High Performance 2-Stroke Exhaust 101
The purpose of an "expansion chamber" is to return to the exhaust port a negative sound wave then a positive sound wave at precisely the right time. If the pressure wave returns too late, you lose some of the fresh fuel charge in the combustion chamber and performance. If the wave returns too soon, it pushes hot exhaust gas back into the combustion chamber contaminating the fresh charge and creating hot spots on the piston.
The challenge to the pipe designer is to arrive at the proper exhaust tuning that will return the sonic waves at the correct time. This challenge is made all the harder by many impeller/nozzle combinations, engine configurations, riding conditions and rider preferences.
Traditionally, if you wanted low RPM torque and high RPM horsepower, it required several pipes. A few of our competitors cast rings into their pipes to achieve pipe tuning by "cut and try". In 1992 Factory Pipe introduced the first truly tunable pipe using our variable water injection system.
This system allows you to modify where and how much water injects into the exhaust by the turn of a set screw. Where our competition had you change the length of the pipe, the Factory Pipe allows you to vary the exhaust gas temperature which in turn changes the sonic wave speed within the pipe. Changing the sonic wave speed within the pipe has the same tuning affect as changing the length of the pipe.
Tuning Your Exhaust System
Note - The following information pertains to Factory Pipe systems that use our "A" or "B" headpipe with three (3) water injection screws. Some systems such as the SeaDoo 580/650/720 and Kawasaki 750SS/XI have only one injection screw. On pipes equipped with ECWI use only the bottom injection screw.
Most Factory Pipe systems have our exclusive "tunable" headpipe which allows you to custom tune the pipe to your riding style. The following information gives a general overview of how this system works and how each adjustment will affect the performance of your watercraft.
Double check all hoses, bolts and clamps from your installation. For the first "on-water" test of your new Factory Pipe we recommend closing the top and middle adjustment screws and opening the bottom screw 3/4 turn out from closed. This setting will be more water than is required but will provide a good starting point to test the pipe.
Ride the watercraft for several minutes while varying the throttle position. Put the boat through a long series of slalom turns keeping the engine loaded. Hit the kill switch or pull the lanyard and let the boat come to a stop. Open the engine cover as quick as possible and check the pipe temperature by splashing water on the chamber body directly after the headpipe coupler. The water should lightly sizzle for the first few inches on the chamber body. If the water does not sizzle, close the bottom adjustment screw 1/8 turn and retest. If the water sizzles rapidly, open the bottom screw 1/4 turn and retest.
This set up will provide the best top end performance for your watercraft. With the pipe adjusted as stated above, open the top screw 1/4 turn. This will cool the exhaust in the headpipe and provide better bottom end performance at the expense of some top-end. This would be an ideal setting for running slalom or a tight buoy course. If you want a change that is somewhere in the middle of the two settings, close the top screw and open the middle screw 1/4 turn or add another 1/8 turn to the bottom screw. Some engines may react differently from the above. For example, while testing the 650 Super Jet we found that we gained top end performance by running the top screw open and the others closed. You may use any combination of the three screws to achieve the desired performance. However, AT LEAST ONE SCREW MUST REMAIN OPEN AT ALL TIMES TO PREVENT DAMAGE TO THE PIPE.