When Polaris introduced the 2002 snowmobiles, two major changes were touted: a new Edge chassis and engines that now feature exhaust valves. The use of exhaust valves on high performance two strokes is not new. Dirt bikes have been equipped with valves for a couple of decades. Rotax has been building Ski-Doo engines with RAVE (Rotax Adjustable Variable Exhaust) valves for years and Polaris implemented the technology on race snowmobiles and watercraft about six years ago. Even so, with the new RMK powerplants, many have asked how and why exhaust valves help performance?

In order to understand the advantages of exhaust valves, a little basic two-stroke theory is required. Two-stroke engines are very simple compared to our more familiar automotive style four-strokes. In a two-stroke engine, all intake and exhaust ports are opened and closed by the piston traveling through its stroke. The design and placement of the ports not only determine the amount of peak horsepower the engine will produce but also the rest of the engine's power characteristics. Generally, large exhaust ports produce more power at high RPM (about 8000 rpm on a twin sled engine) than small ports. Unfortunately, the corresponding increase in top end power is offset by a decrease in power at lower RPM. In the real world, that's a trade off between full throttle hillclimbing and picking through the trees throttle response. Therefore, when an engine is designed, a compromise must be made between peak power and drivability. Enter the variable exhaust valve. The exhaust valve allows the best of both scenarios. At low RPMs, the valve is down, effectively blocking off part of the exhaust port and giving the engine the benefit of great bottom end power. As RPMs increase, the valve is lifted up, letting the engine breathe and make more power. While there are a couple of different ways to actuate the valve, the effect is the same: better bottom end and top end power.

Finally, how does altitude fit into this discussion? At high altitude, our lack of atmospheric pressure essentially makes our stock engines low on compression and over-ported. With 30% less effective compression and ports designed to flow much heavier air than we have, it's no wonder we observe a lack of throttle response on many engines. For us, the advantages of valves will be even more pronounced than for our sea-level counterparts. At altitude, the valves essentially allow the engine to adjust to its lack of air. For years, most have acknowledged that the low-end grunt of the Rotax engines has been hard to beat. The RAVE system is the primary reason that those engines function so well stock at higher altitudes.