Blowoff Valve Question Help

[paulr33]

hi

compressor surge occurs when the pressure past (after) the compressor wheel exceeds what the compressor wheel can physically supply

that is, for a given turbocharger, it's compressor is able to flow xyz amount of air and pressure at xyz speed. this is called the compressor map (it's a chart).

if the pressure after the compressor wheel goes higher than what the compressor map dictates (ie higher pressure than what the compressor can physically supply) compressor surge occurs

this is usually observed by vl turbo flutter or cuhututututut noises

common causes of compressor surge are

poor, damaged, mangled compressor wheel - ie damaged, poor highflow spec, incorrect compressor vs housing mismatch (ie botched highflow)

no or poor bov system - when you close the throttle body, as there is no bov or poor bov control, the pressure in the intake exceeds well beyond what the compressor can supply

typically upwards of 40psi when you close the throttle body without a bov, so the compressor surge's

[GTScotT]

can confirm that with HKS SSQ and bnew BLITZ super sound VD 2009 model bovs (two of each) on a GTR 32 there is excessive backspin no matter the spring preload.

stock is good mmkay

[ryan101]

would a gtr BOV on a rb25det have any advantage over the stock rb25 BOV?

[RB25PWR]

if you read the thread, no.

[paulr33]

GTR BOV on RB25DET is a good upgrade and very suitable

2nd hand GTR BOV's usually sell quickly and are worth $

there is no such thing as "backspin" with no BOV

the physics do not support a compressor "spinning backwards" so please, no more mythbusters

[GTScotT]

there is no such thing as "backspin" with no BOV

the physics do not support a compressor "spinning backwards" so please, no more mythbusters

OK politically correct, myth bust us and tell everyone what the zututu noise is then.

I have no solid knowledge of what it is, but its bad and may as well be the turbo spinning backwards for all it matters.

[JonnoHR31]

still spin forward but the air is moving backwards and putting backpressure on the comp wheel in the opposite direction its spinning, hence the noise. if the comp wheel stopped and went backwards it wouldnt go chututututu on you

[paulr33]

mythbusters ahoy

the vl turbo chutututu is the compressor surge noise. from my post above, this occurs when pressure exceeds well beyond what the compressor can supply. the actual noise comes from the mid air collison of air and pressure that occurs in the pipework. the compressor wheel does NOT spin backwards, this is physically impossible and the physics do not support that behaviour. the compressor does however recieve excessive pressure / force from compressor surge as the wheel surge's each time it gets a massive pressure hit. compressor surge basically shortens the life of the turbocharger compressor wheel.

"apparently" after blow off valves or recirc valves were introduced turbocharger failure (compressor wheel failure) rates dropped and service intervals were changed for turbocharger components which is a direct result of using a blow off or recirculation valve.

[paulr33]

http://www.turbobygarrett.com/turbobygarre...er/faqs.html#t9

What is compressor surge?

The surge region, located on the left-hand side of the compressor map (known as the surge line), is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur. When referencing a compressor map, the surge line is the line bordering the islands on their far left side.

Compressor surge is when the air pressure after the compressor is actually higher than what the compressor itself can physically maintain. This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. In cases of extreme surge, the thrust bearings of the turbo can be destroyed, and will sometimes even lead to mechanical failure of the compressor wheel itself.

Common conditions that result in compressor surge on turbocharger gasoline engines are:

* A compressor bypass valve is not integrated into the intake plumbing between the compressor outlet and throttle body

* The outlet plumbing for the bypass valve is too small or restrictive

* The turbo is too big for the application

What is the difference between a BOV and a Bypass Valve? How do they work, and are they necessary?

A Blow Off Valve (BOV) is a valve that is mounted on the intake pipe after the turbo but before the throttle body. A BOV's purpose is to prevent compressor surge. When the throttle valve is closed, the vacuum generated in the intake manifold acts on the actuator to open the valve, venting boost pressure in order to keep the compressor out of surge.

Bypass valves are also referred to as compressor bypass valves, anti-surge valves, or recirculating valves. The bypass valve serves the same function as a BOV, but recirculates the vented air back to the compressor inlet, rather than to the atmosphere as with a BOV.

[sh@un]

Stock GTR BOV can handle above 300kW's.

Aftermarket BOV's have no gain.

Only reason to change if you are running a ridiculously high amount of boost and the stock BOV cannot handle it any more.

well, they've been used on RB's making over 1000kW at the flywheel...

[GTScotT]

Thank you Paul, a flaming now well justified.

Now let us all rejoice in the knowledge of the backspin!

[T04GTR]

all the expert engineer tubro comp map theorists asside,,,, use the factory ones. you will never have to "upgrade"

[bj stan]

what would cause stock bovs to vent? for some reason mine are now sounding like aftermart bovs i dont believe theres any leaks as the car holds boost and the only things that have changed is a different front bar (alot more 'holes' in it) and pods. Its very annoying and dosnt make sense to me as to why its doing it