BC3135 – Mitsubishi 4B11 Evo X Exhaust Valves (30mm / +1mm) – set/8

For the dedicated Mitsubishi 4B11 builder targeting absolute supremacy in forced induction performance, valve train integrity is the final frontier between a powerful engine and a grenade. The stock valvetrain, while robust for OEM purposes, becomes the critical failure point when pursuing significant horsepower gains and sustained high-RPM operation. The BC3135 exhaust valve set from Brian Crower represents a fundamental upgrade engineered to eliminate this weakness. These are not merely replacement parts; they are a precision-machined component system designed to withstand the extreme thermal and mechanical stresses found in modified Mitsubishi 4B11/Evo X engines. By increasing the exhaust valve diameter to 30mm (+1mm over stock), this set works in concert with appropriate cylinder head machining and camshaft upgrades to unlock improved exhaust flow, reduce cylinder pressure, and provide a monumental increase in valvetrain stability. This is engineering for those who measure success in horsepower retained at the redline and durability measured in seasons, not mere dyno pulls.

Engineering for Extreme Conditions: Material & Design Philosophy

The core challenge for any performance exhaust valve is managing the vicious cycle of heat and fatigue. Brian Crower addresses this with a two-pronged approach: superior material science and optimized geometry. The BC3135 valves are CNC-machined from a high-grade Austenitic Stainless Steel (21-4N) alloy. This material is selected for its exceptional high-temperature strength, corrosion resistance, and critical thermal expansion properties that closely match popular valve seat materials, preventing hot-seating and subsequent failure. The one-piece, solid stem construction ensures maximum integrity, avoiding the potential weaknesses of friction-welded designs under extreme abuse.

Beyond the material, the valve profile is meticulously designed. The +1mm oversize (30mm) head diameter is not just about moving more gas; it’s about doing so efficiently. The backside of the valve head is often contoured or undercut to promote flow characteristics as the valve lifts, reducing reversion and improving scavenging. This is a critical detail for turbocharged applications where efficient evacuation of exhaust gases directly translates to reduced turbo lag and lower exhaust manifold pressure. Each valve is manufactured to exacting tolerances, ensuring perfect consistency in weight and dimensions across the entire set—a non-negotiable requirement for achieving smooth, high-RPM valvetrain operation without destructive harmonics.

Precision Manufacturing & Surface Enhancements

The manufacturing process is where the BC3135 valves separate themselves from lesser alternatives. Each valve undergoes a multi-axis CNC machining process that guarantees stem straightness and concentricity to within ten-thousandths of an inch. The stem is then precision micro-polished to a mirror finish. This serves a dual purpose: it minimizes friction with the valve guide, reducing wear and heat generation, and it creates a superior surface for retaining essential lubrication oil film. Many performance engines fail due to stem galling; this polish is a primary defense. Furthermore, the critical margin—the flat surface of the valve head that seals against the seat—is machined to a precise width and angle, providing an optimal sealing surface that resists cracking and carbon buildup under detonation.

Tangible Performance Benefits of the BC3135 Valve Set

Installing the BC3135 valve set is a transformational modification that yields concrete, measurable results. It is a supporting modification that amplifies the effectiveness of your turbo, cam, and tuning upgrades.

• Increased Exhaust Flow Capacity: The +1mm diameter provides a significant increase in curtain area, allowing the engine to expel spent combustion gases more efficiently. This reduces pumping losses, lowers Exhaust Gas Temperatures (EGT), and enables the turbocharger to spool more quickly.
• Enhanced High-RPM Stability & Power: The superior material strength and consistent weight prevent valve float at elevated RPMs. This allows you to safely extend your rev limit, moving the power band higher and accessing horsepower that is otherwise left on the table due to valvetrain instability.
• Dramatically Improved Durability: The 21-4N stainless steel construction offers vastly improved resistance to heat fatigue, oxidation, and stem wear compared to stock valves. This is essential for engines running increased boost, aggressive timing, or sustained track use where thermal loads are catastrophic for OEM components.
• Supports Major Power Goals: By removing the exhaust valve as a flow restriction and failure point, these valves are a cornerstone build component for 500whp+ power levels. They provide the headroom needed for larger turbos and more aggressive fuel strategies.
• Optimized for Performance Valve Springs: These valves are designed to be paired with high-lift, high-rate performance springs and retainers (such as Brian Crower’s matched kits). Their design accounts for the increased seating forces, ensuring long-term reliability in a stiffly sprung race engine.

Technical Specifications & Fitment Data

The following table details the exact specifications of the BC3135 valve set. Compare these dimensions directly to your stock components and cylinder head blueprint to plan your machining work accordingly.

Vehicle Fitment

Primary Application: This valve set is engineered specifically for the Mitsubishi 4B11T engine platform. Direct fitment includes:

• Mitsubishi Lancer Evolution X (2008-2015) – All model years and trims (GSR, MR, SE, RS).
• Mitsubishi 4B11T Engine Swaps – A core component for any build utilizing this engine in a swapped vehicle.

Critical Installation Note: Installation of these +1mm oversize valves is not a direct drop-in procedure. It requires expert cylinder head machining. The exhaust valve seats must be professionally sunk and recut to the new 30mm diameter, and the valve-to-piston clearance must be meticulously verified, especially when used with higher-lift camshafts. Always consult with your machinist and refer to authoritative resources on valvetrain geometry, such as this technical article on valvetrain geometry from Engine Builder Magazine, to ensure a correct and reliable build.

System Integration & Final Build Recommendations

The BC3135 valves are a keystone component, but their full potential is only realized within a fully optimized system. They are designed to function as part of a comprehensive Brian Crower valvetrain system. For maximum performance and reliability, pair them with matching BC0540 or similar beehive spring kits, titanium retainers (BC1545), and high-lift camshafts. This ensures perfect harmonic compatibility and spring pressure for the intended RPM range. Furthermore, this upgrade should be considered alongside a performance valve job—a multi-angle valve seat cut (typically a 45-degree seat with top and bottom cuts) is highly recommended to maximize the flow benefit of the larger valve. For builders pushing the limits, remember that increasing exhaust flow can shift your engine’s volumetric efficiency map; always correlate this hardware upgrade with appropriate tuning adjustments to optimize air/fuel ratios and ignition timing. This is not a part for the casual enthusiast, but for the builder engineering a legend.