Blow-Off Valve VS. Wastegate

Blow-Off Valve VS. Wastegate

December 12, 2017

What size wastegates do you have and how are they different? What size/kind of blow-off valves are available? 

I get these questions a lot. Here is a quick summary of the two.

First off, know the difference - both of these components are used in turbo setups only.

Let’s start with wastegates.

Wastegates allow the unused exhaust gasses to exit safely into the exhaust or atmosphere rather than through the turbo. For example: if you’re running 10 psi out of your turbo, the remaining exhaust gasses that could push your turbo to a higher psi, is diverted out through the wastegate, ultimately regulating the turbo speed.

Two Types of Wastegates

Internal Wastegate - typically found on OEM setups and smaller sized turbos (ie. GT25/GT28/Td05, Sr20det, RB25det, 1JZ, 2JZ OEM Turbos etc.). Most of these internal wastegates are non-adjustable, and a manual or electric boost controller would be needed for increasing boost levels. Although there are aftermarket internal wastegates that do have an adjustable rod to some degree.

External wastegate - these wastegates are typically found on setups that demand more performance and higher horsepower. External wastegates have an adjustable or replaceable spring for adjusting boost levels. The external wastegates range in size and style. Typically you will find a 2-Bolt flange style, or V-Band style. This is your preference. For sizing, don’t let it get complicated. Below is a rough idea of the general capacities of the most common performance wastegates. Actual specs and capacities may vary between types and brands.*


Engine Displacement  

 Turbo Size


-2.0-2.5 Liter 4cyl

GT30-GT35-6262 or smaller


-2.5+ Liter 4 cyl / 6cyl

or similar size


-V8 or High Horsepower

T67, HX45, HX55, +

In some cases, twin wastegates can be used. Wastegate setups can become pretty extensive if you are doing a crazy custom manifold or something wild. But in most cases, the suggested sizing has always worked in our hands.

Finally, onto something much simpler, the Blow-off Valve.

What does a blow-off valve do?

Well, 9 out of 10 turbo cars on the road will have an intercooler system. Even OEM turbo cars will have a BOV or relief valve of some sort incorporated into their intercooler system. The BOV is most commonly found close to the throttle body, but may also be installed in other areas due to space restrictions. The main purpose of a BOV is to release the built up boost pressure in your intercooler piping. The result of not releasing this pressure will cause a backlash of air against your throttle body and/or turbo itself. Air pressure against your throttle body will give you an inconsistent sensor reading and may also result in a rough idle or other vacuum related problem. Backlash air against your turbo will cause that ‘Flutter’ sound that everyone likes, everyone except your turbo that is. The built-up air must be released, or else the turbine can go from full spool to a drastic change in airflow resistance. Over time, this may ultimately cause premature turbo failure.

There are 2 types of BOVs or at least 2 ways to have them installed. One is a recirculated system. This is when the air released from the BOV is routed back into the intercooler/intake system. The second is Vented to Atmosphere. Venting to the atmosphere is most common and will produce a more aggressive relief sound. Some say they get a better response from their turbo with no BOV, others will say it’s a problem waiting to happen. We suggest running a BOV in any turbocharged vehicle. But do whatever tickles your fancy. As for sizing, BOV are much simpler than wastegate sizing. Here at Grassroots Performance, we have tested 50mm wastegates to be the best performing size BOV from small applications and horsepower such as sr20det and rb20det, all the way up to turbo-powered LSX setups. Piping size for your intercooler is also something to take into consideration, but for the most part, a 50mm BOV properly installed would cover the basics of most turbo setups. If you find yourself with a set of twins, then you’re a lucky guy. And if you are running twin turbos on your setup, then you’re right if you guessed you can run 2 x BOVs.

All in all, this post was made to simplify the differences between a wastegate and BOV. Information and specs may vary on the exact models of components you are using. We hope this blog points you in the right direction and offers some useful insight on suggested sizing we have tested here at Grassroots Performance. The main thing to remember is proper placement/installation of each component, and not overcomplicating. When things get complicated things tend to get expensive. And Expensive + Complicated = Not a fun time.

Learn more about our Grassroots Cooling HERE.
Learn more about our Grassroots Racecar Essentials HERE.

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