Tech / Tech Articles

Quick Tech: Wastegates vs. Blow-Off Valves…What’s the Difference?

Turbocharging continues to gain popularity.

But there are still a few areas of a typical turbocharger setup that remain fuzzy to turbo newbies. One of the most common misunderstandings involves the basic difference between a wastegate and a blow-off valve.

So here goes…

What is a Wastegate?

A wastegate prevents the turbo from creating too much boost.

Once the exhaust pressure from the engine spins the turbo’s turbine wheel fast enough to produce the desired level of boost, the wastegate will open to redirect the excess exhaust pressure around the turbine wheel. This keeps power levels at the desired level and prevents too much boost, which could cause detonation and engine damage.

Turbo Wastegate

Most wastegates operate off of pressure, or pneumatically, using an actuator spring. As pressure reaches a determined level of PSI, the spring is pressed open, actuating a valve which allows the exhaust gases to flow past the turbine and through the wastegate. These exhaust gases are essentially “wasted” because they no do not help spin the turbine wheel. The wasted exhaust gases will travel through the wastegate so long as peak boost is held.

Computer-controlled wastegates operate a bit differently. They use a sensor, which signals the ECM when boost pressure goes too high. When the pressure becomes too high, the ECM signals a solenoid valve that opens the wastegate.

There are a couple different configurations for wastegates. External wastegates are located on the exhaust side of the turbo unit, between the exhaust turbine wheel and the exhaust manifold. At this location, the wastegate can get the exhaust flow upstream of the turbine and then route it back into the exhaust on the other side. Internal wastegates are located within the turbocharger housing itself and offer a more compact option with less piping.

What is a Blow-Off Valve?

Turbo Blow-Off Valve

A blow-off valve (BOV) is similar to a wastegate, but works on the intake side of the turbocharger.

The BOV prevents pressure from building up in the intake tract. Excess pressure in the intake (ahead of the turbo) can back up and then cause compressor surge when the engine speed changes suddenly. The BOV is normally closed, but when a certain pressure level is achieved, the inlet air opens a spring to relieve the backed-up pressure. This reduces stress on the compressor and its bearings.

If you’ve ever heard a turbo car make a hissing sound, that’s the blow-off valve allowing excess pressure to escape when the driver suddenly lets off the throttle.

Tags: , , ,


  1. Hi, what make that loud horn noise …what kind of blow off valve.. it’s sounds awesome.

    • OnAllCylinders Staff says:

      Hey Scotty, we’re not sure what sound you’re referring to, but you’re right, it’s the Blow-Off Valve that makes the “Whhoooooosh!” whistling sound that folks seems to associate with performance turbocharger setups. (Most stock systems are quiet and unobtrusive, aftermarket ones can make the whoosh much more audible.) If you’re looking for a specific sound quality, it’s probably best to go to car meets and hear for yourself, or search videos of Blow-Off Valves in action on the Internet.

  2. nick mccarthy says:

    Hi, im currently building a small v twin 670cc engine with twin turbos. The turbos im going to use have a built in waste gate but im wondering if i need a blowoff valve for the intake side?

    • OnAllCylinders Staff says:

      Hey Nick, without knowing the exact specifics about your build or its intended purpose, we can’t be 100% sure, but the answer is probably–especially in high boost applications and/or engines that see quick throttle input(s).
      More importantly, you’ve got to make sure your BOV is properly suited to your application. A call to the Summit Racing tech department will be a ton of help here: 330-630-0240.

  3. Jesse Ashbaugh says:

    Do you need both or can you use one or the other in a turbo set up

    • You must have a waste gate of some sort on the exhaust side of turbo on any application. If you don’t, it is certain you’ll end up lifting a head over spinning turbo or worse. Waste gate controls exhaust gas pressure from your engine to turbo. Once peak boost psi, air flow, and turbo revolution speed are met, waste gate will open as needed to divert exhaust flow from motor a different direction than past turbo exhaust turbine. Waste gate May only just enough so that just enough exhaust is diverted away from turbo to hold a certain boost level or turbo speed, leaving the rest of exhaust flow on its path though turbo turbine to hold turbo speed and psi. Cars also use what they call a VGT turbo. These turbos use electronic actuators that open and close veins around turbo turbine or slide in and out from around turbine of turbo to either block exhaust flow all together hence exhaust brake or open to let exhaust by spinning turbo turbine. They may also open or move the veins halfway causing extra back pressure from exhaust gasses trying to get by and actually cause turbo to spool up faster at first and then opening all the way to get max desired boost pressure. This helps bigger Diesel engines that tend to have boost lag when first hitting the throttle. Not quite as good as compound turbo set up, but a VGT system definitely can help turbo spool faster when getting into throttle from a dead stop. So a VGT turbo veins and actuator (all on and inside of turbo) are able to control boost pressure like a waste gate does, exhaust brake, (no waste gate can do that) and can also control exhaust pressure from engine forcing turbo to get into boost faster. I can go on for hours on different types and functions. Hope this helps

  4. HI! I ‘m driving Audi A4 DTM and I bought BOV but I ‘m afraid to use it. There is no any complication if I use it on this DTM

    • As long as the right springs are in BOV and vacuum lines are ran right, you won’t have any issues. ( I assume you are removing factory BOV and installing an after market BOV)

    • I want to add, make sure you check you BOV spring/springs!!!! Most BOV’s come with multiple springs. Many people don’t know this because makers of most BOV’s put all springs into BOV during packaging. A lot of people without knowledge and understanding take their BOV out of the box and replace their factory BOV with new BOV and end up lifting a head or heads on their car or truck and don’t know why!!! Low and behold, makers of BOV’s pack all springs inside BOV causing it to be set at 20-40psi!!!!! (depending on brand and type) most factory turbo cars aren’t supposed to go over 4-10psi of boost. Now you took your factory set BOV out and replaced it with a BOV with a spring pressure of 30psi!!! Bye bye heads!!!! And you thought they sent you a BOV to match your year make and model. Bottom line, if your not familiar with parts you want to change out on your vehicle, stop and ask someone or research the shit out of it till you have an understanding of what your about to do, and ALWAYS check inside for the right springs….

      • Hi,
        I know this is an old hthread but something bothers me. If you are running wastegate and bov, where would you plumb in vacuum for the wastegate? Before or after bov? Also, in this case, how would you setup both springs? Same boost level on both or one higher than the other?

  5. Joshua elliott says:

    Hello, you article is very informative. I have a 2011 Ford F-150 EcoBoost. It recently through a code p0230 turbo over-boost condition. I have recently replaced both boost sensors. Would this code be caused by a faulty BOV or the Wastegate? Any tips or suggestions would be extremely appreciated.

  6. It definitely could be a sticky/sticking waste gate, but it could be 20 other things as well. Best thing I can tell you is chat rooms, and threads online specific to your 2011 ecoboost. If you are having an issue, chances are many others before you had same exact problem and can fill you in on what fixed it and how to do it. Side note, those little twin turbo eco boost engines kick ass. Pretty dam peppy for 2 little turbos on assembly line made car I must say.

Leave a Reply

Your email address will not be published. Required fields are marked *


This site uses Akismet to reduce spam. Learn how your comment data is processed.