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A first impression might be that the new Polaris 850 Patriot engine is a hasty response to Ski-Doo’s release of their 850 twin last year, but that’s not the truth. Reality is that they both have been working on 850s for years. Polaris was already working on theirs before the Ski-Doo motor was released. So this is NOT a tit-for-tat situation and certainly not a copycat scenario. The Polaris 850 Patriot engine is a brand new design.
A new engine design can usually take four to six years to complete, starting with component designs all the way through tooling, testing in the field and finally production. This engine is so new that there are no interchangeable components with the trusted former 800 Liberty engine. The Liberty 800 has been changed and updated at least four times over numerous chassis and each time durability and power improved. That is a great testament to the original design!
However, eventually you can only do so much within the constraints of the design you started off with, and if you want to move on to higher performance levels, while also improving durability, it is time to start fresh with a new engine that incorporates all the lessons learned over twenty-five years of Liberty engine development.
¦ Applying what was learned
The most critical parts in 2-stroke engines are cranks and pistons. Both live in highly stressed environments, and when performance requires bigger bores, longer strokes and higher RPM, you have to deal with increased component stresses. A good example of bumping up against this wall was the 1000cc twin cylinder engines introduced by Ski-Doo and put in the Mach Z in 2005 and Arctic Cat with their Thundercat that hit the snow several years earlier. There was the 900 Fusion from Polaris just 13 years ago as well. Many of these sleds ran into crank vibration problems, and as a result, clutch and belt durability problems that could not be solved at the time, and they were all quickly dropped in favor of the more reliable, smaller 800 twins.
The new approach today is much more cautious and precise with smaller incremental changes that may eventually lead to 900cc engines once any possible problems are worked out at the 850 level. It is interesting that both Ski-Doo and Polaris chose to increase the stroke to gain displacement. Ski-Doo kept their 82mm piston and increased stroke from 75 to 80mm, while Polaris kept their 85mm bore and went up from 70 to a 74mm stroke.
A good crank working environment requires a sturdy case that does not twist and this is greatly enhanced if the engine also has a mono-block cylinder design instead of separate cylinders. The new twin mono-block cylinder assembly on the new Polaris Patriot 850 went through a design process using a computational fluid dynamics program to optimize water flow around the cylinder, exhaust port, power valve and through the one piece head and around the combustion chambers. The new crank features a larger clutch taper, and higher capacity rod bearings running on stronger redesigned pins.
Rod bearings are also critical in engines with longer strokes, as inertia load goes up. Beefier big end rods are preferred to keep the rod end round under the high load and prevent the rod eye from deforming and “pinching” the bearings. A lot of development went into the new rod design on the Patriot and the piston pins and bearings were also made stronger.