GX240 • GX270 • GX340 • GX390 (UT2/RT2) Technical Manual
© 2010 American Honda Motor Co., Inc — All Rights Reserved 9
EXHAUST SYSTEM
Recommended Muffler
The recommended muffler and exhaust pipe are matched to the engine in terms of emissions performance, exhaust
backpressure, sound level, and durability.
Consider the following:
• Discharge the exhaust gas directly to the open air. Do not install flammable parts or any parts with poor heat
resistance properties around the exhaust system or near the discharge port.
• The exhaust gas must not enter the cooling-air intake port. Be especially careful when using the exhaust deflector
to change the discharge direction.
• The muffler and exhaust pipe become very hot during operation and remain hot after the engine has been shut off.
Install the muffler and exhaust pipe so the fuel system and other heat-sensitive components are isolated from the
exhaust heat.
Fabricated Exhaust Systems
The muffler type and the shape and length of the exhaust pipe(s) affect emissions performance and engine power. If
you use a muffler other than a recommended Honda muffler, observe the following precautions to maintain the
engine’s peak performance:
• The shape (bends and elbows) of the exhaust pipe can affect exhaust backpressure. If exhaust backpressure is
excessive, it can affect emissions performance and/or cause detonation.
• The exhaust pipe inside diameter must be the same size as the exhaust port diameter.There must be no gap
between the port inside diameter and the exhaust pipe ID.
• The exhaust backpressure increases if the diameter is less than specified. If the diameter is larger than specified,
the effective width of the exhaust gasket is reduced which could cause an exhaust leak.
• When the exhaust pipes are connected together before the muffler, make sure that the exhaust pipe length is as
short as possible to reduce backpressure.
• Muffler volume and design will affect exhaust backpressure. Increase the volume of the muffler if exhaust
backpressure is higher than specified.
Exhaust Back Pressure Measurement
Measure the exhaust back pressure at the exhaust pipe with the engine under
continuous load (WOT), 30 mm (1.2 in) from the exhaust pipe mounting flange
as shown (make sure the test nipple does not extend beyond the inner wall of
the exhaust pipe).
3000 rpm 3600 rpm
Min. Max Min. Max
GX240
GX270
kPa 4.2 9.0 4.9 11.0
mmH
2
O 428 918 500 1122
GX340
GX390
kPa 4.6 10.5 6 12.5
mmH
2
O 469 1071 612 1275
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