BC3135S-8 – Mitsubishi 4B11 Evo X Exhaust Valves (30mm / +1mm) – set/8

In the relentless pursuit of turbocharged horsepower and unyielding reliability, the valvetrain stands as a critical frontier. For the Mitsubishi 4B11T powerplant at the heart of the legendary Lancer Evolution X, the factory exhaust valves represent a calculated compromise between mass production durability and performance. The BC3135S-8 Exhaust Valve Set from Brian Crower obliterates that compromise. Engineered as a direct-fit, +1mm oversize upgrade, this set of eight 30mm valves is a foundational component for any serious 4B11 build targeting elevated boost thresholds, sustained high-RPM operation, and absolute combustion integrity. This is not a mere replacement part; it is a precision instrument designed to transform exhaust flow characteristics and thermal management, directly translating to broader powerbands and enhanced engine security under extreme conditions.

Engineering the Ultimate 4B11T Exhaust Flow

The primary constraint for power output in a turbocharged engine is often the efficiency of expelling exhaust gases. The BC3135S-8 addresses this fundamental bottleneck through a meticulously engineered +1mm oversize design. Increasing the valve diameter from the stock 29mm to 30mm provides a significant increase in curtain area—the available space for gases to pass around the valve head. This reduction in exhaust restriction allows the turbocharger’s turbine to spool more efficiently, reduces backpressure in the cylinders, and facilitates a cleaner scavenging process. The result is tangible gains in mid-range torque and top-end horsepower, as the engine breathes with markedly less effort. Furthermore, the improved flow characteristic helps lower exhaust gas temperatures (EGTs), a critical factor in preventing pre-ignition and protecting the turbine housing and manifold from excessive thermal stress.

Material Science & Construction: Forged Durability

Brian Crower constructs the BC3135S-8 valves from EV8+ stainless steel, a material selected for its optimal balance of high-temperature strength, corrosion resistance, and thermal fatigue life. Unlike inferior alternatives, this grade of stainless is specifically formulated for exhaust valve duty, maintaining its tensile strength and resisting deformation under continuous exposure to extreme heat and cyclical loading. Each valve is crafted using a precision forging process, which aligns the metal’s grain structure to follow the contours of the valve. This forging technique creates a part that is inherently stronger and more resistant to fatigue failure than a machined-from-bar-stock or cheaper cast alternative. The valve stems are then hard-chromed and micro-polished to a mirror finish, achieving exceptional lubricity and wear resistance within the valve guide. This minimizes friction, reduces guide wear over time, and ensures consistent valve timing and seal throughout the engine’s operational life.

Performance Benefits & Application Scope

Installing the BC3135S-8 valve set is a strategic upgrade that delivers systemic benefits to the 4B11T’s performance envelope and longevity. It is a cornerstone modification for builds ranging from aggressive street applications to full-race engines.

• Increased Flow Capacity: The +1mm oversize design directly increases volumetric efficiency, supporting higher horsepower levels by reducing exhaust system restriction.
• Enhanced Thermal Management: Improved flow lowers cylinder and exhaust port temperatures, reducing the risk of detonation and extending the life of surrounding components.
• Superior High-RPM Stability: The forged construction and optimized weight ensure reliable operation at elevated engine speeds, critical for maintaining valvetrain control.
• Foundation for Forced Induction: This upgrade is essential for engines running significantly increased boost pressure, larger turbochargers, or advanced tuning strategies, providing the exhaust flow needed to realize that potential.
• Direct Fitment: Designed as a drop-in upgrade for the stock 4B11T cylinder head with appropriate valve seat machining, simplifying the installation process for experienced engine builders.

Technical Specifications & Vehicle Fitment

The BC3135S-8 is defined by its precise dimensions and material properties. It is imperative to pair these valves with a professional valve job, machining the seats to match the new 30mm diameter and the specified valve angles to achieve a perfect seal.

Vehicle Fitment

This valve set is engineered specifically for the Mitsubishi 4B11T dual-overhead-cam (DOHC) 16-valve engine. Primary application is for the 2008-2015 Mitsubishi Lancer Evolution X (Evo X). It is also a critical upgrade for any performance build, swap, or race application utilizing the 4B11T engine block and cylinder head. Critical Installation Note: Installation of +1mm oversize valves absolutely requires a professional machine shop to perform a corresponding 3-angle valve job on the cylinder head. The valve seats must be carefully cut to the new 30mm diameter to ensure proper sealing and alignment. This procedure should never be attempted without proper machinery and expertise.

System Integration & Tuning Considerations

The BC3135S-8 valves are a component within a system. To maximize their benefit and ensure reliability, complementary upgrades should be considered. A matched set of Brian Crower or equivalent high-performance valve springs and titanium retainers is strongly recommended to control the slightly increased mass at high RPM and maintain positive valve closure under high boost. This upgrade often accompanies porting and polishing of the cylinder head, where the larger valves can take full advantage of smoother, reshaped exhaust ports. From an engine management perspective, while the valves themselves do not require a specific tune, they enable the engine to safely make more power. Any subsequent modifications to boost, fuel, or timing that increase output must be calibrated with a professional ECU tune to exploit the new flow potential while maintaining safe air-fuel ratios and ignition timing. For a deeper technical understanding of valvetrain dynamics and their impact on performance, refer to this authoritative resource on high-performance valvetrain principles.

In conclusion, the Brian Crower BC3135S-8 Exhaust Valve Set is a definitive upgrade for the Mitsubishi 4B11T platform. It represents a fusion of precision machining, advanced material science, and performance-focused design. By addressing the exhaust flow limitation inherent in the factory configuration, it provides a measurable increase in power capacity, improves thermal efficiency, and delivers the forged durability required for competitive use. For the builder who views the engine as a system of interdependent components, these valves are not an option; they are a necessary investment in the foundation of a potent and reliable high-performance powerplant.