Polaris PRO-RMK 850 163″ Project Sled

“The Evil Twin”

Take a stroll down memory lane and you will find that most snowmobilers modified their sleds at one point in time. Through the 70’s, 80’s and 90’s it was common to find machines with twin or triple pipes because after all, it was important to be faster, stronger, lighter, etc, etc. Racing snowmobiles was quite popular and many people raced as well as rode casually. In fact, modifying snowmobiles was in a way its very own sport, and was a past time for many.

Over time, stock machines have evolved and become better – much better. The need to modify has decreased, especially for the mountain rider who can now access areas on a stock machine that were not accessible on a modified machine just 15 years ago. Racing organizations have restricted modifications in an effort to make classes more “fair” or to make racing more “affordable”.

Today we have many young riders who have never been to a race event or have seen a set of twin or triple pipes. They’ve never had the experience of hearing a set of pipes scream around an oval or up a hillclimb course. Seriously, what could be better than the sweet sound of a high-revving set of tuned expansion chambers, especially a triple-piped triple? This gave us the idea to build a mountain sled the old school way. Let’s take what already works great, and throw whatever we reasonably can at it to make it even better

When Polaris announced that they would be coming out with a brand new 850 motor for 2019 we became interested in building a project sled to put it through the paces. Soon after, we started hearing rumors that SLP would be building twin pipes for the Patriot 850. At first it was like, “Really? Twin pipes?” But when we looked long and hard at the all-new engine design and construction, it started to make sense. Here we had an engine that was truly built strong from the ground up and would be receptive to spinning at a higher RPM level and able to flow enough air to make some serious power. Sure enough, SLP made the announcement at Haydays – twin pipes were indeed coming back. This was a no brainer and we knew that we had to try this combination for the project.

We contacted Polaris and arranged to purchase a 2019 PRO-RMK 163” with a 2.6” lug track. We would ride it stock for a good portion of the season and then transform it into a mod sled that would mimic race sleds from days past. And while we were at it, let’s see how it works at a hillclimb event. In fact, let’s compete at the World Championship Jackson Hillclimb with it.

Upgrade #1: A-Arms That Don’t Pick a Side

Breakdowns in the backcountry can ruin a day pretty quickly and in fact, can be quite dangerous if you aren’t prepared to get yourself out. While testing the 850 in stock trim we had the unfortunate experience of taking out three A-arms with a very large rock. We were riding in marginal snow, we knew the risks, and this time it bit us good. In what we think is a great strategy, Polaris engineered the AXYS chassis to protect the vital organs of the machine i.e. the bulkhead, motor and tunnel. This experience put that engineering to the test, and everything worked exactly as intended – all organs were safe and sound. Because we now had to replace broken parts, it was a perfect opportunity to throw on a set of aftermarket arms.

ZRP manufactures high end billet components and every one of them is race test tested – alot! While researching arms we learned that ZRP has designed an upper arm that can be flipped and used as a left or right hand arm. This is a big deal if you find yourself in a spot where you need to replace one. Dealer doesn’t have a left side in stock, no problem, a right side will work. Because they are billet, they are much stronger, lightweight and they can be anodized which is also very durable. Take a close look and you will see visible gussets next to hollowed areas which result in the best combination of lightweight and strength. We like that the lower arm is also a high-clearance arm which offers some performance benefits as well. More clearance reduces drag and also protects the arm from obstacles that could otherwise damage a standard clearance arm.

The arms were supplied with everything needed for proper installation including pivot bushings and upper ball joints. Plus, the upper ball joints are adjustable meaning a custom camber can be used if need be although we chose to stay with the stock camber settings. We found the install to be fairly straightforward and the supplied instructions were adequate for someone with basic mechanical experience.

Upgrade #2: A Ski That Rocks

If we wanted to go fast up Jackson we needed a ski that would be predictable, aggressive and did we mention predictable? It’s not a good thing to have the bars ripped out of your hands while slotting through gates and while we love the stock Polaris gripper ski, we were wanting to find something that would offer more floatation in the backcountry and hold a corner better through the gates at Jackson. We have used the SLP Mohawk skis for years because they do just that. Plus, the mounting brackets offer two mounting positions and several runner options so they can be fine tuned for specific conditions. Racers often use this ski with an aggressive carbide to turn fast on the bottom of the hill in qualifying, then switch to the welded runner for finals in order to prevent the ski from catching on sharp rocks on the upper part of the hill.

For the backcountry, we chose middle ground and used the 4” x 75° carbide as it offers enough grip on the icy trail, but doesn’t take much arm effort to turn. The skis themselves are slightly wider than the stock ski, utilize a rocker keel, plus they have two outside wings to add extra side grip in powder while increasing floatation. The cool thing is the side wings don’t touch the snow on hard pack so they don’t increase steering effort, but they still come into play when the ski gets into softer snow where more side bite and floatation is needed.

Upgrade #3: A Shocking Difference

We have been a bit critical of the stock shock package Polaris has offered on the RMK’s for the last few years but for 2019 we were pleasantly surprised with how well the stock Walker Evans piggyback shocks were calibrated. The rear track shock was still on the soft side for our bigger test riders in the backcountry and this also meant they were definitely too soft for racing.

We happened to be talking to Polaris about our project build when they offered up a great solution in the form of a somewhat new shock that was coming called the Walker Evans Velocity shock. While this shock had been offered on the 600RR, a slightly different version would be featured on one of the 2020 mountain sleds and Polaris was willing to get us a set to put through the paces.

The Velocity shocks are a bypass shock which means that some of the oil is not forced through the piston and is allowed to “Bypass” around it or away from it. Once the bypass is closed off the shock becomes 30% stiffer. The bypass is controlled by the location of the piggyback reservoir on the body of the shock. Compared to the standard piggyback shock the Velocity reservoir is mounted lower on the body.

Our Velocity shocks included both high and low speed compression adjusters unlike the single compression adjuster found on the standard piggyback shock (supplied on the PRO-RMK). These allow the user more ability to fine tune the ride based off shock shaft speed (not ground speed). They do not include a rebound adjuster although in 400 miles we only found a few situations where we wished we had it.

Upgrade #4: Two Pipes are Better Than One

Shoving two pipes into a tiny chassis and making them fit well is a work of art. As the saying goes, the Mona Lisa wasn’t painted in a day, and we knew it would take some time for SLP to cram a highly tuned set of pipes into a tiny chassis with precision fit and finish. These guys are two stroke exhaust experts and have become super detail oriented with fit and finish over the years. Because of this we knew it would take some time for SLP to refine a package that would meet their extremely high expectations.

The finished product was worth the wait and is nothing less than impressive. The fit and finish of the pipes is amazing and we are blown away that SLP was able to make them fit in the chassis so well. Because the chassis is so tight, SLP had to get creative with the fit and designed a double jointed headpipe in order to make them easier to install and be removed. And the sound – it brings back memories of open mod hillclimb sleds at Jackson. Thanks to SLP, these pipes are quiet enough to be ridden in the backcountry. The 2-into-1 silencer still has the raspy sound of a glasspack but at the lower noise level a responsible backcountry user should be emitting. Incredibly, the twin pipes and silencer are actually lighter than the stock single pipe/can combo by six pounds. An additional 17 ponies comes from these bolt on beauties making them the ultimate combination of power-to-weight.

SLP also provided clutching recommendations to properly calibrate the CVT clutches with the new found power. The pipes require some mapping adjustments (ECU re-flash) and while we were at it we threw on an SLP intake just to make sure we never ran the 850 lungs out of air. The pipes are ceramic coated to help reduce heat in the chassis and also to help get the pipes reach operating temperature quickly. While the ceramic coating does help with heat management we know that adding a second pipe is going to create some additional heat so we are running the SLP chassis vents as a way to help with under hood temps. Ringing out any machine with the flipper to the bar for long stretches will build up under-hood heat so we have made it a habit to stop and open our side panels occasionally to help cool off clutches and prevent heat saturation. This practice works to help control the heat from the twin pipes as well.

Upgrade #5: Gear Down to Go Up

Jackson doesn’t require gobs of top speed, but it is important to be fast corner-to-corner. Playing in the tight trees and in the creek bottoms also can benefit from quick throttle response and the ability to generate track speed on demand. A lower gear ratio can be a good way to improve both so we decided to bolt on the billet aluminum gear down kit from Kurt’s Polaris out of Seely Lake, Montana. Kurt recommends special clutching with this kit however because we were combining the kit with SLP twin pipes we decided to tackle the clutching on our own. We didn’t get this kit until the end of the season so we need to put more miles on it to give a full report but so far the SLP clutching worked quite well with the combined parts and we didn’t have to do much adjusting.

Upgrade #6: Feeding the Beast

Ok, so it isn’t so much of an upgrade but more of a preventative necessity. Since we turned up the horsepower we needed to make sure that we were running adequate oil and fuel to prevent future problems. SLP recommended that we run 91 octane non ethanol pump gas and to mix in some Lucas Oil Octane Boost as cheap insurance. If we happen to get a bad batch of fuel the octane boost will give us some buffer and help prevent detonation. Polaris recommended that we run Polaris VES Extreme oil as it has been designed to work with the 850 engine and would offer extra protection even with a modified engine. To date, all is well and the engine has run flawlessly with no reliability problems whatsoever.

Upgrade #7: Shiny Stickers = Armor

We wrap most of our machines to protect the plastic and components. Since we are going to race Jackson we decided to have a little fun with the twin pipe theme of the project. ArcticFX designed and printed our “Evil Twin” wrap and the finished product is amazing. One side benefit is if we ever happen to damage a panel, ArcticFX will reprint individual panels so we don’t have to purchase the entire wrap again. We ride in some pretty tight trees so the vinyl helps protect the plastic and keeps it looking good. We even had several situations where sticks would scratch the plastic but wouldn’t even leave a mark on the vinyl…..This Stuff Is Tuff!

Upgrade #8: The British call it a Bonnet

We prefer to keep the weight off our backs except for the essentials such as probe, shovel, spare gloves, first aid kit, fire starter, etc. You know, emergency stuff that we hope we never have to use. That means we need enough storage on the machine itself to carry the rest of our backcountry safety items. Polaris makes several bags that allowed us to store everything we needed, plus some. The machine shipped with the Pro-Ride under-seat bag but we decided to add the Burandt Adventure tunnel bag as well as a Protaper handlebar bag. Spare goggles go in the handlebar bag along with a GPS (preferably one that has emergency capabilities) for quick access. Under the seat goes our drink and homemade custom tool kit, lightweight gloves and balaclava. In the tunnel bag goes a spare mid layer and very warm gloves (in a ziplock bag), lunch, rope, saw, goggle lenses, hat, spare bolts, emergency LED headlamp, long pencil magnet and belt if needed. And yes, we have used every one of those items at one point or another.

The Verdict

To be honest, after we put 500 miles on the stock 850 we were starting to wonder if we should even modify it because the sled just worked so good. The saying “don’t mess with a good thing” was ringing in our heads but the urge to try out the pipes, shocks and a-arms was driving us crazy. Good thing too because the entire package came together much better than we could have ever hoped. The throttle response was lightning fast and the power curve was wider than any machine we have ever ridden. Most surprising was that we actually gained bottom-end power with the twin pipes – which was unheard of in the old days.

The power delivery starts out strong, revs fast and keeps pulling all the way to 8600 RPM. It has torque similar to a long stroke big bore, but the response of a short stroke race motor. The mapping and clutching worked perfectly, and the peak RPM stayed consistent even when we varied in almost 3000 feet of elevation. Backshift was quick and the RPM recovered well, even if we set the track down hard into a drift under full throttle.

At one point we learned a valuable lesson when we accidentally tried to install a Polaris #3211183 belt (for an 800), and then an aftermarket belt. Neither would pull proper RPM and we instantly lost 300-500 RPM. We found that it is very important to use the recommended #3211216 belt from Polaris as that is what the machine is calibrated for. Any other belt will definitely require a different clutch calibration and even then may not work properly.

Our early season rock encounter had made us somewhat hesitant to be aggressive. The addition of the ZRP a-arms gave us the confidence to hit the big holes and charge through the rough stuff, which was especially necessary at Jackson. The PRO-RMK React front-end is very agile and as the name implies, reacts quickly to rider inputs. The Velocity shocks and Mohawk skis magnified these traits even more, but what impressed us most was how tunable the entire package became.

We could calibrate the machine to stay flat and be aggressive for hillclimbing up Jackson or we could set the shocks softer, move the skis back and use less carbide to make the machine drop in easier and turn with less effort. We prefer something in the middle with less transfer while still keeping the suspension plush and predictable. Here are the settings we found worked best for us in the backcountry:

We are saving the race results from Jackson for another article but we plan to continue the build over the upcoming season and will again race this machine at Jackson in 2020. We will be playing with some more lightweight parts from ZRP including the lightweight flywheel and clutch cover. We will also be playing with more parts from SLP such as billet heads and torque arm. We are hoping to put over 2,000 miles on this machine which makes it the ultimate long-term test for a mountain sled. Curiosity just keeps getting the best of us!

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