Blow-Off Valves

[ How It Works | Other Types | Installation | Quick Release Valves ]


This is page describes what a blow-off valve (BOV) is, how it works, and how to install one.

What is a BOV?

There is a lot of confusion over what a BOV is and what they are good for.  First of all, the name "blow-off valve" is not a very good name.  In fact, the Chrysler factory services manuals call it by it's real name: the "turbo bypass valve".  Somehow this valve earned the name "BOV" in the turbocharged world and it has stuck.  A real blow-off valve is the same thing as a "pop-off valve", which is a valve that will suddenly open and a certain pressure to maintain that pressure (kind of like a pressure regulator).  To avoid this obvious confusion, while maintaining the nomenclature of the turbo world, I will refer to the turbo bypass valve as a blow-off valve, or BOV.  In a way, the turbo bypass valve does work like a pop-off valve, since they will open at a certain pressure and maintain it (about 15 psi for Chrysler's valves).  This may be how it earned that name, however that is not how it is used, so who knows.  So in summary, turbo bypass valve = BOV on this page.  :)

On a turbo engine, the BOV is used to relieve the pressure from the turbo output when the throttle is closed.  These valves are only used on engines with the blow-through turbo setup.  For more information on this, see the Turbocharger Concepts page.  The BOV is basically a vacuum-actuated valve that opens when sufficient vacuum is present.  Vacuum is supplied by a connection on the throttle body, while the BOV inlet is connected to the turbo output hose.  On 1988 or later Chrysler Turbo I and Turbo III engines, the BOV is placed inside that airbox to suppress noise and prevent the BOV from sucking in dirty air.

While in boost, the valve remains closed and the turbo pumps air into the engine normally.  Without the BOV, when the throttle is closed the turbo is suddenly trying to pump air against a closed throttle plate.  This creates a pressure spike in the turbo output hose and sends a pressure "wave" crashing back and forth between the throttle plate and the turbo compresser blades.  The pressure spike quickly slows down the turbo and the pressure wave can actually damage the turbo.  On intercooled engines, this pressure wave effect is suppressed but the pressure spike still occurs.  When the throttle is opened again, the turbo has to spin up again, creating turbo lag.  If a BOV is present, the BOV will open as soon as the throttle is closed, releasing the pressure spike into the airbox and avoiding the pressure wave phenomena.

How a BOV Works

As stated above, the BOV is a vacuum-actuated valve.  When the throttle is closed (not necessarily all the way), a vacuum is created between the engine and the throttle plate.  Near the throttle plate is a hose nipple that supplies vacuum to the BOV through a device called a "Quick Release Valve".  Basically the quick release valve supplies the BOV with vacuum when there is vacuum in the throttle body, and vents the BOV to atmosphere when there is boost in the throttle body while prevent boost from reaching the BOV.  Below is a diagram of a typical BOV while in boost with the throttle open.  The red arrow indicates that boost pressure supplied to the BOV by the 3/4" hose that connects from the bottom of the airbox and BOV to the turbo output hose.

The spring holds the valve closed, but if enough pressure reaches the BOV inlet, it will be forced open (about 15psi).  When the throttle is closed and vacuum is supplied to the front of the BOV diaphram, it releases the pressure from the turbo:

As you can see, the operation of the BOV is fairly simple.  While this diagram looks similar the the stock Chrysler BOV, there are other BOVs available that look different, but work the same.

When the throttle is opened again, the BOV will close because the quick release valve will vent the vacuum inside the BOV.  There is a fraction of a second between when the throttle is opened and the BOV closes.  When the throttle is suddenly opened, the turbo is usually not spinning very fast and is not providing much boost, if any.  There is often a moment where there is actually a vacuum between the throttle body and the turbo because the turbo hasn't "caught up" with the engine's demand for more air yet.  Since the BOV is still in the process of closing, it will often suck some air through the pressure release connection because of this vacuum inside the turbo output hose.  This is part of the reason that the BOV is located inside the airbox on the clean side of the filter.  If some air is sucked in, it will always be clean air.  If you are installing you own BOV, be sure to either put a small filter on the release side of the BOV, or connect it to the clean side of the airbox.

Other Types of BOVs

The Bosch BOV, used on the SAAB 9000 Turbo, is smaller but flows better because of the 7/8" hose connections.  Unfortunately, this BOV has a weaker spring and is forced open at about 9psi.  The BOV can be held shut by supplying the vacuum nipple with some boost.  Gus Mahon has a vacuum bleed configuration that can accomplish this.  You cannot send all of the boost to the BOV because it will rupture the diaphram, which is why his design bleeds off about half of the boost, while supplying it with full vacuum.  Here is the diagram, courtesy of Gus, and a picture of mine made using VacuTite parts:

The Mitsubishi BOV that was used on all Mitsubishi Eclipse, Eagle Talon, and Plymouth Laser cars equipped with a 2.0L Turbo engine is a very high quality BOV.  It is all aluminum and has a VERY stiff spring.  It will not forced open until over 23psi!  In 1995, the BOV was changed and is of poor quality so be sure to get a pre-1995 (1st generation DSM) BOV.  They are about $95 from the dealer, or around $50-$75 from the DSM Parts Trader.  This BOV has a 1" inlet and outlet, but only has one hose connection because it is designed to be mounted to a manifold.  It can be retrofitted for hoses, thanks to Gus Mahon.  See Garry Donovan's web page in it.  His web page also has other BOV info and shows some pictures of other, aftermarket BOVs.  The Mitsubishi BOV also has a faster response time than the stock or Bosch BOVs.  This is because of the stiff spring in conjunction with the diaphram vent.  The vent is a small tube that feeds the back of the diaphram with boost from the outside of the valve.  So when you are in boost, the pressure is pushing on the valve and on the back of the diaphram to try and open the valve, but the stiff spring holds it shut.  When the throttle is closed, vacuum appears on the front of the diaphram and the combination of the vacuum on the front of the diaphram, the pressure on the back of the diaphram, and the pressure on the valve causes it to open quickly.  Once the valve is open, the pressure from the turbo is released along with the pressure on the back of the diaphram.  Now, only the vacuum on the front of the diaphram is holding it open.  Once the throttle is opened, the vacuum disappears and the BOV slams shut because of the stiff spring.

How To Install Your BOV

If you have stock Turbo II engine, or have intercooled your Turbo I, then you probably don't have a BOV.  If you have a 1988 or later Turbo I, then you already have a BOV in your airbox.  If your have a 1987 or earlier Turbo I engine, then you do not need a BOV.  The pull-through intake system of the early Turbo I engines don't suffer from the problems of the blow-through setup.  The reduction in lag provided by the BOV makes its installation worthwhile on a blow-through engine.

The first question is where to put the BOV.  The most popular setup is to mount a hose connection somewhere on the upper intercooler ducting.  If you have a stock Turbo II airbox, one idea would be to rivit and RTV a hose connection somewhere on that upper duct.  You would have to grind off some of the ribs and use a hose connection with some sort of flange on it.  You don't want anything protruding into the duct because it will restrict the air flow to the throttle body.

Another idea is to weld a hose connection on a smooth-curved piece of 2 1/4" exhaust pipe.  You can use this, along with two short pieces of 2 1/4" truck raditator hose as the connection from the intercooler to the air box (or right to the throttle body, if you don't have the Turbo II airbox).  You will have to cut the curved pipe such that it has the correct angle.  If you won't want to weld it, you can use pop-rivits and RTV.  This is how I did it on my 1988 Dodge Shadow ES.  The picture to the right shows the pipe with the 1" fitting rivited and RTVed on.  This size is for the Talon BOV or the Bosch BOV (I shoved the 7/8" I.D. hose on there).  I cut the fitting out of the bottom of a crankcase breather filter for a Dodge Ram V8 (Fram BA3632 or CA3632).

If you have a straight run from the intercooler to the throttle body, then mount the BOV somewhere on the piping there.

No matter which way you choose, some thought and some fabrication will be necessary to do it right.  Also, you need to be concerned about the pressure release side of the BOV.  Make sure that you put a small filter on it, or that you connect it to the clean side of your air box.  You don't want it to suck any dirt or debris into the engine.

Also keep in mind the blow-off point of the BOV you are using.  The stock Chrysler BOV will blow off (be forced open) at about 15psi.  The Bosch BOV will blow off at about 9psi, while the Mitsubishi BOV won't blow off until about 23psi.  If you are running boost levels that are higher than the blow-off point of your BOV, then you will be leaking boost through the BOV.  You may need to supply the BOV vacuum nipple with some boost using the design shown above.

What is a "Quick Release Valve"

A quick release valve is a device used on BOVs to provide it with vacuum, but prevent it from seeing boost while venting the BOV.  It is the combination of a check valve and a bleed valve.  It has two connections, one to the BOV and one to the throttle body.  It also has a felt pad that acts as a filter on one side for the vent.  When there is vacuum from the throttle body, the vacuum is directed to the BOV connection and the vent is closed.  When there is boost from the throttle body, the boost is blocked by the internal check valve.  The closing of the check valve also opens the vent on the other side, which vents the vacuum in the BOV, allowing it to close.  Without the venting by the quick release valve, the BOV would not close and the boost generated by the turbo would be dumped into the airbox.
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This page is maintained by Russell W. Knize and was last updated 02/09/99. Comments? Questions? Email

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