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All the RAVE
The “RAVE” valve was a much different design than the Yamaha rotating barrel. This powervalve was a flat plate that pulled out of the way as the engine speed increased. It was quickly named a “guillotine valve” because of its obvious action. All high performance snowmobile engines now have the same “guillotine” valve design, but as the design developed, the activation and control of the valve went through changes as new technology in computer controlled activation motors evolved.
The first “RAVE” valves were activated by the increased pressure in the exhaust system as engine speed increased. This pressure came from a small passage leading from the exhaust port and activating a rubber bellow that moved the slide out of the way. Opposing this movement was a small spring. Depending on the length and rate of the spring, the mechanism could be “tuned” to open at different engine speeds, depending on the motor’s size and the type of exhaust. Two into one exhausts require stiffer springs than individual pipes, due to the quicker build up of pressure in the exhaust which is now fed by two cylinders.
When Polaris introduced their version of the 2-1 exhaust the pressure was taken through a passage above the exhaust port directly linked to the cylinder pressure. One after-market system (PSI) actually ran the passage up the center of the powervalve stem. All these systems can be tuned and are working successfully.
Yamaha introduced an interesting powervalve system on its 700 SRX triple, it was not a rotating barrel, but also a “guillotine’ valve, but the activation system was different. Rather than a bellow activated by pressure and tuned with springs, the Yamaha system used a computer controlled “Stepper Motor” that pulled the valves through cables.
Computer control has many advantages as rpm is only one variable. You can now actually introduce several other features such as cleaning cycles where the valves “cycle” to prevent oil deposits from building up. You can also pull the valves away during starting to make the engine easier to pull over. Both Arctic Cat and Rotax now also have complicated computer controlled powervalves, and the programming is becoming steadily more sophisticated.
One problem with powervalves is that deposits build up over time, and valves can get stuck in the down closed position. Top end power will then be reduced, and the valves need to be removed and cleaned. Oil manufacturers are aware of this problem and are offering new products that claim to be cleaner and easier on powervalves. Yet it may be a good idea to clean your powervalves once a year as part of your fall “get ready for winter” routine, to make sure everything works flawlessly throughout the season.
Powervalves lower the exhaust port outlet edge, and this is a great advantage when it comes to trapping fresh gasses in the cylinder and avoid unburned hydro-carbon to exit out the exhaust, and has therefore been a plus in the emission control department. Exhaust sound is also now measured in a stationary test at 4000 rpm or below, and with power valves in full closed position at this engine speed, the pressure pulse is reduced in strength, which may lower sound readings by as much as 4 dBA compared to a non-powervalve older machine.
Instead of controlling just power as first intended, the valve is now a 3 into 1 technology. It has made the popular 2-stroke engine able to meet new EPA legislative challenges. The powervalve is now an integral and important part of the modern 2-stroke engine that will be with us for a long time in the future.