SKI-DOO: AFTERMARKET MUFFLERS OR "CANS"
As most everyone is aware as a manufacturer of snowmobiles all of our unit's must meet stringent noise or sound levels before they can be certified. This present's unique challenges for engineering when trying to keep weight and under hood heat to a minimum. Even though our after muffler's are fairly large, they are relatively light for their size as seen below, they have only as many parts and baffles in them as needed to ensure the vehicle pass's noise, performance, and emission's regulations.
One parameter that is extremely important and more so with the introduction of knock sensors is the overall muffler system's back pressure.
Rotax and Valcourt work closely together to determine the maximum back pressure the muffler system can have and not affect the engine's reliability or performance.
This backpressure will vary with each engine type and the "pressure" we are talking about is measured in millibars.
Normally as a manufacturer we would not test after market parts such as "cans" or silencers, however we are aware that these items are often used especially in western markets in both the US and Canada. With the introduction of the 800 R last season and the various RPM fluctuation issues we experienced, a decision was made to dyno test and validate fuel flow and backpressure on a number of the after mufflers or cans that we have seen on dealers or customer's units in the field.
The following chart depicts horsepower and back pressure curves for the standard after muffler and various aftermarket items. You will note that a number of these "cans" actually loose as much as 6-7 HP! The increased back pressure that some of these silencers produce can actually cause the engine to go into detonation. If the knock sensor is performing properly it should sense the abnormal vibrations stemming from the detonation occurring and will change the fuel and ignition timing parameters via the Power Tek ECM until the detonation stops. One side affect of these changes will be a decrease in horsepower and throttle response. The exact opposite from what some of these aftermarket silencer makers claim.
