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FUEL CONSUMPTION- EXCLUSIVE INFO!
I’M IMPRESSED BY the double-gated exhaust valve system on this new Polaris 800 HO, and by the Brake Specific Fuel Consumption (BSFC).
I first work with variable-exhaust-timing engine in 1981, that was Yamaha’s TZ250-H road race engine. It had a spool-valve, which lowered the top of the single large oval exhaust port by about 6mm. This gave it an additional several hundred usable rpm on the bottom end.
Yamaha eventually moved from the spool-type valve to an angled sliding gate, allowing the gate to be closer to the piston in its lowest position. Now, I see a similar thing in Polaris’ double gate. The same advantage is doubled because the lower gate can close the port to a greater degree, making the Exhaust Open (EO) point later, and broadening the engine’s pulling range for quick throttle response.
With no carburetor, the new 800s injection timing has an additional control measure – the ability to delay the addition of fuel until that is added to a part of the air charge that will not reach the exhaust port at all.
4-stroke engines in snowmobile sizes typically display BSCFs around 0.50, but carbureted 2-strokes, because of their fuel short-circuiting, have generally operated around 0.65 (30% of fuel lost to the exhaust). Some exceptional examples, especially when high compression is used, manage to squeak just under 0.60. Yet in the dyno test Jim Czekala conducted we saw 0.38 – 0.40 at part throttle from Polaris’ new 800 double-gated, finger-port-injected engine.
For more in-depth 2-stroke engine info see amsnow.com. -
By Kevin Cameron