Boosted Four-Strokes. The Sled of the Future?

When two-strokes make their departure from the sport of snowmobiling, now I know that the 4-stroke future is even better.

Jan. 28, 2008 By Stephen Clark

Two-strokes are on the way out, and as much as the snowmobiling community may stamp their feet and moan, it is a fact. The streetbike market has been forced to give up two-strokes, followed by most of the motocross market, the atv market, and it is only a matter of time before the snowmobile market is forced to give up oil-burning two-strokes. Sorry to break the news all you two-stroke lovers.

So where is the future of snowmobiling after legislation brings an end to the two-stroke motor? Well, just like streetbikes, mx bikes, and ATVs, the future is in four-stroke motors.

Yamaha has been on the four-stroke program for several years. (Remember how they were with the YZ400F mx bike back in '98?) The difference is the snowmobile market is much less accepting of the heavy four-stroke motors than the bike and quad communities. Yamaha nailed it with the YZ400F when Doug Henry won a AMA Supercross, but the four-stroke snowmobile crusade has not been quite as easy. The RX-1 had impressive power but was only marginal in a mountain chassis, the Apex was better, but the Nytro is the first four-stroke to compete with similar two-strokes right out of the crate. When Robbie Malinoski won a WPSA Open SnoCross on a prototype Nytro, we knew we were in for a treat.


As you may have read on our Nytro review, the machine is very capable even in stock form and will hold its own against any 600 and some 700 class machines in the mountains. But in stock form it is never going to run with a 800 or 1000 class machine, which is fine - it is rated at 130hp. Also, it would be fine with me if our riding group consisted of stock 600s but it doesn't. It's not fun to watch your buddies play in areas you can't even get to - so we took our Nytro over to Mountain Performance to see if they could help.

Like Yamaha, Mountain Performance has been on a four-stroke performance crusade since the beginning, building supercharger systems and lightweight components for Yamaha four-strokes to make them more capable in the mountains. The supercharger kit for the Nytro is the latest in their vast inventory of parts, and the kit has been under development since roughly the first prototype Nytro landed in the states over a year ago.


Abe installed a billet stand-off on to the end of crankshaft, this will drive the belt and pulley system.

Over the past few years, I have ridden stock machines and have grown to detest mod sleds. Many riding buddies have owned mod machines and I have witnessed their pain first-hand as the machines burn down and require lots of maintenance and tinkering to keep running. Sure, they run well when they do run - but they just don't run often enough. So I was a little hesitant to hand over a brand new, capable machine to be bastardized with performance parts.

how to

The Rotrex supercharger is mounted on the inside of the frame.

But I did it anyway. There were the concerns of using a supercharger versus a turbo; turbos with their explosive top-end power, the complexity of the supercharger system, and excessive heat from the turbocharger. But I was willing to sacrifice some overall boost (25psi+ available for most turbo kits) for the more favorable characteristics of the Rotrex centrifugal supercharger used by MPI.


The throttle bodies were removed and modified slightly before being reinstalled onto the engine. The billet plate on the throttle bodies is used to secure the intercooler to the throttle bodies.

The stage 2 supercharger kit was decided upon and Abe and Mac began to tear into the Nytro. It became immediately apparent that this was not a one evening installation, mainly because of the lack of space under the Nytro body panels. Unlike its two-stroke counterparts, the Nytro has almost no space under the hood or seat making the task of fitting a supercharger, intercooler, and piping difficult. But MPI had spent countless hours developing a kit that would fit underneath the stock body panels and fuel tank leaving the supercharged Nytro looking completely stock externally.

step by step

The oil reservoir is mounted in the nose cone of the Nytro in the space that was used for the tool bag, this reservoir is hooked to a small cooler and is a self-contained system to keep the supercharger correctly lubed.

The Nytro had to be disassembled. The fuel tank, headlight assembly and steering column had to be removed to begin installation. Thankfully, Yamaha made the job much easier by using dzus fastners on body panels and push pins on many of the connections. These fasteners can be removed quickly and make the process much easier.


The intercooler is bolted to the throttle bodies and secured with springs, the other end of the intercooler is hooked up to the supercharger via a rubber hose.

We drilled a hole in the aluminum frame for the Rotrex supercharger, and the Rotrex C30 charger bolted into location. Next the belt and pulley system used to drive the supercharger mounts on the outside of the frame. Some slight modifications were made to the throttle bodies to make them withstand the boost and the intercooler was bolted into location and hooked to the supercharger. The Rotrex supercharger requires specially formulated oil for the traction drive system inside the supercharger, so a separate oil system with a reservoir and cooler was also installed.

work hand

Both pulleys were mounted, aligned, and torque. One pulley is mounted onto the crankshaft and the other on the end of the supercharger.

With the main basic ingredients of the supercharger system installed, all that was left to do was wire in a 02 gauge, boost gauge, supercharger oil/water temp, and the GEMS module. The GEMS module is an electronic control box that hooks into the stock injector harness and reads boost from the intercooler, with a little user input the module adjusts the fuel for the increase in air from the supercharger.


With the supercharger installation complete all that was left was to re-assemble the machine.

The MPI kit is very impressive. All the components have been carefully machined to work around the stock machine, and the fit and finish of the kit is on par with how the stock machine is built. Abe and Mac re-assembled the machine and it was ready to go. The whole installation took roughly six hours and although many of the steps can be done by anyone with basic mechanical knowledge, a few steps do require patience and mechanical finesse. Anyone installing a kit must always keep in mind that one small error during installation could compound and turn into a much greater problem once the machine is on the snow. (And repairing machines in waist deep powder is definitely not ideal, it is much easier to take it slowly and do it right the first time in a nice warm shop.) MPI has several experienced technicians available by phone from 9-5 every weekday to assist customers with installation and tuning issues, should you decide to tackle it on your own.

With the installation complete we were eager to get out on the snow and see how the machine ran. To be honest, I was expecting the Nytro to be brutally fast - to the point of not being rideable. I was surprised to find that the machine was incredibly rideable and much easier to maneuver than stock. The power from the engine is definitely impressive, but more impressive is how well the chassis works with more power.


The machine is so easy to ride, you can put it almost anywhere you want on the mountain without ever feeling like you are in a sticky situation. There is absolutely no lag and the power is always right there when you need it, if it weren’t for the additional noise from the supercharger you would think that it was just a very powerful naturally aspirated engine. The MPI supercharged Nytro is the most fun, maneuverable, and capable mountain snowmobile I have ever ridden. All my concerns about riding a mod are completely gone since riding the machine. The Nytro still starts and runs like stock, only much faster.


As for maintenance, when snowmobiling is this much fun, I'm prepared to sacrfice a little more time checking the details to keep it running. So when the sad day comes that two-strokes make their departure from the great sport of snowmobiling, at least I now know that the four-stroke future is even better.

Mountain Performance

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