Polaris VES
Several Polaris models are equipped with the Polaris patented Variable Exhaust System. This unique exhaust valve management system changes the effective exhaust port height in the cylinder to provide maximum horsepower at high rpm without sacrificing fuel economy and engine torque at low to midrange throttle settings.
In order to understand the operation and function of the VES 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 effect 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 horsepower 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 touring snowmobile will have a relatively low exhaust port compared to an engine of the same displacement range designed for racing or high performance.
The main components of the VES are the exhaust valve, valve housing, bellows, piston, return spring and valve 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 valve cover.
At idle or 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 drilled passage. This forces the exhaust valve upward against spring pressure. The valve continues to move upward toward the high port position as cylinder pressure, horsepower and rpm increase.
VES Maintenance
Due to the simplicity of the VES design, maintenance is limited to periodic inspection and cleaning of system components. The VES should be disassembled, inspected and cleaned (remove carbon deposits) every 1,000-2,000 miles, depending on operating conditions.
Source: Polaris 2002 Snowmobile Field Update