2004 Polaris Pro X Factory Service Manual, Page 253Get this manual
ENGINE Variable Exhaust System (V.E.S440 PRO XR 440 PRO XR Variable Exhaust System:
The exhaust valve system on the new 440 PRO XR is electronically controlled by solenoidsPressure from the cylinders is routed through the solenoids to the atmosphereDuring this time the exhaust valves are downWhen the CDI senses 6700 RPM, it triggers the solenoids to shut off the bypassed air going to the atmospherePressure then builds inside the exhaust valve bellows, causing the valves to fully openWhen RPM drops below 6700, the solenoids open to allow air to bypass through to atmosphere and the exhaust valves close
Below 6700 RPM