Polaris introduced the first sport version of a side by side in 2008 the Ranger RZR800. The RZRs more compact sport chassis offered an alternative to the top-heavy feel of the more utility based Yamaha Rhino Kawasakis burro speed Mule and even Polariss own bus-like Ranger 700. With a low center of gravity and sporty steering feel the RZR was the choice for side by side speed junkies. While Polaris installed a compact 760cc parallel twin motor they left a lot of room for improvement. The RZR powerplant is by no means a feat of technological wonder. Lets take a look at a few of the 760cc twins downfalls.
While many manufacturers build parallel twin motors not many build one quite like the Polaris motor. Look inside of the Polaris motor and you will find that it looks more like a Chevy 350 than any motorcycle engine you have seen before. The old school pushrod hydraulic valvetrain is the first challenge to the RZR motor. Digging deeper into the 760cc powerplant and removing the cylinder head will reveal the fact that both pistons are moving through their cycles together unlike typical parallel twin motors. By having the pistons cycle together Polaris supposedly created more mid-range horsepower and torque. Unfortunately with that supposed additional power comes a decreased rev-limit due to so much mass being thrown up and down at the same time. As if Polaris was trying to make it worse they forgot (or chose not) to add a support bearing to the center of the crankshaft thus further limiting the peak RPM of the 760cc twin. All of this combined creates what we call the generator motor effect. Like a generator the RZR motor seems to lack any sort of powerband. To the right is a dyno chart comparing the horsepower of an 08 RZR 09 RZR S 11 RZR S 11 RZR 900XP(stock) 11 RZR 900XP(un-corked) in that order from bottom to top. Dont panic yet there are plenty of mods you can perform to wake up that generator motor.
Soon after releasing the RZR Polaris introduced the S model which was their answer to power hungry enthusiasts around the world. Polaris engineers redesigned the exhaust system to flow like it should have in the first place. Combine that new exhaust with a larger camshaft and you now have a bit of a powerband and about another 6 horsepower. To get the S model a few mph ahead of the base model Polaris removed a limiting spacer inside of the clutch to allow for more travel in the sheave. The good thing here is that these parts could be swapped to the base model to gain the same power. Let us warn you though; in order to change the camshaft on a RZR you must remove the motor due to the fact that this is not an over head cam engine while the exhaust system is a pretty straight forward swap. Owners of base model RZRs can also disassemble the primary clutch on their units and remove the spacer that robs them of about 5 mph. To many owners this will simply not be enough power and thats where the aftermarket world comes in.
The RZRs rapid popularity created a giant demand for performance parts. With everything from big-bore kits to turbo systems being offered how is one to decide which route to take? The old saying that you can go as fast as you want to spend can get pretty extreme when it comes to a CVT vehicle so here are a few tips on what to spend your hard earned money on. The essential components to purchase are a set of quality high-compression pistons a nice camshaft and a good set of hi-rev valve springs. A RZR that will be used recreationally should not exceed 11.0:1 compression unless you like paying $7 a gallon to run it. While that compression may be safe with pump gas in a quad imagine the additional weight and load you are pulling in a SxS. Camshaft selection is crucial. Go to big and be prepared for pushrod and lifter nightmares; go to small and you might as well have left it alone. When trying to build a normally aspirated RZR motor keep in mind your final result will be something near 50 rear wheel horsepower. You can spend insane amounts of money trying to squeak out a few more ponies but in the long run you are going to be stuck somewhere around there. Building a generator motor can only result in one thing a built generator motor. The only answer to rear wheel horsepower numbers above 60 is boost.
While adding a turbo to a RZR may sound like a very complicated process todays turbo kit manufacturers make it quite simple. With a bolt on kit RZR owners can nearly double their rear wheel horsepower in less than a day worth of work. Reliability comes down to keeping boost pressure at a safe level. Polaris designed the fuel system and engine on the RZR as a normally aspirated only system. With the additional demands of the compression ratios achieved by adding boost catastrophic failure can occur if the boost pressure is raised above what your fuel system can supply or what your pistons and crank can hold. The majority of the turbo kits on the market include a boost sensitive fuel controller which does what the factory Engine Control Unit (ECU) can not; they read positive boost pressure and add fuel to compensate. These fuel controllers by themselves can support about 6 pounds of boost. If that is not enough power for you then you will have to upgrade the existing RZR fuel system. Step 1 is to replace the Polaris fuel pump with an upgraded model and depending on the pump you choose some modifications may be needed inside the tank. Step 2 is to add a return line from the fuel rail to the tank. This will allow you to add a rising rate fuel pressure regulator which is step 3. Under boost a rising rate fuel pressure regulator increases the fuel pressure feeding your RZRs fuel injectors. On our dyno we have supported up to 12 psi of boost with safe air fuel ratios with this setup. Horsepower freaks wanting over 75 rear wheel horsepower will have to add a third injector to their fuel system to keep air fuel ratios under check. With the fuel system handled engine internals are next to be addressed.
While compression and lightweight cast pistons are great for normally aspirated engines once boost is introduced factory pistons will collapse and cause engine failure. Typically we have seen ring landings collapse around 9 pounds of boost. Many RZR owners are using shims to lower the compression of their engine. Not only does this require special pushrods to compensate for the change in height but it does nothing to strengthen the pistons. A pair of low compression forged pistons will not only take more abuse but will also extend engine life and besides if you are going to tear down your motor why not do it right the first time. The cost of pistons and gaskets as a preventative measure will be far less than the cost of an engine failure. The chances that only your pistons will have to be replaced after a failure are slim to none. When ring landings collapse or pistons melt there is almost a guarantee that cylinder damage will occur. Seeing as how the cylinders of a RZR are not able to be bored replacement is needed at quite a cost. In a nutshell if you add a turbo to your RZR change your pistons. As far as the crankshaft goes there is a simple answer dont rev your motor beyond the factory set rev-limiter unless you plan to spend big money on an aftermarket billet crank. Many aftermarket ECUs integrate a 7000 rpm rev-limit; while this will definitely give you additional mile per hour it also exposes you to the risk of crank failure. Many RZR owners flirt with disaster when they run their engines beyond the stock rev-limit.
Choosing which route to take in building your motor should be carefully thought out. The terrain you plan on riding is a major factor. If mud bogging and deep water crossing are in your future a turbo may not be the best choice for you. On the other hand if your home is the dunes which requires substantially more horsepower a turbo is by far the best choice. Racers must remember that depending on the class you race in rules may dictate that a turbo is not allowed. Individuals with turbo restrictions have no choice but to build a high compression normally aspirated setup usually accompanied by extreme cylinder head modifications and high-end valve train. It is never good for the bank account to change plans part way through a build so think hard about where you want to be in the end.
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