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Two-cycle exhaust power
valves (11-04)
OBJECTIVE:
To bring to light the existence and purpose of a
special mechanism referred to as an exhaust power valve used in the two-cycle
industry to control the size of the exhaust port.
ISSUES:
The need in certain two-cycle engine designs to
service the exhaust power valve.
TECHNICAL DISCUSSION:
When dealing with basic two-cycle engine design, the
size of the exhaust port can be extremely important. A large port tends to
provide freer flow offering additional performance at higher engine speeds. Due
to loss of fresh air/fuel charge at lower speeds however, horsepower and torque
are often compromised when a larger exhaust port is utilized. Smaller ports on
the other hand, tend to offer better low speed performance which diminishes as
engine
speeds increase due to their restrictive nature. The most desirable situation
would be one in which the size of the exhaust valve would change with engine
speed. Smallest at low speeds with a gradual increase in size as the
engine RPMs increase.
The concept of an adjustable exhaust port size
is nothing new. Variations have been around since the 1970s and in mass
production in the motorcycle industry since the early 1980s. In recent years,
this type of exhaust port has become increasingly popular within the motorcycle
and snowmobile industries.
Most commonly referred to as a power valve,
its operation is quite simple. Though the actual function of the valve will vary
with design, the general principle is the same. At low engine speeds, the power
valve is in a closed position, covering a portion of the exhaust port. In this
position, low and lower speed performance is maximized. As the engine reaches
higher operating speeds, the valve is retracted creating a larger exhaust port.
The end result is a maximization of high-speed operation without sacrificing low
end and mid-range performance.
The movement of the valve can be controlled
inseveral differing ways. Some designs use exhaust or cylinder pressure to
activate the valve while others use an electro/mechanical method. A third style
uses a direct cable or mechanical linkage to operate the valve. Though each
design has its own benefits and disadvantages, all suffer from a similar
problem. That problem, is that they all require periodic maintenance.
With the valve being placed in the exhaust
stream, it must function in an extreme environment. Not only must it deal with
high temperatures but also exposure to a constant flow of contaminates.
Carbonaceous material, un-burnt fuel and heavy oil residue can form deposits on
the valve. As the level of these deposits build, the movement of the valve will
become sluggish. Eventually the deposit level will reach a point at which the
valve will stop moving.
One should keep in mind that the level of
force available to activate these valves is minimal. Even moderate restriction
due to deposit formation can render the valve in-operable. In order to maintain
peak performance, periodic maintenance is required. Within the snowmobile
industry, some recommend that valve maintenance (cleaning) be performed as
frequently as after every 500 – 1,000 miles of operation.
This may seem quite frequent but in many cases
it is not. The length of time however, will vary depending on two major factors.
The first is how the equipment is used. power valve equipped engines that are
subjected to extensive idle or operation at continuous low speeds tend to
require more frequent service. Secondly, the type and brand of oil used in the
equipment can be a significant factor affecting the frequency of needed service.
AMSOIL
Interceptor synthetic two-cycle oil for instance, has demonstrated superiority
in the area of maintaining power valve cleanliness. Not only has this shown
through in bench tests but also in the real world of field demonstration.
At intervals designated by the equipment
manufacturer or when engine performance issues arise, the power valves
should be serviced. The actual service process will vary from one design to
another hence one should refer to the individual manufacturer servicing
instructions. If you are a consumer and you have any doubt as to how to properly
service the valve, it should be left to a qualified technician.
As the valve is being disassembled, one should
pay close attention to it’s ability to move within it’s housing. The valve
should move freely and with little or no resistance. If a high level of
resistance is noted or the valve is frozen in place, you can rest assured the
valve was negatively effecting engine performance. Also keep in mind that if a
valve has been in use for any length of time, it will have some discoloration
and deposit formation. The presence of deposits is not of great concern. The
type of deposit and the degree to which they hinder movement is more important.
Hard dry carbon is worse than soft oily deposits. In other words, do not expect
the valve to appear shinny and new.
When removing or cleaning, care should be
taken not to mare or scratch the surfaces of the valve. Damage to the surface of
the valve can result in restricting it’s movement once reinstalled. Residue and
deposits should therefore be removed using a non-metallic scraper and/or a
cleaner such as AMSOIL
Power Foam. Cleaners such as this help to dissolve residue and loosen deposits
without damaging critical surfaces.
RECOMMONDATIONS:
In high performance two-cycle engines, power valve
maintenance will always be a concern. The use of
AMSOIL Interceptor synthetic
two-cycle however can dramatically reduce the need and ease of such servicing.
When servicing is required, AMSOIL
Power Foam can assist in providing safe and effective cleaning. The end result
is more consistent performance with less effort and cost.
 
T's Advanced Synthetics
Greg and Marcy Thurman
Your Nationwide
AMSOIL
Authorized Independent
Dealer
Nationwide US Warehouses,
Canada and Palm Harbor, FL
727-798-8552
E-Mail:
greg@tsadvancedsynthetics.com
If you
experience problems or have questions or comments about
our website please email us at
greg@tsadvancedsynthetics.com
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