water intrusion in the
exhaust system. Do not use more than a 10.2 cm (4 in.) inside diameter exhaust hose
within the first 45.7 cm (18 in.) of each exhaust elbow.
A higher output engine will cause a greater back pressure using the same size exhaust
system as an engine with less output. The boat's exhaust system should be designed and
tested for the highest horsepower engine to be offered in that boat.
Ensure that both cylinder banks of any engine are tested. Record the highest and lowest
back pressure readings. If pressure is out of specifications, changes must be made to the
system to reduce the pressure. These changes include:
• Reduce exhaust system length.
• Eliminate sharp bends.
• Increase exhaust system diameter.
• Reduce muffler restriction.
• Ensure that through the transom or through the hull fittings are above water and are
not restrictive while retaining internal and external flappers.
Exhaust Back Pressure Test
The boat should be in the water and underway. No special loading of the boat is necessary.
The engine must be capable of reaching the specified WOT RPM as verified using an
accurate service tachometer or scan tool. Both banks of each engine must be tested. Use
the highest reading.
1. Drill 3/8 in. hole at the 12 o'clock position (facing up) in the exhaust hose
31–61 cm (12–24) inches from the engine exhaust elbow.
2. Assemble the Schraeder valve and brass adapter. Insert the 1/8 in. NPT male end of
adapter into the hole in the exhaust hose.
18579
a
a
b
b
c
c
Testing back pressure
a - Brass adapter
b - Schraeder valve
c - Exhaust hose