2005 Ski-Doo Racing Handbook, Page 131Get this manual
SECTION 05ENGINE PREPARATION
For two-stroke-cycle engine to have high power capacity at high crankshaft speeds, high volumetric or breathing efficiency is required and the fresh charge losses must be minimizedThe result is achieved by opening the exhaust port early (94.5 BBDC) and utilizing the resonant effects of the tuned exhaust system to control fresh charge lossesWhen an engine of this design is run at medium speed, efficiency falls off quicklyThe relatively high exhaust port effectively shortens the useful power stroke and because the exhaust system is tuned for maximum power, there is large increase of fresh charge lossesAs result, the torque decreases along with dramatic increase of the specific fuel consumptionHigher torque along with lower fuel consumption can be obtained at lower engine speeds if the time the exhaust port is open is shortenedBOMBARDIER-ROTAX has patented remarkably simple system to automatically change the exhaust port height based on pressure in the exhaust system1 2
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1Squish area