BC1120-1 – Valve Spring – Dual (Mitsubishi 4G63 Eclipse/Evo) – 1 only

For the dedicated tuner or engine builder seeking to extract every last ounce of performance and reliability from the legendary Mitsubishi 4G63 powerplant, the valve train is a critical battleground. The factory valve springs, while adequate for stock power levels and conservative redlines, become a liability under the increased demands of forced induction, aggressive camshaft profiles, and sustained high-RPM operation. Valve float, resonance, and premature fatigue are the enemies of power and engine longevity. The Brian Crower BC1120-1 Dual Valve Spring Kit is engineered from the ground up as a direct, performance-oriented solution to these limitations. This is not merely a replacement part; it is a calculated upgrade designed to provide the precise valve control necessary for high-output 4G63 builds in applications ranging from the DSM platforms (Eclipse, Talon, Laser) to the revered Lancer Evolution series. As a cornerstone of a robust valve train, this kit ensures your engine’s breathing apparatus operates with absolute authority, translating directly to reliable power delivery and enhanced durability under extreme conditions.

Engineering Analysis: Material and Design for Extreme Duty

The BC1120-1 kit exemplifies a purpose-driven approach to component design. Each spring is constructed from premium chromium silicon alloy steel, a material chosen for its superior fatigue resistance and ability to maintain its elastic properties at elevated temperatures compared to standard spring steels. This metallurgical advantage is crucial within the hostile environment of a cylinder head, where heat soaking is inevitable. The dual-spring design itself is a masterclass in redundancy and performance. The primary (inner) spring carries the initial load and is responsible for the majority of the seat pressure, while the secondary (outer) spring acts as a reinforcing partner. This architecture serves two primary functions: it dramatically increases the system’s overall resonant frequency, pushing it far beyond the operational RPM range of the engine to eliminate harmful harmonic oscillations that can lead to valve bounce. Secondly, it provides a critical safety factor; in the highly unlikely event of a primary spring failure, the outer spring can temporarily maintain valve control to prevent catastrophic piston-to-valve contact.

Precision Specifications: The Numbers Behind the Performance

Performance is quantifiable, and the BC1120-1 kit delivers its promise through meticulously calculated specifications. The springs are designed to provide a significant increase in seat pressure over stock, which is essential for maintaining positive valve closure against high cylinder pressures generated by turbocharging. Equally important is the open pressure, which must be sufficient to prevent the valve from “lofting” off the rocker arm or lifter at peak lift, especially when paired with high-lift camshafts. Equipped with the necessary titanium retainers (sold separately, such as Brian Crower’s matching BC1125 set), this spring package is engineered for compatibility with a wide range of aftermarket camshafts. The following table details the exact technical parameters that define this component’s capability.

Installation Imperatives and System Compatibility

Installing the BC1120-1 dual springs is a precision operation that demands attention to detail. It is absolutely mandatory to verify coil bind clearance with your specific camshaft’s maximum lift. This involves measuring the compressed length of the spring at full valve lift and ensuring it exceeds the spring’s solid height by a safe margin—typically a minimum of 0.060″. Failure to do so will result in the spring compressing into a solid block, which will instantly destroy the spring, the camshaft lobe, and likely the valve itself. Furthermore, the increased seat pressure of these springs requires the use of upgraded titanium retainers (BC1125). The stock steel retainers are not designed for this load and can fail, with disastrous consequences. For a deep dive into the science of valve train dynamics and the importance of proper spring selection, refer to this authoritative resource on valve spring harmonics and engine performance. Always perform a thorough valve train geometry check after installation, including checking for proper retainer-to-seal clearance.

Comprehensive Vehicle Fitment

The Brian Crower BC1120-1 Dual Valve Spring Kit is engineered specifically for the Mitsubishi 4G63 engine platform. Correct application is paramount for performance and safety.

• Mitsubishi Lancer Evolution: Evolution III (CE9A), Evolution IV (CN9A), Evolution V (CP9A), Evolution VI (CP9A), Evolution VII (CT9A), Evolution VIII (CT9A), Evolution IX (CT9A).
• Mitsubishi Eclipse / Eagle Talon / Plymouth Laser: All 1990-1999 models equipped with the turbocharged 4G63 engine (6-bolt and 7-bolt configurations).
• Mitsubishi Galant VR-4: 1991-1992 models.
• Application Note: This kit is designed for use with standard diameter valves. It is a direct replacement for the factory valve spring and retainer assembly, but requires matching titanium retainers (BC1125) and appropriate valve stem seals. Always confirm compatibility with your specific cylinder head and valve specifications.

Performance Benefits Summary

Integrating the BC1120-1 kit into your 4G63 build delivers a cascade of measurable advantages that directly contribute to a more powerful, responsive, and reliable engine.

• Eliminates Valve Float: The high spring rate and dual design provide the force necessary to maintain positive cam follower contact at elevated RPM, preventing power loss and valve train damage.
• Enables Higher RPM Operation: By pushing the system’s resonant frequency beyond the engine’s operational range, stable valve control is maintained, safely allowing for a higher redline.
• Supports Aggressive Camshaft Profiles: Provides the necessary open pressure for high-lift, fast-ramp camshafts, ensuring the valve accurately follows the lobe profile.
• Enhances Durability Under Boost: Increased seat pressure ensures the valve stays sealed against extreme combustion pressures from turbocharging or high-compression builds.
• Built with Premium Materials: Chromium silicon steel offers superior heat resistance and fatigue life compared to OEM-style springs, for long-term reliability.
• Redundant Safety Design: The dual-spring system provides a critical fail-safe layer of protection against complete valve train failure.

Conclusion: The Foundation for a High-Output Valve Train

The Brian Crower BC1120-1 Dual Valve Spring Kit is a fundamental upgrade for any 4G63 engine destined for performance beyond factory limits. It addresses the core weaknesses of the stock valve train with a solution rooted in proven engineering principles: superior materials, intelligent dual-spring architecture, and precision-calibrated pressures. When installed correctly with the required supporting components—most notably titanium retainers—it transforms the valve train from a potential point of failure into a asset of unwavering control. For builders of DSM street monsters, dedicated Evo track weapons, or anything in between, this component represents a non-negotiable investment in the integrity and potential of the legendary 4G63 platform. It is the silent guardian that allows the turbo to spool, the cams to breathe, and the tachometer to climb with confidence.