ENGINE
2.6
Variable Exhaust System (V.E.S.)
Some snowmobiles are equipped with the Polaris
Variable Exhaust System (V.E.S.)
This unique exhaust valve management system changes the effective exhaust port height in the cylinder to pro-
vide maximum horsepower at high RPM without sacrificing fuel economy and engine torque at low to midrange
throttle settings.
Bellows
Cover
Spring
Exhaust Valve
Housing
In order to understand the operation and function of the V.E.S. we must first consider the characteristics of a two
stroke engine. The height of the exhaust port in a two stroke engine cylinder has an affect on the total power output
of an engine, as well as the RPM at which the power occurs.
Exhaust systems are “tuned” by design to match engine exhaust port configuration and desired power delivery
characteristics. Engines with relatively “high” exhaust ports (and exhaust pipe to match) produce more horse-
power at high RPM, but only at the expense of low to midrange fuel economy and torque. On the other hand,
“low” port engines provide good fuel economy in the midrange and make their power at relatively lower RPM, but
will not produce as much peak horsepower for a given displacement range. In general, an engine designed for
a racing or high performance snowmobile will have a relatively high exhaust port compared to an engine of the
same displacement range designed for touring.
Although the V.E.S. does not in itself increase horsepower, it does allow an engine to be designed for maximum horse-
power without the inherent disadvantages of a high exhaust port.
The main components of the V.E.S. are the exhaust valve, valve housing, bellows, return spring, and cover.
A guillotine style exhaust valve is connected to a moveable piston. This piston is attached to a flexible bellows,
forming two chambers. The lower chamber is connected to the cylinder by a drilled passageway located just above
the exhaust port. The upper chamber is vented to atmospheric pressure. A valve return spring is located in the
upper chamber between the piston and cover.
At idle and low speeds, the exhaust valve is held in the “low port” position by the return spring. When throttle is
applied (and RPM begins to increase) rising cylinder pressure is applied to the under side of the bellows via the
actuation port. This forces the exhaust valve upward against spring pressure. The valve continues to move up-
ward toward the “High Port” position as cylinder pressure, horsepower, and RPM increase.
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