The Mitsubishi Lancer Evolution IX, or Evo 9, remains one of the most celebrated turbocharged platforms in automotive history. Its 4G63T engine, combined with a sophisticated all-wheel-drive system and aggressive factory turbo plumbing, delivers an exhilarating driving experience. For owners looking to sharpen throttle response, reduce turbo lag, and produce a more pronounced whoosh sound, one of the most popular upgrades is replacing the factory blow-off valve (BOV) with a performance unit. The Forge Blow-Off Valve is widely regarded as a top choice due to its robust construction, reliable sealing, and ease of tuning. This guide provides a complete, step-by-step installation process, along with deeper insights into the why and how of upgrading your Evo 9's BOV.

Understanding the Role of a Blow-Off Valve on the Evo 9

Before diving into the installation, it helps to understand what a blow-off valve does and why upgrading from the factory unit can make a significant performance difference. On a turbocharged engine like the Evo 9's 4G63T, when you lift off the throttle after accelerating, the throttle plate closes. However, the turbo is still spinning and compressing air, creating a pressure spike in the intake tract. Without a relief path, that pressure can crash against the closed throttle plate, causing compressor surge – a harsh fluttering sound that can damage the turbo bearings and reduce performance.

The stock BOV recirculates this excess pressure back into the intake system before the turbo. While functional, the factory plastic and rubber unit is prone to leaking under high boost (especially on tuned cars). A Forge BOV uses a billet aluminum body and a high-quality piston valve that seals much more reliably, holding boost pressure without bleeding. This means less wasted air, quicker spool on the next shift, and improved throttle response. Many owners also prefer the louder, more aggressive sound when the valve vents to atmosphere, though this may require tuning adjustments (more on that later).

Why Choose a Forge Blow-Off Valve for Your Evo 9?

The Forge Motorsport blow-off valve has become an industry standard for the Evo community. Unlike some competitors that use diaphragm-based designs prone to tearing, Forge uses a piston-based system with an o-ring seal. This design is more durable and holds boost more consistently. The valve is also fully serviceable, and Forge offers different spring rates to match your boost level (factory springs are typically blue or red, with red being stiffer for higher boost). The Forge BOV comes with multiple adapters and fittings, allowing easy installation whether you choose to recirculate or vent to atmosphere.

Additionally, the Forge valve is machined from aluminum and hard-anodized to resist corrosion and heat. Over years of use, it will remain leak-free, whereas stock valves often develop cracks or torn diaphragms. For tuned cars running 25–30 psi, a Forge BOV is essentially a reliability upgrade as much as a performance one.

Step 1: Gather Tools, Materials, and Safety Precautions

Proper preparation prevents a frustrating installation. Set aside about an hour and a half, ensure the engine is completely cool (or wait at least 30 minutes after a drive), and have the following items ready.

Required Tools and Parts

  • Forge Blow-Off Valve Kit (specific to Evo 7/8/9; part numbers include FMCLOTH, FMCLOTR, or FMCLVAC depending on orientation – verify with your supplier).
  • Socket set with 8mm, 10mm, 12mm sockets (standard and deep) and a ratchet.
  • Wrenches: 10mm and 12mm combination wrenches.
  • Flathead screwdriver (medium) for prying off hose clips.
  • Small pick or pliers for removing vacuum lines.
  • New hose clamps (if not included in the kit, pick up some miniature worm-drive clamps).
  • Electrical cleaner or brake cleaner (to degrease the intake pipe threads).
  • Anti-seize compound (optional, for bolt threads).
  • Shop rags and a flashlight.

Safety First

Disconnect the battery negative terminal. The Evo 9's ECU can store learned fuel trims; disconnecting the battery for at least 10 minutes will reset them and allow the car to relearn with the new BOV. Also, wear safety glasses – you will be working near hot engine components, and a stray drip of coolant or grease can be unpleasant.

Step 2: Removing the Factory Blow-Off Valve

The factory BOV on the Evo 9 is located on the charge pipe, which runs from the intercooler to the throttle body. On US-market Evos, the stock BOV is a plastic unit mounted near the intake manifold, typically with a small vacuum line attached to its top port.

Locating and Accessing the Valve

Open the hood and look at the passenger side of the engine bay. You will see a black plastic pipe (the charge pipe) connecting to the throttle body. The stock BOV is bolted onto a flange on this pipe, about 6 inches from the throttle body. The vacuum line runs from the valve to a nipple on the intake manifold. On some models, a larger recirculation hose connects from the BOV to the intake pipe just before the turbo inlet.

Step-by-Step Removal

  1. Ensure the engine is cold and the battery is disconnected.
  2. Carefully disconnect the small rubber vacuum line from the top of the factory BOV. Use a pick or squeeze the clip if equipped. Note how the line is routed – you’ll reuse it.
  3. If your car has the recirculation hose (a 1-inch rubber hose from the BOV to the intake), loosen its hose clamp with a flathead screwdriver and slide it off.
  4. Using a 12mm socket or wrench, remove the two or three bolts that secure the factory BOV to the charge pipe flange. They are usually M6 bolts threaded into the plastic valve or into a metal bracket. Be careful not to overtighten or strip threads during removal.
  5. Once the bolts are out, gently pull the factory BOV straight off the flange. It may be snug; a slight twisting motion helps. Do not pry against the charge pipe as it can crack the pipe.
  6. Inspect the factory BOV’s sealing surface and the O-ring (if any). Compare the condition to your new Forge unit – you will see why an upgrade is beneficial.

Note: If the bolts are stubborn, apply penetrating oil (e.g., PB Blaster) and wait 10 minutes. Avoid using excessive force as the charge pipe is aluminum and can be damaged.

Step 3: Installing the Forge Blow-Off Valve

Now the fun part – fitting your new Forge Blow-Off Valve. The installation is straightforward, but attention to two details makes the difference: proper sealing and correct spring orientation.

Pre-Installation Preparation

Before mounting the Forge valve, check that the included O-ring is seated in its groove on the valve base. Apply a thin layer of silicone grease or oil to the O-ring so it slides onto the flange smoothly and seals without binding. Some Forge kits come with an adapter plate; align it with the valve first. If your kit uses a separate adapter, confirm that the flange holes match your charge pipe’s bolt pattern. Older Evo 9s may have a different bolt spacing than newer ones, but the Forge kit is generally universal.

Mounting the Valve

  1. Position the Forge valve (with adapter, if needed) onto the charge pipe flange. Ensure the piston (the center moving part) is not hitting anything and that the valve’s orientation allows the vacuum port to point toward the engine (or upward, as per instructions). The usual orientation has the vacuum port facing the driver’s side or straight up.
  2. Insert the factory bolts (or new stainless ones from the kit) through the holes. Hand-tighten them first, then torque to 10–12 N·m (7–9 ft-lb). Do not overtighten – the adapter or flange may crack.
  3. Reattach the vacuum line. If your Forge valve uses a push-on fitting, simply press the line onto the barb. If it uses a threaded fitting, tighten carefully using a small wrench. The vacuum line is critical; a leak will cause the BOV to bleed boost or fail to open.
  4. If you are recirculating the BOV (keeping it plumbed back into the intake), reconnect the large recirculation hose to the outlet port on the Forge valve. Secure with a hose clamp. If you choose to vent to atmosphere, cap the recirculation port on the valve (some Forge kits include a cap) and block off the intake pipe nipple with a rubber cap and clamp. Be aware that venting to atmosphere on a MAF-based car like the Evo 9 can cause rich or lean conditions under high vacuum – tuning may be needed. Many owners run the Forge valve in recirculating mode for optimal drivability.

Adjusting the Spring Preload

One advantage of the Forge BOV is adjustable spring preload. If you are running stock boost (around 19 psi), the standard blue spring is usually correct. For higher boost (25+ psi), you may need the red spring or to add shims inside the cap. To adjust, unscrew the cap by hand, remove the spring, add or remove shims, and reassemble. Follow the instructions included with your kit.

Too much preload can cause the valve to stay closed under harsh deceleration, leading to compressor surge. Too little preload can cause the valve to leak boost. If you hear fluttering (surge) after installation, increase preload. If you hear a whistling or leakage sound under boost, decrease preload or check the O-ring.

Final Checks and First Startup

With the Forge BOV installed, reconnect the battery negative terminal. Double-check all hose connections and ensure no tools or rags are left in the engine bay. Start the engine and allow it to idle for 30 seconds.

  • Listen for vacuum leaks – a hissing or whistling sound suggests the BOV isn’t sealing or a vacuum line is loose.
  • Check the BOV’s operation at idle: put your hand near the valve (careful of moving engine parts) – you should feel a slight suction (if recirculating) or no movement if the valve is closed. Some Forge valves have a slight audible "ping" or "tick" at idle – that’s normal as the piston moves.
  • Rev the engine gently to around 3000 RPM and then release the throttle. You should hear a clean, sharp "psssh" sound (either vented or recirculated, depending on your setup) without any fluttering. If you hear fluttering, the valve may need more preload.

Troubleshooting Common Issues

Boost Leak

Symptoms: poor performance, high idle, or a hissing sound under boost. Check the O-ring and bolts. Also verify the vacuum hose is not cracked or loose. A common mistake is not tightening the vacuum line barb enough – the plastic factory line can be brittle; replace with silicone hose if needed.

Fluttering / Compressor Surge

This indicates the BOV is not opening quickly or fully. Increase spring preload by adding a shim or installing the stiffer spring. Also ensure the vacuum source is manifold vacuum, not boost reference. Even at idle, the manifold is under vacuum, which should pull the BOV open slightly. If the valve is stuck closed, clean it and lubricate the piston with light oil.

Valve Stays Open / Misfire at Idle

If the BOV is stuck open at idle, you will have a vacuum leak and a rough idle. Check the vacuum line – if it is blocked or the valve’s piston is jammed due to dirt, clean it thoroughly. Also ensure the spring preload isn’t so light that the valve is held open by vacuum.

No Sound

If you are running recirculating mode, the sound will be much quieter than an atmospheric vent. That’s normal. If you are venting but hear nothing, check that the vacuum line is connected to the intake manifold – some Evo 9s have multiple vacuum ports; use the one that sees manifold vacuum (not boost). Also verify the valve’s piston moves freely by pushing it with your finger (engine off).

Tuning Considerations for Vented Operation

Venting a Forge BOV to atmosphere on the Evo 9 can be done, but it may alter the air-fuel ratio. The MAF sensor measures incoming air, but when the BOV vents metered air to atmosphere, the ECU thinks there is more air entering the engine than actually does, causing a rich condition at shifts. This can lead to hesitation or sooty exhaust. Many tuners can compensate with ECU adjustments (e.g., reduce injector duration during high vacuum shifts). For most daily-driven Evo 9s, running the Forge valve in recirculating mode is recommended to maintain factory drivability.

If you must vent to atmosphere, consider a recirculating kit (some Forge valves can be converted) or a blow-through MAF setup, but that’s beyond this basic install guide.

External Resources and References

  • Forge Motorsport Official Site – Check for specific part numbers and tuning guides: Forge Motorsport
  • EvolutionM.net Forums – Extensive community discussions on BOV tuning and installation tips: EvolutionM.net
  • LancerRegister.com – Another great resource for Evo DIYs and troubleshooting: Lancer Register

Conclusion

Installing a Forge Blow-Off Valve on your Mitsubishi Evo 9 is one of the easiest and most rewarding performance modifications you can do. It improves throttle response, protects the turbo from surge, and gives your car that unmistakable turbo sound – all while being a straightforward 30-minute job for anyone with basic tools. By carefully following the three steps outlined above (gathering tools, removing the stock unit, and mounting the Forge valve), you can enjoy a significant upgrade in drivability and reliability. Whether you keep it recirculated or choose to vent to atmosphere, the Forge BOV is built to last and backed by a company with decades of turbo-system expertise. After installation, take your Evo 9 for a spirited drive and feel the difference – you won’t want to go back to stock.