Understanding the Forge Motorsport Blow Off Valve Upgrade for the Audi S5 3.0T

The Audi S5 3.0T is already a well-balanced performance coupe, but many enthusiasts look for ways to sharpen its character. One of the most popular and cost-effective modifications is the Forge Motorsport Blow Off Valve (BOV) upgrade. Priced around $350, this component replaces the factory diverter valve with a fully mechanical, atmospheric blow-off valve. The result is a dramatic improvement in turbo sound, a slight but noticeable increase in throttle response, and improved long-term reliability under higher boost conditions.

This article provides a complete, in-depth look at the Forge BOV upgrade for the S5 3.0T. We will cover how it works, installation details, real-world performance gains, sound characteristics, and considerations before purchasing. Whether you are a seasoned tuner or a first-time modifier, this guide will help you decide if the Forge BOV is the right upgrade for your Audi.

What Is a Blow Off Valve and How Does It Work?

To understand the benefit of upgrading, it helps to know the function of a blow-off valve in a turbocharged engine. When you accelerate, the turbocharger compresses air and forces it into the intake system. When you suddenly lift off the throttle (for example, when shifting gears or entering a corner), the throttle plate closes. However, the turbo is still spinning and continues to pressurize air between the compressor and the throttle plate. Without a relief path, this pressure can slam back into the compressor wheel, causing a phenomenon called compressor surge or "turbo flutter." Sustained surge reduces turbo life and can damage the internals.

A blow-off valve is a pressure-relief device that opens when manifold vacuum (or boost pressure difference) signals that the throttle has closed. It vents the pressurized air to the atmosphere (atmospheric BOV) or recirculates it back into the intake system ahead of the turbo (recirculating or diverter valve). The stock Audi S5 3.0T uses a recirculating diverter valve that sends the air back to the intake, which is quiet but efficient. The Forge Motorsport BOV is an atmospheric valve that releases the air with a distinctive "psshh" sound, adding an audible performance cue.

Why Upgrade from the Stock Diverter Valve?

Factory diverter valves are designed for quiet operation and adequate reliability under stock boost levels. However, many S5 owners eventually upgrade their engine software (tuning) to increase boost pressure. The stock plastic diverter valve can fail under higher-than-stock boost, leading to boost leaks and performance loss. The Forge BOV is CNC-machined from billet aluminum with an anodized finish, using a diaphragm and piston design that is far more robust. It can handle boost pressures well beyond what the stock engine will ever produce, making it a perfect companion for stage 1 or stage 2 tunes.

Key Benefits of the Forge Motorsport BOV Upgrade

The Forge BOV provides several advantages over the factory valve. Below we break down each benefit in detail.

1. Enhanced Turbo Sound

This is the primary reason most enthusiasts buy the Forge BOV. The atmospheric venting produces a crisp, loud "whoosh" or "psshh" sound every time you lift off the throttle. The sound is more aggressive than a recirculating valve and gives the S5 a true performance car character. You can choose from different insert options to alter the pitch and volume — Forge supplies a standard trumpet insert and a softer "quiet" insert (sold separately) for those who want less noise.

2. Slight Power Increase and Improved Throttle Response

While the BOV itself does not directly add horsepower, it contributes to faster turbo spool between shifts. Because the valve vents instantaneously and completely, it reduces the backpressure on the compressor wheel when the throttle closes. This allows the turbo to maintain more RPM, so when you get back on the throttle, boost builds quicker. Many S5 owners report a measurable improvement in throttle response, especially after installing a stage 1 tune. Some dyno tests show gains of 5-10 whp and similar torque gains when paired with a tune, but the primary benefit is drivability.

3. Improved Reliability Under High Boost

The factory diverter valve uses a plastic housing and rubber diaphragm that can distort or crack under sustained high boost (above 18-20 psi). Tuned S5s often run 22-25 psi. The Forge BOV is all metal with a high-grade silicone diaphragm, rated for up to 30 psi continuous and even higher burst pressure. This means you can safely push your engine without worrying about a blown diverter valve leaving you stranded.

4. Easy Installation and Plug-and-Play Design

Forge designs its BOV kit to be a direct replacement for the factory diverter valve on the 3.0T engine (code EA837). The kit includes the valve, adapter plate, vacuum lines, and all necessary hardware. No drilling, cutting, or permanent modification is required. Most DIYers can complete the installation in about 1-2 hours with basic hand tools. The valve mounts in the same location as the stock valve, using the existing bolt holes.

Installation Process: Step-by-Step

Before starting, ensure the engine is cool. You will need a socket set (10mm, 13mm), flathead screwdriver, pliers, and a torque wrench (optional but recommended). The following steps provide a general overview; always refer to the official Forge instructions included with your kit.

Tools and Supplies Needed

  • Socket set with extensions
  • Flathead screwdriver
  • Needle-nose pliers
  • Torque wrench (for final bolt tightening)
  • Shop rags (to catch any coolant if needed)
  • Optional: trim removal tools for engine cover

Installation Steps

  1. Remove the engine cover: The S5 has a plastic cover over the top of the engine. Lift it upward to release the retaining clips, then set it aside.
  2. Locate the stock diverter valve: On the 3.0T supercharged engine (note: the 3.0T is actually supercharged, not turbocharged; however, the BOV still works the same way on the intake air, but many enthusiasts refer to it as a "turbo" sound upgrade — the system functions identically for compressed air). The valve is mounted on the front of the engine near the intake manifold, attached to the charge pipe.
  3. Disconnect the vacuum line: Carefully pull off the small rubber hose connected to the top of the stock diverter valve. Use pliers to release the clip if necessary.
  4. Unbolt the stock valve: There are two or three bolts (depending on the model year) holding the valve to its mounting bracket. Remove them with a 10mm socket and pull the valve out. Some coolant or oil residue may be present; wipe clean.
  5. Prepare the Forge BOV: The Forge kit comes with an adapter plate that matches the mounting surface. Apply the provided o-ring or gasket to the adapter. Then bolt the Forge valve onto the adapter plate.
  6. Install the assembly: Position the Forge valve with adapter onto the charge pipe mounting boss. Tighten the bolts to the specified torque (typically 8 Nm). Reconnect the vacuum line to the barb fitting on the Forge valve. If the hose is too short, use the supplied extension.
  7. Double-check all connections: Ensure the vacuum line is firmly attached and no kinks exist. Verify that the valve is securely mounted and does not contact any other components.
  8. Reinstall engine cover: Snap the cover back into place.
  9. Test the installation: Start the engine and let it idle. Listen for any vacuum leaks (hissing sounds). Rev the engine gently to hear the BOV operation. For a more complete test, take the car for a short drive and accelerate to 5-10 psi of boost, then lift off. You should hear a distinct "psshh" sound.

Tips for a Smooth Installation

  • If you have a 2013 or newer S5, the engine cover might have additional clips — use a trim tool to avoid breaking them.
  • Some aftermarket intakes may require repositioning of the BOV for clearance; the Forge kit includes a 90-degree adapter option in some versions.
  • If you experience a check engine light (very rare), ensure the vacuum line is connected to a proper vacuum source; some customers have mistakenly connected to a boost reference.

Performance Impact and Real-World Results

Many users on forums like AudiWorld and Audizine report that the Forge BOV transforms the driving experience. The sound alone makes the car feel more exciting, but the improved throttle response is equally appreciated. For those who have also installed a stage 1 tune, the BOV helps maintain boost between shifts, leading to faster acceleration. Independent dyno testing on a stage 1 tuned S5 showed a gain of 6 hp and 8 lb-ft of torque at the wheels after swapping from the stock diverter valve to the Forge BOV. While these numbers are modest, the subjective improvement in responsiveness is more significant.

It is important to note that the Forge BOV does not increase peak boost; it simply recovers boost faster after each shift. The power increase is most noticeable in aggressive driving, such as track days or spirited back-road runs. For daily commuting, the enhanced sound is the main draw.

Sound Characteristics: What to Expect

The Forge BOV produces a loud, clean "whoosh" when you lift off the throttle, especially under medium to high boost (10 psi and above). The sound varies based on the boost level and the type of insert used. The standard insert gives a sharp, high-pitched noise. The optional "quiet" insert reduces volume by about 50% and deepens the tone, similar to a traditional blow-off valve on a turbocharged car. Many owners prefer the standard insert for maximum drama. Note that the sound is also affected by your intake system; a cold air intake will make the BOV louder and more distinct.

On the 3.0T supercharged engine, the BOV vents air that has already passed through the supercharger and intercooler. The sound is slightly different from a turbo car, but still very satisfying. There is no negative impact on engine performance or drivability.

Long-Term Reliability and Maintenance

Forge Motorsport is known for high-quality manufacturing, and the BOV is no exception. The valve body is made from billet aluminum with a hard anodized coating to resist corrosion. The internal diaphragm is aramid-reinforced silicone, rated for hundreds of thousands of cycles. Forge recommends inspecting the diaphragm annually, but in practice, they last many years. If the diaphragm eventually wears, Forge sells replacement rebuild kits for about $40. This is a major advantage over the stock diverter valve, which is a sealed unit that must be replaced entirely if it fails.

To maintain optimal performance, occasionally check that the vacuum line is secured and not cracking due to heat. The valve should operate silently when closed; any fluttering or constant hissing indicates a leak or stuck piston. Cleaning is rarely needed, but you can spray mild brake cleaner on the piston and wipe with a lint-free cloth if it gets oily.

Cost Analysis: Is the Forge BOV Worth $350?

At $350, the Forge BOV is more expensive than some generic BOVs on eBay, but it is still one of the most affordable upgrades for the S5. The price includes the valve, adapter plate, hardware, and vacuum lines. Compare that to a full aftermarket intake system ($500-$700) or a cat-back exhaust ($1000+). The BOV delivers a unique audible experience and a reliability upgrade for a fraction of the cost. If you are planning to tune your S5, the BOV is almost mandatory to prevent the stock valve from failing. In that context, the $350 is a preventative maintenance investment that also adds excitement.

Some owners choose to buy a used Forge BOV from forums or classifieds to save money. That is acceptable as long as the valve appears undamaged and the diaphragm is in good condition. Forge does not void warranty for secondhand units, but you must be comfortable with the lack of a seller guarantee.

Potential Downsides and Considerations

While the Forge BOV is generally a trouble-free upgrade, there are a few things to consider before purchasing:

  • Increased noise: Some drivers may find the BOV too loud for daily use, especially if you live in a neighborhood or have a long highway commute. The quiet insert is a good alternative.
  • Emissions and inspection: In some regions, including California, any modification that vents to atmosphere can be considered tampering with the emissions system. While the BOV does not affect tailpipe emissions, it might not pass a visual inspection in strict jurisdictions. Check local laws.
  • No significant power gain without a tune: If you have a completely stock S5 with no software upgrade, you will mainly get the sound improvement. The throttle response improvement is minimal on a stock tune.
  • Potential for over-boost if incorrectly installed: If the vacuum line is connected to a boost source instead of a vacuum source, the valve may stay closed at idle, causing engine hesitation. Always follow the instructions.

Comparison with Other BOV Options

The S5 3.0T community has several aftermarket BOV options. Here is a brief comparison:

  • Forge Motorsport (atmospheric): As discussed, robust metal construction, great sound, easy rebuild. The go-to choice for many.
  • GFB (Go Fast Bits) DV+: This is a hybrid diverter valve that keeps recirculation but with a stronger spring and metal piston. It maintains the stock sound but prevents boost leaks. It is quieter than the Forge. Some prefer it for a sleeper look.
  • Boomba Racing BOV: Another atmospheric option, but cheaper (~$200). Users report it is slightly louder but less reliable in cold climates due to sticking. The Forge is generally considered more durable.
  • Stock valve with upgraded spring: Some tuners just replace the spring inside the stock valve for a few dollars. This is a budget fix but does not give the sound.

For most S5 owners who want the iconic blow-off sound and reliability, the Forge is the best balance of cost, quality, and performance.

External Resources and Further Reading

For more detailed information and community feedback, we recommend the following resources:

Conclusion: Should You Buy the Forge Motorsport BOV?

The Forge Motorsport Blow Off Valve upgrade for the Audi S5 3.0T is one of the most rewarding modifications you can make for $350. It delivers an unmistakable turbo sound that transforms the driving experience, while also improving throttle response and ensuring your engine can handle higher boost levels from a tune. The installation is straightforward, the build quality is excellent, and the valve is serviceable for years to come.

If you are a driving enthusiast who enjoys the mechanical symphony of a turbocharged car, the Forge BOV is a no-brainer. Even if you never plan to tune your S5, the sound alone makes every drive more engaging. For those who prioritize silence, you might consider the GFB DV+ or a quiet insert, but for the vast majority of S5 owners, the Forge BOV is the gold standard. Install it, enjoy the whoosh, and feel the sharper response every time you shift.