BC5911-75 – Unbalanced Polaris XP Turbo (16-up) Stroker Crankshaft 75mm w/270° Crank Pin

In the relentless pursuit of power and durability for the Polaris XP Turbo platform, the foundation of any engine build is non-negotiable. The BC5911-75 Unbalanced Stroker Crankshaft from Brian Crower represents a paradigm shift in rotational assembly design, engineered explicitly for the racer and builder who demands maximum performance without compromise. This isn’t merely a replacement part; it’s a calculated upgrade that redefines the engine’s displacement and combustion characteristics. By extending the stroke to a significant 75mm and incorporating a sophisticated 270° crank pin configuration, this crankshaft transforms the engine’s power delivery, moving the torque curve substantially downward for explosive, tractable power that dominates from mid-range to top-end. Crafted from premium 4340 chromoly steel, heat-treated, and finished with micron-precision, the BC5911-75 is the definitive choice for constructing a forged, high-horsepower ProStar engine capable of withstanding the immense pressures of forced induction and aggressive tuning.

Engineering the 270° Advantage: A New Power Stroke Philosophy

Traditional OEM and many aftermarket crankshafts utilize a 180° crank pin configuration, where opposing pistons reach top-dead-center (TDC) simultaneously. The Brian Crower BC5911-75 disrupts this convention with its advanced 270° crank pin offset. This design staggers the piston firing order, creating an uneven firing interval. The result is a profound change in the engine’s harmonic balance and exhaust pulse sequencing. This technical maneuver provides two monumental benefits for the XP Turbo: a dramatic increase in low-end and mid-range torque due to improved cylinder scavenging, and a distinct, aggressive exhaust note that signals its performance intent. This configuration effectively tunes the engine’s inertial forces differently, which, when paired with a properly balanced rotating assembly, yields a broader, more usable powerband—critical for applications in dunes, trails, and high-speed runs where tractable power is king.

Unbalanced Design for Precision Engine Building

The designation “unbalanced” is a feature, not a shortcoming. An unbalanced or “zero-balanced” crankshaft like the BC5911-75 is supplied without any counterweight correction. This is the professional standard for serious engine builds, as it allows the master engine builder to dynamically balance the entire rotating assembly—crankshaft, rods, pistons, and flywheel/clutch—as one unit. This process ensures perfection for your specific component combination, eliminating destructive harmonics at high RPM that can destroy bearings and components. It provides a level of precision and smoothness unattainable with a generic, pre-balanced crankshaft, guaranteeing longevity and stability in engines destined for high-boost or high-RPM operation.

Direct Benefits of the BC5911-75 Stroker Crankshaft

• Substantial Displacement Increase: The 75mm stroke significantly boosts engine cubic centimeters (cc), creating more displacement for greater air/fuel mixture volume and a foundational increase in torque and horsepower.
• 270° Crank Pin for Enhanced Torque: Staggered firing order drastically improves low-end and mid-range throttle response and pulling power, ideal for technical terrain and heavy vehicles.
• Premium 4340 Chromoly Steel Construction: Offers an exceptional strength-to-weight ratio and superior fatigue resistance compared to OEM or lesser 4140 steel crankshafts.
• Unbalanced (Zero-Balanced) Provision: Enables custom dynamic balancing for your specific rotating assembly, ensuring buttery-smooth operation and maximum durability at extreme RPM.
• Micron-Precision Machining: All journals and surfaces are CNC-machined to exacting tolerances, ensuring perfect oil clearance, alignment, and reliability.
• Designed for Forged Internals: The intended partner for Brian Crower’s H-beam connecting rods and forged pistons, creating an indestructible rotating assembly for high-boost applications.

Vehicle Fitment & Application Notes

Primary Fitment: This crankshaft is engineered specifically for the 2016 and newer Polaris XP Turbo (ProStar) 4-Stroke engine. It is a direct replacement for the OEM crankshaft but requires complementary components and expert installation.

Critical Installation Requirements: Utilizing this stroker crankshaft is a system-level modification. It must be installed alongside a compatible stroker piston (with a raised wrist pin location) to maintain proper deck height and compression ratio. It is incompatible with stock or standard stroke aftermarket pistons. Furthermore, the use of performance-grade H-beam or I-beam connecting rods, such as those from Brian Crower, is mandatory to handle the increased stroke and inertial forces. Engine control unit (ECU) retuning is absolutely essential to properly manage fuel and timing for the increased displacement and altered combustion characteristics.

Technical Specifications: The BC5911-75 in Detail

For a deeper technical dive into the physics and benefits of stroker crankshafts and crank pin angles, we recommend reading this authoritative resource from Engine Builder Magazine on stroker crankshaft fundamentals.