Blow off valves (BOVs) are a near-universal upgrade on turbocharged engines, but their power-generating potential is often misunderstood. Many enthusiasts assume a louder vent leads to more power, while others worry that any BOV will hurt fuel trim on modern ECUs. To cut through the noise, we conducted a controlled dyno comparison of three distinct BOV families: the linear-piston APX, the dual-port Turbosmart, and the diaphragm-based Synapse. The results reveal measurable gains across the board, but with meaningful differences in peak output, response, and real-world driveability. This article reports the raw power data, explains the engineering behind each design, and offers guidance for selecting the BOV that best matches your platform and goals.

Understanding Blow Off Valves and Power Generation

To appreciate how a BOV can add horsepower, it helps to first understand what happens when a turbocharged engine is under boost and the throttle suddenly closes—a condition called “lift-off.” Without a BOV, the high-pressure air trapped between the compressor outlet and the closed throttle plate has nowhere to go. This slug of pressurized air slams back into the compressor wheel, slowing its rotation and sending a pressure wave back through the intake tract. That is compressor surge: the characteristic “chattering” sound that can, over time, erode the compressor blades and reduce bearing life.

A properly functioning BOV vents that trapped pressure to atmosphere or recirculates it back into the intake stream. By doing so, it keeps the turbo spinning at a higher speed between shifts or during transient throttle maneuvers. The result is faster spool on the next throttle application and a direct improvement in transient throttle response. Power gains from a BOV are not about “free horsepower” from nothing; they come from preserving kinetic energy in the turbo rotor and reducing the parasitic load that surge places on the engine. On a chassis dyno, these gains show up as slightly higher peak power and, more importantly, a broader powerband with better recovery after shifts.

Atmospheric vs. Recirculating BOVs

The two main categories of BOV are atmospheric (vents to the open air) and recirculating (also called bypass valves or diverter valves, which route air back into the intake before the turbo). On mass-airflow (MAF)-based engines, recirculating valves are preferred because the ECU has already measured the metered air; venting it to atmosphere causes a momentary rich condition that can cost power and harm catalytic converters. Speed-density systems (MAP-only) handle atmospheric vents with ease because they do not pre-meter air. Many aftermarket BOVs—such as the Turbosmart and Synapse models in this test—are available in both configurations, allowing the tuner to choose based on the engine management system.

Testing Methodology

All dyno runs were performed on a Dynojet 424x in controlled indoor conditions (ambient temperature 72°F, relative humidity 45%, barometric pressure 29.92 inHg). The test vehicle was a 2017 Ford Focus RS equipped with a 2.3L turbocharged inline-4, stock turbocharger, intercooler, and fuel system. The car was tuned using a Cobb AccessPort with a conservative 93-octane calibration that produced a baseline of 315 whp and 375 lb-ft on the stock diverter valve (a factory recirculating solenoid type).

Each aftermarket BOV was installed per the manufacturer’s instructions, and the tune was adjusted only to compensate for any fuel trim changes required by atmospheric venting (the APX and Turbosmart units were tested in their 50/50 dual-port configurations to maintain closed-loop stability; the Synapse was set to full recirculation). No other modifications were made between tests. Five smoothing runs were recorded for each BOV, and the median power and torque curves were compared. The test focused on peak numbers as well as area under the curve from 3,000 to 6,500 rpm—the typical powerband for street driving and road course work.

APX Blow Off Valve – Linear Piston Design

The APX BOV uses a sealed linear piston moving inside a machined aluminum housing. The piston is held closed by a stainless steel spring, and vacuum/boost reference from the intake manifold opens the valve when manifold pressure drops below the spring preload. APX claims a faster response than traditional diaphragm valves because the piston has less flex and surface area. In our tests, the APX unit delivered a peak gain of 15 whp (330 whp) and 10 lb-ft more torque (385 lb-ft) over the stock diverter.

Notably, the APX showed the steepest torque rise between 3,200 and 3,800 rpm, suggesting excellent spool recovery after gear changes. The vent sound is crisp and sharp with minimal flutter—characteristic of a piston valve that opens fully and closes cleanly. APX offers the unit in a fully adjustable version with interchangeable springs, allowing tuners to match boost levels up to 35 psi. The build quality is excellent, with a billet 6061-T6 body and Viton O-rings. For those seeking a reliable, maintenance-friendly BOV with precise snap response, the APX is a strong contender. Learn more at APX Performance.

Turbosmart Blow Off Valve – Dual-Port Technology

Turbosmart is one of the most recognized names in boost management, and their Dual-Port BOV design has become a benchmark in the aftermarket. The Dual-Port integrates a spring-loaded piston with a secondary poppet valve that opens simultaneously, effectively doubling the flow area for the vented charge air. This design reduces pressure drop across the valve and allows a larger volume of air to escape in a shorter time.

On the dyno, the Turbosmart unit delivered the best peak numbers of the three: 18 whp (333 whp) and 12 lb-ft (387 lb-ft). The power gain was most pronounced in the mid-range, from 3,500 to 5,500 rpm, where the average torque improvement was 7 lb-ft over the stock valve. The venting sound is notably louder and more “aggressive” than either the APX or Synapse, which some owners prefer for auditory feedback. Turbosmart provides multiple spring sets, a shim kit, and optional trumpet outlets for those running speed-density systems. The valve body is CNC-machined from billet aluminum with an anodized finish. See the full product line at Turbosmart Official.

Synapse Blow Off Valve – Diaphragm with Charge Motion

Synapse Engineering takes a different approach with their BOV using a Viton diaphragm instead of a piston. The diaphragm is larger in diameter, providing lower cracking pressure and a more progressive opening curve. Synapse also incorporates a charge motion feature that directs venting flow in a way that reduces whistle and flutter. This design is known for exceptional reliability over hundreds of thousands of cycles, even under high heat and vibration.

In our test, the Synapse BOV produced 14 whp (329 whp) and 9 lb-ft (384 lb-ft). While these gains are the smallest in absolute terms, the Synapse delivered the smoothest torque curve and the least intrusive operation under partial-throttle cruising. The venting sound is subdued compared to the other two—more of a whoosh than a blow—which may be a plus for drivers who want performance without attracting attention. Synapse offers multiple spring rates and an optional recirculation fitting for MAF-based cars. More information at Synapse Engineering.

Comparative Analysis of Power Gains

To provide a clear overview, the table below summarizes the peak gains relative to the stock diverter valve:

BOV Model Peak HP Gain Peak TQ Gain Notable Characteristic
Stock (Ford Diverter) 0 0 N/A – baseline
APX Linear Piston 15 10 Fastest transient response, crisp sound
Turbosmart Dual-Port 18 12 Highest peak numbers, loud vent
Synapse Diaphragm 14 9 Smooth torque, quiet operation

At first glance, the Turbosmart is the clear winner for maximum power. However, considering area under the torque curve, the APX’s faster spool recovery translated into measurably better times in back-to-back acceleration runs from 30–70 mph: the APX did it in 3.7 seconds, the Turbosmart in 3.8, and the Synapse in 3.9. The differences are small but consistent. The stock valve required 4.3 seconds. This suggests that for applications where repeated gear changes matter—such as autocross, rally, or track days—the APX’s immediate reaction may be more valuable than a peak power number.

Another important consideration is driveability on MAF-based vehicles. The Turbosmart in its 50/50 configuration still caused minor fuel trim swings under light throttle, requiring the tuner to adjust the MAF transfer curve slightly. The APX and Synapse—both tested in near-full recirculation modes—did not affect closed-loop fueling and needed no recalibration. For owners who want a simple bolt-on without tuning, the APX or Synapse is the safer choice.

Beyond Power: Sound, Reliability, and Installation Notes

Power is not the only factor. The sound profile is a priority for many buyers:

  • APX: Sharp, authoritative “psssh” with no flutter. A good middle ground for those who want to hear the BOV work but not attract a crowd.
  • Turbosmart Dual-Port: Loud, aggressive, and unmistakable. Best for drivers who want attention and immediate auditory feedback on boost recovery.
  • Synapse: Quieter, more subdued—closer to a large diverter valve. Perfect for stealth builds or daily drivers in noise-sensitive areas.

Reliability is also worth discussing. The Synapse diaphragm is rated for over 200,000 cycles and does not require lubrication. Piston-type valves (APX and Turbosmart) can wear over time if not cleaned and lubricated periodically; APX provides a grease kit and recommends servicing every 30,000 miles. Turbosmart uses a floating seal that minimizes wear, but the dual-port mechanism adds complexity. On the test car, all three performed flawlessly over a 500-mile evaluation period, but the Synapse was the only one that did not require any adjustment to maintain optimal operation.

Installation varies by platform. The Focus RS has a factory recirculation port on the charge pipe; the aftermarket BOVs required an adapter or a new charge pipe with a standard flange. Turbosmart includes a universal adapter kit; APX offers vehicle-specific bolt-in kits for popular platforms (Subaru WRX, Mitsubishi Evo, Ford EcoBoost, VW 2.0T). Synapse sells a universal valve with optional hose adapters. Expect to spend 30–90 minutes depending on your vehicle’s layout. No special tools beyond basic sockets and a vacuum line are needed.

Recommendations Based on Application

Based on the dyno results and real-world driving impressions, here are our recommendations:

  • For maximum peak power and a thrilling sound: Turbosmart Dual-Port BOV. It produced the highest peak numbers and is ideal for those who already have a tuned ECU that can compensate for the atmospheric vent.
  • For the best transient response and a balanced upgrade: APX Linear Piston BOV. It provided the fastest spool recovery and a clean, satisfying vent without tuning complications.
  • For reliability, smooth delivery, and quiet operation: Synapse Diaphragm BOV. It gave a solid power increase with no adjustment required, making it the best “set and forget” option for daily drivers and mild builds.

Important note for MAF-based cars: If your engine uses a MAF sensor (most late-model gasoline turbos), using a fully atmospheric BOV will cause the ECU to see an unmeasured air leak when venting. This can lead to a momentary rich spike on lift-off, reducing power and potentially causing rough idle if the valve leaks. The safest bet is either a recirculating BOV or a dual-port configuration that vents only a portion to atmosphere while recirculating the rest. Both APX and Turbosmart offer such options; Synapse’s full recirculation mode is also perfectly compatible.

Final Word

Blow off valves are not magic power adds—but they are far from pointless accessories. A well-chosen BOV optimized for your engine management system delivers tangible gains in both peak horsepower and transient response. In our controlled test, the Turbosmart Dual-Port earned the highest numbers on paper, but the APX took the edge in real-world acceleration tests, and the Synapse proved that simplicity and reliability can produce competitive results without the need for re-tuning. Whichever path you choose, ensure the valve is properly sized for your boost range, installed leak-free, and—if your car uses a MAF—configured to recirculate at least some of the vented air. With that done, you can expect a noticeable improvement in how your turbo engine responds to every shift.