Building and riding a no-compromise modern snowmobile that weighs less than 400 lbs. has long been a goal of ours here at Rocky Mountain Xtreme. Our goal has been realized with Project Xtreme Mountain King, a highly modified 800 Pro RMK that weighs less than 400 lbs. without sacrificing any day-to-day usability.
We'd like to share our ideas on what lightweight sled modifications can do for you. We'll also discuss why lightweight mods are worthy of consideration, take a look at the components that comprise our latest project and compare this XMK to previous lightweight sleds we've built.
It is a well-known fact that the two most critical predictors of snowmobile performance are weight and horsepower. If we decrease weight or increase power, performance is very likely to improve. As snowmobiles evolved over the years, features like independent and long travel suspensions, bigger tracks, larger engines and more sophisticated engine management systems have all generally conspired to add weight to snowmobiles. Those improvements have made our sleds much more enjoyable to ride. Most of us wouldn't trade all that modern technology for a sled that's simply lighter but probably wouldn't run, ride or handle as well. We have learned, though, that building lighter versions of current sleds always makes them perform better and they are more fun to ride. 
I was introduced to the virtues of lighter snowmobiles by my good friend Jon Bayne not long after becoming a Polaris dealer in 1994. Bayne was a customer of the dealership and was working on getting as much weight as possible off his XLT at the time.
In addition to teaching me a bunch about powder riding, his sleds always worked better in deep snow and consistently demonstrated the virtues of lighter weight. Many other folks also noticed those advantages and in 2000, we built our first custom XMKs for several customers. Those sleds started with a Polaris Gen II bulkhead that was mated to a custom-built Holz tunnel and then finished with all the lightest components that were available.
It was riding mine for a couple seasons that really taught me how much of an advantage riding a lighter sled really gives you. Not only is it possible to climb higher and go further before getting stuck, but most riding can be done with considerably less effort than your buddies riding heavier machines. While that advantage may be less apparent leaving the truck with everybody refreshed and recharged, it normally becomes very evident by mid-afternoon when herding around their snow barges has taken its toll.
Back then, our sleds had only 141-inch tracks and weighed about 410 lbs. dry. Since then, we've built mod 800s in the Edge era, a Summit 1000 and 900 RMK that each lost a staggering 80 lbs., as well as several Dragons. My `09 Dragon 800 was about 50 lbs. lighter than a stock one which put its dry weight in the 430-pound range. That weight loss combined with some extra horsepower made it the most enjoyable sled of any I had thrown a leg over.
Throughout all of those projects, the technology of the base sleds was improving and the resultant project sleds got better as well, but we never could quite get back to the weight of those early machines.
While I think it's safe to say that the manufacturers have known how important weight is, especially for mountain riding, other engineering considerations resulted in new sleds often weighing more than their predecessors in the `90s and early 2000s. While it was true that the top sellers in the West were always the largest displacement engines, the birth of the 900-1000cc class sleds did remind us that having that much weight planted in the nose of the sled is not a recipe for making it more fun to ride. Even with 80 lbs. removed from them, the weight distribution still limited them from being quite as much fun to ride as the smaller twins. It is still true that a majority of riders opt for the biggest engine they can buy but only if that engine does not result in a large weight disadvantage. In 2008, Ski-Doo introduced the Rev XP platform, and thus really deserves credit for bringing the weight discussion back to the forefront. Although the initial XPs had some chassis challenges that actually masked how light they were, any fairly conducted test between different brand sleds generally proved that the lighter sled did float and perform better, especially in deep snow. When Polaris was ready to introduce its latest chassis, the Pro RMK, weight loss was once again a key selling point.
With a dry weight of a claimed 431 lbs. on a 155-inch 800 Pro RMK, we couldn't help but wonder what it would be like after having been put on a diet. Once we rode the Pro and got to appreciate both the weight loss compared to a Dragon and the rigidness of the chassis, we felt even more confident that the sled would work even better with less weight.
The sled that we built and rode last year was about 408 lbs. and did confirm we were headed in the right direction. With some more work and even lighter components, the Pro XMK weighs in at just more than 386 lbs. dry. Even though we're not ready to reveal the whole recipe for the secret sauce, the major ingredients for this much weight loss include replacement of the rear suspension, replacement of the entire front suspension and skis, replacement of the chaincase with a belt drive, lightweight exhaust components, a lighter seat and modification of the driven clutch.
While we have been accused in the past of prioritizing weight loss over functional considerations, you'll notice that this sled still has both the stock headlight and taillight at that weight, along with both original bumpers as well. In addition, we could have lost more weight by running Walker air shocks on the front and a simpler shock on the rear. We've chosen to leave the coil shocks on the front and the superior shock on the Holz Alpha X rear skid to ensure great ride and handling characteristics along with lots of adjustability. We did have the opportunity to display this sled at both the Denver and Salt Lake City snowmobile shows and a common question was, "Where did you lose most of the weight?" To Polaris' credit, there is not a single spot on these sleds where it's possible to lose a lot of weight. Instead, it takes losing a little weight in a lot of spots.
I am often asked at the shop about upgrades and specifically in what order I would do them. Even though we are obviously proponents of losing weight, at our altitude in Colorado, the best initial upgrade is almost always a moderate increase in power and drivability from some engine mods. It simply provides the biggest improvement for the money spent. However, when the discussion turns to making big investments, I am less convinced that spending it all in the engine compartment makes the most sense. My experience is that almost every rider, regardless of his base skill level, is a better rider on a lighter snowmobile.
Alternately, I don't believe that every rider's skill benefits from having significantly more horsepower. In actuality, the trade-offs normally associated with big bore or turbo engines can make riding them even more challenging than a stock sled. While the permutations of mods performed and dollars spent is endless, it's at least worth considering that building a lightweight sled with moderate engine work can get you a similar weight-to-horsepower number as a high horsepower engine mod on a stock chassis.
Those who know me well would probably classify me as a numbers guy. After all, we built an engine dyno to quantify horsepower changes because trying to describe mod work in terms of sled lengths was too imprecise. Given that, it's worth considering whether my earlier assertion about a lightweight sled performing similarly to a pump gas turbo actually makes sense.
On our dyno at altitude (and not corrected to sea level), a stock Polaris 800 makes about 110 hp. For comparison, that's more than an original M8, about the same as an Apex and less than the Ski-Doo 800R. With our head mod and an SLP pipe, the horsepower increases to just more than 120. Combining the two, 386 divided by 120 means this sled will be carrying about 3.22 lbs. per horsepower.
In our experience, a stock Pro weighs between 434 and 438 lbs. If we add a relatively simple pump gas turbo, that's still likely to increase weight by at least 20 lbs. On the dyno, we've generally found that 6 psi of boost is worth about 30 hp. For the sake of round numbers, let's call our turbo horsepower 140. Again, combining the numbers, we have 456/140=3.26 lbs. per horsepower. Now, before anybody gets too invested or excited about these numbers, please keep in mind that I'm simply making the point that a light sled with some engine work can get you to a weight-to-horsepower ratio in the ballpark with a pump gas turbo. These numbers are general estimates based on previous turbo projects and are not the result of testing a specific kit. In fairness, we are also talking about a turbo at relatively low boost levels. We have certainly had higher boost 800s on the dyno making in the 180-200 hp range and it's quite clear that it's impossible to lose enough weight to achieve the kind of weight-to-horsepower ratios that high boost turbos produce.
If you are fortunate enough to be able to invest several thousand dollars in building big power or a seriously light sled, which one makes more sense? For me, the lighter sled is more fun to ride more of the time. The weight loss is more noticeable as the riding gets more technical. If you consider the physics, the lightweight sled will be more efficient. If two sleds have the same weight-to-horsepower ratio and one is heavier, it must be producing more horsepower. In a two-stroke, more power means it must be burning more fuel to produce that power. Given the amount of fuel it takes to make big horsepower, the efficiency of the light sleds means having a longer fuel range and perhaps being even lighter than the big power sled which may have to haul extra fuel.
Lastly, the long-term ownership costs of light sleds tend to be lower than big power sleds. Generally, the cost of lightweight parts is fixed and they don't continue to cost you money in the same way that a high horsepower sled is likely to. While it may be possible to bend or break light parts, they don't have the potential or tendency to blow up like modified engines do. Even if the turbo or big bore engine doesn't suffer a catastrophic failure, extra horsepower does shorten the life of the engine, wear drive clutches out faster, etc.
In fairness, hitting that hidden rock early in the season on the lightweight sled will be more expensive to fix when the front end is made of titanium.
There is also no feeling quite like going uphill with the skis dangling and listening to that special sound that a turbo makes.
If Mindy (my wife) will agree, I think the solution is to own one of each.
(Facey is owner of Rocky Mountain Xtreme in Colorado. For more information, contact 303-654-0867 or visit www.rmxtreme.com.)