Purpose of a blow off valve?

[PacmanTravis]

What is the purpose of a blow off valve besides "pssssssst"?

[DanG]

Pac:

The blow-off valve is a safety device. When you're sitting besides a truck at a traffic light, and all of the sudden it scares the crap outta you 'cause it spontaneously goes "Pssssssssh"- Same principle. The truck has a safety valve in its air brake system that lets off pressure if there's too much.

Let's look at the turbo blow-off, also called a wastegate. When you're accellerating, the turbo spins up and starts producing boost. When you're running all the way up to redline, the turbo is spinning in some cases at over 100,000RPMs- No joke. When you take your foot off the gas to shift, your throttle plate snaps shut. Trouble is, the turbo is still spinning, and still producing boost. That boost has to go somewhere until you open the throttle plate again, so they built in a valve that opens if the boost gets too high.

That's the most noticeable time when a wastegate opens because of the audible "pssssssh". You can also get this if you're just running WOT for a while- If the turbo spools up and produces boost greater than what the wastegate is set to control, the wastegate will "waste" the remaining boost by venting to atmosphere.

Disclaimer- Come to think of it, there may be a difference between wastegates and blow-offs after all... A blow-off may vent to atmosphere, whereas a wastegate just bypasses the incoming exhaust gases to slow down the turbo. Both achieve the same effect.

Hope this helps,

[PacmanTravis]

damn, now THATS an answer

[EdgeCat]

If you haven't figured it out yet, Dan is one of the Duratec performance GODS. ;)

[Draxas]

Also:

Some turbos come with a wastegate built into them.....others, you buy an external wastegate (like a TiAL)...... Some people have the tube coming out of the wastegate directed back into the exhaust stream and it flows out with the rest of the exhaust that goes through the turbo.... other people make it so the wastegate exhaust just goes right into the atmosphere as well.... a wastegate has a very distinct sound to it, and you may often here it while the car equipped with one is accelerating.....whereas you hear a blowoff valve go usually when the person shifts (or, like Dan said, the boost hits the point that the blow off has been set to, to let off the excess boost).....

You can see in the attached picture what an external wastegate might look like. This one is directly bolted to the exhaust manifold. When the exhaust "pressure" between the engine and the turbo gets too high, a spring loaded mechanism in that really shiny part on top of the wastegate opens, and vents the exhaust gases out of that gaping hole you see. You can attach a pipe to that opening and have the gas routed back into the rest of the exhaust aft of the turbo, or send the pipe just straight down letting the "wastegated exhaust" right into the atmosphere....... Obviously this isnt SMOG legal... ;) It can be quite loud when a wastegate lets the exhaust out, and it makes a very distinct noise......

Nikolas

[FastCougar]

The difference between a wastegate and a blowoff valve is where they sit in relation to the turbo.

A wastegate is associated with the exhaust side of a turbo and is used to regulate exhaust flow to the turbo. "The wastegate is a valve that allows the exhaust to bypass the turbine blades. The wastegate senses the boost pressure. If the pressure gets too high, it could be an indicator that the turbine is spinning too quickly, so the wastegate bypasses some of the exhaust around the turbine blades, allowing the blades to slow down." - HowStuffWorks.com

A blowoff valve is associated with the intake side of the turbo and is exactly as Dan described it ... a pressure relief valve for when boost is reached or when shifting.

[DemonSVT]

A blow off valve is a safety device to prevent overboosting. Just like stated. It does not control boost on a turbo, only vents boost past it's preset safety limit to save engine damage.

However it is NOT a Bypass valve!

A bypass valve is used to vent boost when the throttle plate closes to prevent compressor surge. It can be venting to the atmosphere or back into the intake.

A wastegate is just that. A gate (internal or external) that "wastes" the excess gas so the turbo does not over spin. It is controlled via an wastegate actuator (vacuum/boost actuated diaphragm assembly that has a preset boost level)

Edit: oops - old post...

[Kinger]

So what does a MBC do? I mean if a blow off valve is calibrated for 10 psi, wouldn't a manual boost controller be the same thing except its adjustable? I'm lost on the diff between MBC and BOV. Last question would someone use all three or are all three a way of acieving the same result? Thanks guys!

[ChrisHightower]

MBC "Manual Boost Controller" is just another little valve that regulates the airflow to the wastegate actuator. If you want more boost produced, you tighen this valve. This restricts some of the air pressure going through the small tube, in turn, makes the turbo spin faster by holding the wastegate closed making more of the exhaust gases go around the turbine.

In addition, the wastegate usually opens and stays partially open at WOT making sure the boost keps coming. The BOV pops open usually when you let off from WOT, like during shifting.

Regulate the boost with the wastegate, and prevent turbo damage with the BOV.

[exigent]

Um... a wastegate and blowoff valve are NOT even close to being related guys. The wastegate allows exhaust gas to escape or rerouted back the the exhaust tract without going through the turbo... the blowoff valve goes on the intake side to prevent overboost. Its for safty as well as when you let off the the throttle and the throttle plate closes.... the turbo still spins and builds pressure in the intake tract between it and the throttle plate... if that boost had no place to go it would break somthing... so... the blow off valve releases the extra boost presure to prevent this.

Well... if you dont have a VATN turbo this is true anyways.

[Kinger]

I'm confused....time to search the net.

[Grav1ty]

well since i red all this, and so here is picture. I underlined what should be the waste gate controler and the blow off valve. Im not sure if they use the same sysyem on cars or not, but it does give you the general idea. Sorry about the picture quality its alittle fuzzy. Basically the blow off valve is a pressure relief valve, The waste gate basically regulates the amount of boost you want. So when there is a rapid increase of pressure in the intake due to closing the throttle valve, the wastegate cant dump pressure fast enough, so the blow off valve will dump pressure so you dont break something. forget about that second picture its a different system, i just got picture happy.

[heliumrush]

I am checking the forum for a friend.

Sorry, guys lots of mis-information passed around. Wastegate is what controls boost by bypassing the exhaust. External and Internal WGs work the same pretty much.

BOV bleeds off the boost when the trottle plate closes. To prevent the air from coming back hitting the compressor. It helps with spool up and keeps the turbo from making funny "turkey" sound. No damage will happen without one, unless you are runing over 15 psi of boost, then you can have problems with the compressor shaft down the road.

BOV and By Pass valve are pretty much the samething. Just how each one is setup is the difference.


A pop off valve is what you guys where talking about if the turbo overboost. I don't know of anyone that really runs one though. If you have good boost control system you shouldn't ever really overboost.

I hope this clears up a few things.

[s2kyellowsol]

Wastegate:

The wastegate is just a valve that opens when you have exceeded your desired boost level, and allows exhaust to flow around the turbine, instead of through it. This lowers the pressure differential across the turbine, less work is done, and the turbo slows down.

The only "gotcha" with the wastegate is that it must be able to flow enough gas to let the turbo slow down. If it can't, the you get "boost creep" where boost levels slowly grow as the car remains under boost. Bad.

BOV:

You're under boost, the turbo is fully spooled, and life is good - then you shift. That means that you foot comes off the gas - and the throttle plate slams shut. Suddenly, instead of flowing in a continous stream through the engine, the intake air smacks into a closed throttle plate. The turbo, which is still spinning and producing boost because if it's rotational inertia keeps producing pressure, and the intake stream, caught between a rock and a hard place, jumps in pressure. In fact, you get a high-pressure shockwave that travels from the throttle plate back to the compressor vanes, that once it gets there, is a little like poking a stick into the spokes of a bicycle wheel.

The repeated shock is hard on the compressor vanes and the shaft bearings, and in any case acts like a brake, slowing the turbo, and requiring it to be spooled up again.

The BOV sits in between the turbo and the throttle plate, and if it detects the shockwave created by a shift, vents it elsewhere - either to the atmosphere, or back into the inlet side of the turbo.

[FastCougar]

I thank you guys for coming here and clearing things up, but I think that DanG and I already said what you two said. If you reread our posts, you will see that Dan explains the BOV perfectly and I describe the wastegate just as accurately. I think what caused the confusion was in my post, I alluded to the fact that wastetgates and blow off valves serve a similar function. What I meant to say was that they function in a similar way ... diverting flow. A wastegate diverts flow of exhaust to protect the turbo and to also slow it down. A BOV diverts flow of intake air to protect the turbo and also allow it to slow down. Although the function isn't the same, the same principles apply, thus my statement "The difference between a wastegate and a blow off valve is where they sit in relation to the turbo.", which is factually true since they are both diverters with one on the intake side and the other on the exhaust side.