Blow-off Valves Vs. Wastegates

What Are They & Do You Need Them?

Blow-off valves and wastegates are likely parts you’ve heard of and might have a basic understanding of, but the specifics can be somewhat confusing. Both are valves that play an essential role in regulating pressure and airflow in turbocharged vehicles, but the specifics of what each does are drastically different. So let’s build your knowledge of these components so you can determine how best to make use of them for your build.

What’s a Blow-Off Valve (BOV)?

A BOV is a vacuum-operated valve that helps prevent engine damage by venting excess boost. A BOV is not responsible for controlling the maximum boost levels. It’s a common misconception that BOVs vent excess charge pressure once the boost threshold is met. Instead, BOVs vent charge pressure when the throttle is closed, and intake manifold pressure (the space behind the throttle plate) reaches a specific vacuum threshold. In short, a blow-off valve vents boost pressure when you let off the throttle, resulting in a distinctive “pshh” sound that can rile up just about any automotive enthusiast. Most factory-turbocharged vehicles will come with a recirculation valve, which serves the same basic function as a BOV but instead routes the air back into the intake and is much quieter.

Do I Need a Blow-Off Valve?

Technically, a BOV is not necessary for the normal operation of a turbocharged engine, but there is a big caveat here. Especially in the case of mid to high boost pressures, the lack of a BOV or an undersized BOV can cause something known as compressor surge. Compressor surge, more commonly called turbo flutter and verbalized as “stu-tu-tu-tu,” occurs when air backs up into the turbocharger and is chopped up by the compressor blades. Now, there are two kinds of compressor surges. Closed-throttle compressor surge occurs when the throttle plate closes, blocking the charge air from entering the engine. The turbocharger will still produce boost for a moment, and that air will back up into the turbo since it can’t pass the closed throttle plate. This type of compressor surge happens when the turbocharger isn’t under load, meaning it’s less prone to damaging the turbocharger than one might think. It will, however, reduce the turbine speed and can increase turbo lag. This can be alleviated with a blow-off valve. To synthesize the above, you may not necessarily need a BOV, but it would behoove your performance and reliability goals to have an appropriately sized BOV fitted to your vehicle. Contact the performance vehicle experts at Boostin Performance to learn more about how we can help you fit the right BOV to your vehicle.

What’s a Wastegate?

Turbochargers are driven by exhaust gasses that pass over a turbine after exiting the engine. The more exhaust that flows through the turbine housing, the faster the turbine will spin and the more boost it will create. That’s great, but what happens when the turbocharger starts making too much boost? There needs to be some way to control the turbine speed, and that’s where the wastegate comes in. A wastegate diverts exhaust gasses away from the turbine housing, limiting the turbine speed and, ultimately, the boost level. For more control of your boost level, you can also fit a boost controller to your wastegate.

Internal Vs. External Wastegate

Just about every factory wastegate will be an internal wastegate, meaning it’s an integrated part of the turbine housing. These will work fine for stock and moderately modified vehicles, but they don’t offer the same adjustability as an external wastegate. An external wastegate is mounted outside the turbine housing (surprising, we know) and is positioned between the exhaust manifold and turbine housing. With an external wastegate, you can easily adjust the spring rate (which adjusts boost pressure) and have more flexibility with the routing of the excess exhaust gasses. Being able to control the route of exhaust gasses means you can reduce turbulence in the turbine outlet and improve overall turbo performance.

Do I Need a Wastegate?

A wastegate is necessary in nearly all turbocharged setups. There may be some exceptions with carefully-sized turbochargers that would reach the maximum desired boost at the maximum RPM, but this isn’t typically feasible. So why do you need a wastegate? For one, over-boosting an engine can destroy it. Next, it comes back to compressor surge. If you’ll remember, we mentioned there are two types of compressor surges. While a closed-throttle compressor surge isn’t necessarily the worst thing for your turbo, an open-throttle compressor surge is a whole different beast. When the throttle is open, and the turbocharger is making more boost than the engine can use, this causes the charge air to back up into the turbocharger while under load. As you might imagine, this can cause incredible stress on the turbocharger internals and can lead to premature failure if severe enough. The wastegate is the sole solution; a BOV will not solve open-throttle compressor surge.

Shop Boostin Performance for Blow-Off Valves and Wastegates

While we’ve covered the different applications for both blow-off valves and wastegates, you might still have questions about them specific to your build. If that’s the case, don’t hesitate to reach out to the experts at Boostin Performance with a call today. Then, browse our selection of performance blow-off valves and wastegates on our online performance parts store.