The turbo system boosts engine power by forcing more air into the engine. While more air converts to more engine power, too much boost from the turbocharger could damage the engine. The turbo uses a blow-off valve and wastegate to keep the turbo boost in check.
While both devices come with some cars, others install them. However, both devices are often confused for each other. Follow as I explain what they do and which you need by comparing blow-off valve vs. wastegate side by side.
Blow off valve
The blow-off valve, abbreviated as BOV and sometimes called dump or compressor bypass valve, sits at the intake side of the turbo. And helps ward off excess pressure in the intake to avoid compressor surge when engine speed changes suddenly. A typical example when an engine speed changes is when you release the throttle.
Generally, when you take your foot off the throttle, the engine reduces the fuel sent for combustion, so it will need less air. The problem is, at this point, the intake already has much-pressured air stored, which is supposed to be released into the engine. Since the engine doesn’t need the air, it remains in the intake.
However, such high-pressure air isn’t supposed to stay in the intake and will blow up the intake pipes. In a worst-case scenario, it will cause turbo stalling or move pressure back into the turbo, causing potential harm to the turbo.
The turbo blow-off valve removes this unwanted pressurized air from the intake and dumps it elsewhere, preventing the intake from blowing up and eliminating turbo damage. Generally, the BOV stays closed, but when it detects excess pressure in the intake, it opens a spring, releasing residual air pressure.
Blow-off valves come in two types — recirculating and atmospheric valves. The recirculating blow-off valves dump pressure air into the non-pressurized side of the turbo. In contrast, an atmospheric BOV releases pressured air into the atmosphere.
Typically, which valve type is best for your turbo depends on how much boost your turbo has and the space available to install it. Except you want to damage the engine, don’t drive a turbo without BOV.
The wastegate, also known as the diverter or bypass valve, found by the exhaust flank of the turbo helps reduce the boost created by the turbo. It does this by regulating turbo speed.
Turbos help pull more air (boost) into the engine, helping the engine to generate more power. However, some engines need only a certain amount of turbo boost. Once it’s too much, it can lead to an explosion or engine damage. The only way to reduce the amount of air the turbo generates is to reduce the turbo speed.
Usually, how fast the turbine spins is regulated by exhaust gas. More exhaust gas means more speed and less exhaust gas means less speed. So, to slow down turbo speed is to limit the exhaust gas flowing to the turbine. This is where the wastegate comes in.
The wastegate helps divert excess exhaust gas from the turbine. With less air entering the turbine, the slower it spins and the less boost it generates for the engine. Wastegate stays close and opens up to release excess exhaust air into the atmosphere or exhaust.
Wastegate operates using different mechanisms. While most wastegates work pneumatically (with air pressure), others are computerized.
Pneumatically controlled wastegate work using an actuator spring. When air pressure gets to a certain psi level, the spring opens up and actuates a valve which allows exhaust air to bypass the turbine and leave through the wastegate.
In contrast, computerized wastegates utilize a sensor. This sensor signals the ECM when the pressure is too high. The ECM, in turn, signals a solenoid to open the wastegate to let out exhaust gas.
There are two wastegate positions depending on your configuration—internal and external wastegates. External wastegates are positioned on the exhaust side of the turbo and sit between the turbine wheel and exhaust manifold. In contrast, internal wastegates are placed in the turbine housing itself, featuring a portable design with fewer pipes.
Not all turbocharged engines require a wastegate. Typical examples are old model turbos that feature a variable geometry turbine housing working in a similar capacity with a wastegate.
Blow-off valve vs. Wastegate: Differences
Both the BOV and wastegate work to vent air from the turbo system. The major difference is how and where they do the job. While the BOV controls air inside the system, the wastegate regulates air yet to enter the system. But to be more direct, here are the major differences between blow-off valve vs. wastegate in turbo.
Position: While blow-off valves work at the intake side, the wastegate works at the exhaust flow side of the turbo.
Main task: The blow-off valves remove pressurized air from the intake, whereas the wastegate diverts exhaust gas from the turbine wheel.
Blow-off valve vs. wastegate sound
Both devices make a distinctive sound that helps you know they are functional. Blow-off valve vs. wastegate sound; here are the differences. Typically, a BOV makes a hissing sound when working. You should hear the blow-off valve sound when the valve is removing excess pressured air from the intake.
However, if you hear a rattling or chipping noise, the BOV is probably not working, or it’s too small. A non-functional BOV can cause compressor surge, straining the compressor and its bearing, invariably shortening the turbo’s life span.
In contrast, a wastegate is silent; it doesn’t make any noise. If you hear any, there is probably an issue with the wastegate or a compressor surge. It could be a loud whooshing noise or a hard-hitting brap that would pop when the wastegate is diverting excess air from the turbine into the exhaust or atmosphere.
Moreover, a bad external wastegate causes a fluttering sound when the throttle closes. When the external wastegate is bad, it may prevent the turbo from pushing air into the engine when it should. Hence the flutter sound.
A failing wastegate might also make a rattling noise, and this is due to the fact the wastegate is not sealed properly. So, for those asking, do wastegates make a sound? Now you know.
Blow-off valve vs. wastegate turbo
In turbos, the BOV and wastegate work synchronously to keep the engine and turbo intact. If you are wondering, can you run a wastegate and blow-off valve together? Yes, you can.
With the BOV helping to remove excess pressure from the intake to avoid blowing up, the wastegate ensures the engine doesn’t get an excess boost from the turbo. In fact, the wastegate and blow-off valve combo is very necessary for turbo engines.
So, the question of blow-off valve vs. bypass valve, which is needed, shouldn’t even be a topic of debate unless you want to damage your engine or turbo. However, for turbos with variable geometry turbine housing, you don’t need a wastegate since it functions in the same ability as a wastegate.
Blow-off valve and Wastegate pros and cons
While both devices work to keep your turbo in perfect health, they can sometimes also be a pain in the neck. Let’s explore the blow-off valve vs. wastegate pros and cons.
Blow off valve
- Prevent pressure from building up in the intake tube
- Prevent compressor surge
- BOVs can make noise that makes one uncomfortable, especially when there is a compressor surge due to nonfunctional BOVs
- Faulty BOVs can damage an engine
- Controls turbo boost
- Prevents engine damage
- It can be very noisy when faulty
- Wastegate actuator replacement is expensive
BOV vs. Wastegate: Which is better
With all said blow-off valve vs. wastegate, which is better? None is better than the other; you need both. So, the question of blow-off valve vs. diverter valve, which you should get, shouldn’t even be a thing to debate.
While a BOV removes excess pressure from intake to keep intake intact, the wastegate helps control turbine speed to prevent engine damage. The Bov and wastegate combo is necessary if you want your engine or turbo to last. I believe this should answer those asking, do you need a wastegate and blow-off valve?