By Jon Crowley
The original Polaris RZR XP 900 generated plenty of attention when it arrived for the 2011 model year—and for good reason. After driving it at the 2011 Polaris RZR XP Media Event, it was clear that the new machine represented a major step forward in sport side-by-side performance.
Polaris described the RZR XP as the “ultimate combination of power, suspension and agility.” Much of that performance came from the newly developed ProStar 900 Twin, an engine designed in-house specifically for the RZR XP platform.
The 875cc Polaris ProStar 900 Twin engine introduced in the RZR XP 900.
A Purpose-Built Sport UTV Engine
The ProStar 900 was more than an existing ATV engine with a larger bore, different camshafts and revised CVT calibration. Polaris developed the 875cc twin-cylinder engine as part of the complete RZR XP powertrain.
In its original configuration, the engine produced a claimed 88 horsepower and had an 8,250-rpm limit. Combined with the RZR XP’s claimed 1,190-pound dry weight, that output gave the machine an impressive power-to-weight ratio for its era. Polaris claimed the RZR XP accelerated 29 percent faster than its closest competitor at the time.
DOHC Cylinder Head and Dual Throttle Bodies
The ProStar engine uses a dual-overhead-cam cylinder head with four valves per cylinder. This arrangement allowed Polaris to use high-flow intake and exhaust ports while reducing valvetrain mass and the number of moving components.
Fuel and air are delivered through two 46mm throttle bodies. Polaris calculated the engine’s specific output at approximately 100 horsepower per liter—an impressive figure for a naturally aspirated production side-by-side engine in 2011.
180-Degree Crankshaft Design
The ProStar 900 uses a 180-degree crankshaft, meaning its two pistons move in opposite directions. This configuration helps counteract some of the engine’s internal forces and reduces the amount of rotating inertia that must be managed by the balance shaft.
The result is a relatively compact twin-cylinder engine capable of delivering smooth, high-rpm power. Its oversquare 93mm bore and 64.4mm stroke also reflect the engine’s performance-oriented design.
Compact Transmission and Final Drive
Polaris developed a compact transmission specifically for the RZR XP and its new ProStar engine. The powertrain was engineered to transfer the engine’s torque to the wheels efficiently while controlling driveline noise.
A helical-cut final drive and optimized gear geometry helped improve efficiency and reduce power losses through the drivetrain. The transmission, engine and CVT calibration were designed to work as a complete package rather than adapting a utility-oriented powertrain for sport use.
Cooling for Demanding Conditions
High-performance UTV engines must cope with sustained loads, slow technical terrain and hot riding conditions. Polaris equipped the ProStar 900 with an integrated oil-cooling and liquid-cooling system intended to manage engine temperatures and support long-term durability.
This thermal-management strategy was particularly important in the RZR XP, which was designed for demanding environments ranging from high-speed desert terrain to sand dunes and rocky trails.
Polaris ProStar 900 Twin Specifications
- Engine type: Twin-cylinder, four-stroke
- Displacement: 875cc
- Claimed output: 88 horsepower
- Valvetrain: Dual overhead camshafts
- Valves: Four per cylinder
- Bore and stroke: 93mm x 64.4mm
- Compression ratio: 10.5:1
- Throttle bodies: Dual 46mm
- Intake valve diameter: 37.7mm
- Exhaust valve diameter: 32.4mm
- Exhaust header diameter: 1.875 inches
- Engine-speed limit: 8,250 rpm
Polaris RZR XP 900
Why the ProStar 900 Mattered
The ProStar 900 helped establish a new performance benchmark for production sport UTVs. Its 88-horsepower output, high-revving design and purpose-built drivetrain gave the original RZR XP 900 a level of acceleration that separated it from the utility-derived side-by-sides that came before it.
More importantly, the engine signaled Polaris’ commitment to developing dedicated powertrains for high-performance side-by-sides. The ProStar name would eventually expand across much of the company’s off-road lineup, but the 875cc twin in the RZR XP 900 was one of the engines that helped define the next stage of the sport UTV market.
