Celica Forums General Discussion Engine and Performance Blow Off Valve

Ok, I know they sound sweet, and that you need a turbo for it, but do they acutally serve a functional purpose?

Most everything in life serves a purpose.

i'm no turbo expert but i believe blow off valve is same thing as wastegate, i will stand corrected if i am wrong but read this.

http://auto.howstuffworks.com/turbo6.htm

^yes. wastegate recycles, atmospheric blow off valve dumps it (making the noise everyone creams their pants over), and there are hybrid ones that will recirculate some and discharge the rest.

it is sort of like a wastegate... it lets off excess exhaust... that's what the 'pshhhhhhhhhhhhhh' is, lol. in n/a cars... you want back pressure... in turbo you want NO backpressure... that's why you get a 3'' exhaust. and the BOV lets off more excess exhaust

kills un-needed "waste pressure" from the turbine

BOV is on intake side. It has nothing to do with exhaust.
It vents out the air to prevent compressor surge due to the
throttle is slammed shut when you shift.

Bypass valve works the same way as BOV. The only different is
it recirculates the air back into the intake (before the compressor wheel)
Bypass valve is common to car that come with AFM or MAF
(non-MAP type or non-speed density air measuring system)
This is to prevent venting metered air out.
If the metered air is vented out too much, the car will run too rich
and may stall.

On the other hand, wastegate lets the "exhaust" bypasses the turbine wheel.
The main purpose of the wastegate is "to limit boost"
So, the wastegate is more like boost controller than the BOV or BPV.

By the way, BOV, bypass valve and wastegate will not work on N/A car.

The bov release presure from the compressor when you let off the throttle. If the presure is held in the air presure can get to high and break the shaft connecting the compressor and the turbine, bov also helps get rid of turbo lag by letting the turbo freewheel in between shifts.

Nah...Not that bad. It just shortens life of the turbo, but not breaking the shaft. The pressure will not get too high due to the throttle is shut and there is not enough air in to the cylinder, so there is no enough exhaust velocity to spin the turbine and compress more air in.

It's just the compressor surge at the wheel that is not good. When the wheel spins at 100,000 or more rpm, coming to hault instantly is not a good idea.

hks sounds sick

Just out of curiosity, which is best: BOV, bypass valve, or wastegate?

Read my post up there.
BOV and wastegate are two different things.
They do different jobs.
You cannot compare them.

nice info gt4, i stand corrected. Just for clarification boost is limited at the wastegate correct?

Yup.. boost is limited at the wastegate.
It does that via the wastegate actuator.
If the boost pressure on the actuator diaphram exceeds
the wastegate spring setting, it open up the wastegate door and
let the exhaust gas bypasses the turbine.

Unlike the wastegate, BOV act on "vacuum"
If it senses the vacuum (due to throttle shuts),
it opens up and releases the compressed air out.

Let me add more info on aftermarket boost controller.
Aftermarket boost controller (manual or electronics one) allows
you to boost more than stock wastegate setting by manipulating
the boost pressure signal to the wastgate actuator.
The actuator will not sense the real boost pressure.
Hence, the wastegate door is kept shut longer and the turbo
builds up more boost.
(up until you hit the desired boost set by boost controller)

very informative, thanks

just a question: if the turbo car is in park and you rev the engine would you hear the blow off valve go off or does the car have to actually be moving? might sound dumb but ive never experienced a turbo car yet.

Some yes, some no.
For example, on alltrac, I can let off BOV if I rev the engine high enough. But it'll not be as loud as when engine is under load and boost.

thats cool i was just wondering i hope i can hear mine if i just wanted to show it off sometime

eek not the backpressure demon again

only reason you can't do a 3 inch exhaust on are na 4cyls is because of heat the air in the exhaust will get too cold cold ai is denser there for flos slower a 2.25/2.5 exhaust will keep the temp up which means the exhaust is less dense and flos better...

what flows better thru a straw hot oil or cold oil

backpressure is a missunderstood read the exhaust post

I cannot add any more to this, well said before. Just here is the sound of my R2D2
Video [c2design.hu]

to add to that, even when you have a 3" exhaust with no cats on a turbo car, you will experience boost creep.

and a BOV releases the pressure inside and is usually recycled
usually, 100% atmospheric BOV's will cause you too lose boost = loss of power
depending on the setup

and will run rich when the bov goes (instead of the air the maf reads gonig in to the engine half of it goes out the bov)

That's why AFM/MAF suck.
It's much better to change to speed density system.

Only true with some turbos with internal wastegate setup.
No problem with the external wastgate setup.

lol my friend had MAD boost creep on his DSM. Good thing he had a gauge or he would not have realized he was running 28psi on all stock stuff

exactly, in which you would want to port the wastegate on the internal setup
but that does not always fix the problem.