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Liberty 700 EngineThe first question I asked Polaris engineers about this motor was: Why build a 700 version of the 900 engine with the same long stroke and smaller pistons to boot? Wouldn't it have made more sense to bump up the 600 H.O. instead?
Another option would have been to enlarge the 900 to a 1000cc model and challenge Ski-Doo's Mach Z for the trail king title. But all I had to do was try this one, and my questions were answered in a hurry.
First, Polaris' target was Arctic Cat's lively F7.
While the Liberty 700 delivers exciting power, we should note that it's really a 755cc engine, compared to 698cc for the F-7. With that extra size comes extra weight. The Liberty 700 is more than 60 lbs. heavier than the F7's engine, but still performs surprisingly well.
But why stick with the long stroke and smaller piston. With a 77.5mm bore and an 80mm stroke, this engine is actually "under square," clearly different than conventional wisdom.
There are some good examples of quick acceleration with under-square engines. Have you ever checked out a 250cc motocross bike? They all have long strokes and smaller bores. If you watch those guys launch for triple jumps at a Supercross race, you get an idea of their engine's explosive acceleration, and that is exactly what you get in the new Liberty 700.
With a light 600-size piston and a 900-size stroke, this engine revs quicker than expected, and the torque roosts you hard out of corners.
I checked it out against an F7, and although top speed was close, the Polaris flew faster out of corners. The Liberty 700 simply pulled harder.
Part of the credit for the quick response can be attributed to the new 4-injector version Polaris' Cleanfire fuel injection system.
This new version has a second injector placed in the scavenging back port, while the original injector location remains in the crankcase. At lower rpm, only the back port injector works, assuring that the crankcase doesn't build up with fuel. When the crankcase injector comes on at higher loads, there's no fuel buildup to clean out, so throttle response is crisp and clean.
In terms of response, fuel economy and emissions performance, this new 4-injector system is clearly a large step forward in Cleanfire's evolution.
I hope we see more of this new under-square concept soon. Shorter strokes are good for reliability in higher revving engines, but there are some interesting advantages with under-square long-stroke engines.
First, you'd think that there would be less wall area for ports, but since the port actually ends up being taller, analysis of transfer flow area shows that you end up with more mixture flow. A longer, tall cylinder also burns gases more efficiently, and there's less tendency for fresh gasses to short circuit the exhaust port. This delivers emission advantages, and you end up with both increased torque and a cleaner burn.
For the 2006 Snocross racing season, new rules dictate a 600cc engine in the Open Class, and factory engineers are looking into longer stroke engines to gain back some of the explosive torque they had with the earlier 800 Open Class engines.
My guess is that we'll see a trend toward under-square engines for torque, emission and fuel efficiency reasons. The Liberty 700 may be the first step in this direction.
I was pleasantly surprised by the 700 and it's a very different animal than last year's 900 Fusion.