View Full Version : Blowoff Valve FAQ

08-29-2010, 03:50 AM
Blow Off Valve Frequently asked questions


Pressure release valve or more commonly known as a "Blow Off Valve", releases turbo pressure when the throttle plate is closed. The turbo is still spinning and still creating pressure. The forced air will hit the throttle plate and return where it came from. When a BOV reads vacuum from the manifold, it either opens a valve, or softens the valve. Which lets the pressure escape from it's opening. In order to work correctly the air must go back into the intake before your turbo because the Air Flow Meter has accounted for it. If not you will have a temporary rich condition which will upset your idle slightly. Proper tuning can get around this hassle. Most choose to vent to the air, as they love the sweet sound the air makes when it runs to the atmosphere. If the pressure release valve (as some call it) contains a horn or small holes/vents it will cause the air to make a louder/higher pitched sound depending on what is used.

Q: What is the difference between a blow off valve and a bypass valve?

A: A blow off valve will release pressure to the air, while the bypass valve will release

the pressure into the intake system between the turbo and AFM.

Q: I installed a blow off valve, it sounds really cool but my car stalls when I let off. Why?

A: First of all, the reason this is happening is because the Air Flow Meter on our cars measures

the air coming in through the flapper door. It senses this air and adjusts the fuel mixture

accordingly. When you let big rush of air out of the intake system, you are letting out a

bunch of air that was just measured. The fueling will still dump the fuel associated with

that air and cause an over rich condition. This will cause your car to stall momentarily or

in some cases actually turn off. Once the BOV has closed and the intake system will return

to it's normal state and work again.

To fix this, you need to adjust your BOV. Most BOVs have a screw or nut to adjust the

tension. You can turn the screw/nut to the right and increase the tension. Do this several

times with a test drive in between. If your BOV stops letting out the pressure, you have

adjusted it too tight. You will then need to start adjusting soft again. You must keep

playing with this adjustment until you get it just right. A good adjustment will allow the

BOV to release pressure after slight boost, and not stall afterwards. It is not necessary for

the BOV to release pressure when you rev the car.

Q: I have heard of a BOSCH or other bypass valves being vented to air, is this possible?

A: Yes, but not right away. If you vent the bypass valve to air, your car will stumble and

most likely turn off between shifts. The reason for this is that once all the pressure is

released the bypass valve will still be open and create a huge air leak which the AFM

does not like. If you put a one way check valve on the end of the output, this will only

allow the air to go out, but not in. Be sure to seal the other side of the intake where the

air would normally be vented. This makes for some interesting sound effects!

Q: How can I make my blow off valve LOUDER?

A: There are a few possibilities. The first thing to remember is that there is more boost located in

the piping between the turbo and intercooler, yet most blow off valves are placed near the throttle.

Now, you want it by the throttle for response, but you can place it closer to the turbo for a louder

sound output. Another thing to consider is to amplify the sound. You will notice that blow off

valves with basic air holes are not as loud as blow off valves with horns (blitz) or air splitters (HKS).

If you have a greddy type S, the most common BOV out there, you can find something similar in

style to the horn on a blitz BOV and that will make the sound output far greater. Finally, if you

(or the people on the street) to hear your bov, then you need to place it in a location that would

let the sound travel out. In most cars the sound is muffled by the hood lining. Placing them out

side of the engine bay on intercooler piping or similar methods will make your BOV loud and scary!

Q: Where do I get the vac/boost feed for my BOV?

A: While using the small nipples on the left side of the intake manifold may work, I suggest

that you use the larger A/C idle adjuster hose. This will increase the response and thus

the sound of your BOV. In addition to this, this hose is much closer to your BOV location.

ST165 - Using a large tee, tap into the large vacuum hose coming from the rear of the intake manifold.

This is the hose that runs up to the A/C idle adjuster on the firewall.

ST185 - Using a large tee, tap into the hose that runs from the right side of the manifold to the A/C

idle adjustment. Facing the engine from the front, it will be very obvious how much larger

this vacuum hose is that comes directly out of the intake manifold.

ST205 - Simple use the stock vacuum hose supplied from the factory.

Q: Can I use my new BOV in conjuction with my stock type BOV

A: Yes you can but it requires some fancy fabrication. This would be the ideal setup. The stock bypass valve could handle low boost / flow situations. Which means you can tighten the main blow off valve so that it only opens under high boost. This way you have created a "double valve" system. This is not as necessary these days since twin valve designs exist.

Q: How Do I install a blow off valve??

A: Here are some BOV installation notes:

Blow Off Valve

This is an old style HKS Sequential BOV Mated to a T style Flange. This is the easiest way to mount a BOV to your ST165. It would rest in between the Turbo outlet and I/C. You could weld on the I/C but this option gives you the ability to go back to stock.

Notice the small air filter on the BOV. This prevents any dirt or debris from entering the chamber after a long pressure release. A good safety feature.

Bypass Valve

Here we can see how the T Pipe should be mounted. There is very little room for tolerance on ST165. Here we see the Bosch bypass valve installed. The output of the valve is rerouted back into the intake to maintain a perfectly sealed system as Toyota had originally desired. You can see how the bypass valves usually use a tube design.


Mounting on the ST185 is not so easy. Here is a blitz bov that has been custom mounted. Universal bov kits usually come with a flange that can be welded to a pipe for usage. Make sure that you get the aluminum flange. You must weld a tube your your ST185 intercooler pipe that is roughly the size of the circle hole on your flange. Then weld the flange to the top of that pipe. From that point you can simply mount it and go.


You can see here on Herman's ST185 how close the bov comes to hitting other parts in the engine bay. Blitz Super Sound, Greddy Type S, and HKS Super sequential are all recommended for ST185 as they fit perfectly. Note that in both pictures, the bov is mounted almost exactly in the same spot. Make sure you mount it right on the first bend coming out of the other bov, as that is the known spot for the bov to fit nicely. If you wanted to mount a Bosch bov as the above 165, you would simple weld a 2" tube to the same place, no flange is necessary.



If you are lucky enough to own a ST205, you can easily buy a BOV kit. The ST205 comes with a bypass valve similar to the Supra TT. The input and output of the BOV is a 2" tube. So the kit will include a hose that you can attach to another tube which contains the BOV flange on it. The other side which normally routes the air back into the intake is sealed off. Blitz Super Sound is pictured here.

08-29-2010, 04:25 PM
Q: Where do I get the vac/boost feed for my BOV?

A: While using the small nipples on the left side of the intake manifold may work, I suggest

that you use the larger A/C idle adjuster hose.

wheres this at ? because i use the small nipples on the intake.

08-29-2010, 09:13 PM
I'll have to look at my car. I'm posting up as much stuff as I can for AT. net in case it takes a shit.

I've got mine connected to the vacuum port on the back of the manifold. If I find what they're talking about, I'll post a pic.

09-01-2010, 03:05 AM
If you put a one way check valve on the end of the output, this will only allow the air to go out, but not in.

technically, it's the cracking pressure of the check valve that prevents the very small amount of air that the turbo is producing from leaking out of the valve. the fact that it's a check valve is irrelevant, and any valve with 1psi of cracking pressure would work.

really, the only way air can enter the valve is cracking the throttle quickly on a setup with a turbo that's laggy enough to not keep up with the drop in pressure as the manifold and charge pipes equalize.

the "leak" induced from the valve being open is slowly venting air, since the turbo will spin anytime the engine is running. the spinning turbo is creating airflow, and the closed throttle gives that air only one place to go- out the valve.

i argued this with billwot over on the other board until i was blue in the face, but he never acknowledged that his original article was mistaken (despite this being observable fact).

i suppose it's all a moot point for me now, since i don't even run a valve.