To Blow Off Valve or not to Blow Off Valve??

[Muzz]

My opinion - If the 90% of turbo oem cars come with a recirculation valve, when the manurfactures are all focoused on keeping costs down, then theres obviously reason for them using one. They dont just go sticking random parts in there cars, they do testing and evaluating, if it was worthless it wouldnt be there.

BOV on a everyday car and no bov on a race car.

Tons of top level race cars use a BOV, for example:


If they didnt believe the benifits out weighed the extra weight, they wouldnt run one, simple.
Its a car by car basis, on some it may be basically completly pointless, while on others it plays an important part.

Heres a good read, seems unbiased, unlike alot of other BOV talk - http://hrc.s10.forumsplace.com/viewtopic.php?p=15

IMO this argument is completly and utterly pointless, unless youve studied fluid dynamics to a very high level, and you can actually understand how this surging air acts on the turbos mechanicals, which none of us have.

Aimre, you seem dead set trying to convince everyone theres no benifit, do you understand how the surging air effects the turbines mechanicals? Im guessing not, which really dosnt make what your saying any more than an uneducated opinion, the worst kind there is.

To run one, or not to run one, simple question.

Run one if you like the noise or you want your turbo to be working in the best possible conditions, and the extra cost dosnt bother you.

Dont run one, if you dont like the noise, are super paranoid about weight reduction, are scared of it malfunctioning, you like the dose sound, or your happy to risk maby reducing your turbos working life, to save some cash.

Statement :- Compressor surge will not damage a turbocharger. It's only air being chopped by the compressor wheel!

Fact :- DEAD WRONG! People who state this fact should have their licences taken off them for being a danger to a perfectly good car. Compressor surge is one of the leading reasons for premature turbocharger failure. It runs a close second place to problems such as poor oil delivery to turbo bearings, high boost with ceramic coated wheels and hitting a turbocharger with a 9 pound hammer because it's too big for the space in the engine bay. But regardless of how many BOV companies put special notes on their turbo spec sheets and their websites, people who believe that the fluttering sound coming from their turbo when they back-off is only air being chopped by the compressor wheel, continually destroy turbochargers and then blame the turbo for being either too small or not built well enough to handle their engine. Believe me, I've seen stock standard CA18DET's with a bit more boost and no BOV destroy turbo bearings, shafts, wheels and in some extreme cases even the housings for one reason, COMPRESSOR SURGE.

That fluttering noise isn't just the excess pressurized air trying to rush back past the compressor wheel in the wrong direction. It does some horrible things that a lot of people aren't aware of. This is a tech article on BOV's not compressor surge so I will outline the other hazards here but leave the full explanation for the Compressor Surge article.

Hazards associated with compressor surge :-

1 - The air rushing back through the compressor wheel actually applies end load to the shaft. Although a turbocharger can handle some end load their primary direction of movement is spinning around. When this compressed air hits the wheel and creates the fluttering noise it actually pulses against the wheel. This pulsing at speeds reaching over 100,000 rpm causes the shaft to hammer backwards and forwards on the thrust plate. Eventually the plate wears out and the wheels hit the housings. How does it do this when th air is coming from one side of the wheel? Well providing the wheel dosn't completely stop it's still trying to push air into the inlet tract, the air trying to get out forces it's way around the front of the wheel pushing it back, onto the thrust plate.

2 - When a compressor wheel is being spun by the turbine wheel it has air being sucked into the smaller side of the wheel and being pushed out the larger side. Where it comes out of the comrpessor wheel it's at a tangent to the wheel itself, which means it's coming from one side of the wheel, not the middle. When this compressed air rushes back into the housing it hits the side of the wheel and creates a lot of side load against the wheel and shaft. This side load is what causes bush bearing turbos such as the T25 and T25G to destroy themselves rather quickly when subjected to compressor surge. The amount of side load on the shaft actually chews into the bush, regardless of how good the oil feed is. Ball bearings aren't as prone to this problem, but it does mean that eventually the bearing will have to be replaced, and a lot sooner than they realistically should be.

3 - When this air rushes back into the comrpessor wheel in the wrong direction it actually starts working the same as an exhaust brake on a truck. It puts pressurised load on the wheel and slows it down. Depending on the boost this slowing down effect can be very quick. There have even been instances where the compressor wheel has come to such a sudden stop with so much reverse pressure that it will try to spin backwards. Not a good thing to happen. It will result in a shattered wheel or snapped shaft and in the worst case an exploded turbo housing due to one of the previous two outcomes.

4 - When you back-off, if the car is fitted with a boost controller (be it electronic of mechanical) the wastegate will start to close. This means that the turbine wheel will begin to increase speed. Now knowing that the compressor when hit with this excessive back pressure of compressed air slows down rather quickly, we can clearly see that if one end of the shaft is trying to speed up and the other is slowing down, eventually something has to give. Normally after it does, it's the hip pocket that starts giving the most.

5 - This is a problem which isn't as big as the others, but it's a problem none the less. On Nissan's and some other vehicles fitted with "hot wire" AFMs, the air coming back through the compressor housing has sometimes reach the AFM. This can result in damage to the sensitive resistors which measure the air coming into the engine. AFMs are not really designed to handle pressurized air although some American cars do have "blow through" AFMs which are in the inlet tract after the turbocharger outlet. AFM's aren't a cheap item to replace if it does get damaged though. If it doesn't damage the AFM then you've come out of it not too bad, but now we see another problem involved in this as well. If air comes through the AFM the opposite way to the proper direction of air flow, then it can in some cases actually cancel out the incoming air flow. This will result in the AFM reading no flow what so ever and suddenly trying to shut down the injectors to stop the engine from over-fuelling. Due to safe guards such as knock sensors and O2 sensors the quick instanance of this happening won't normally damage an engine. So as I said, it's not a major problem, but still a problem you need to be aware of.


To Fit or Not To Fit, That is the question!

[Bludger]

sorry, my brain doesn't function as well as your brain, i just love dose

LOL

[Muzz]

honestly why wouldnt u BOV it, its not like its expensive, and it can be done legally. at least then u dont have to be paranoid about it.

Exactly my thoughts, i got a stock recirculation valve from an s15 with 25k on the clock for $9, for my future setup, cheap piece of mind.

If somone were to get a nice new expensive turbo, i think its definatly worth the slight extra cost, to get a bov, so your not trying to surge air back through it. Treat it right and give it the best possible working conditions.

The only reasons i can see to skimp on this part, is because your doing a setup on the cheap, and are using a cheap 2nd hand turbo.

EDIT: Or cause you love dose^^^

[Lukezen27]

Exactly my thoughts, i got a stock recirculation valve from an s15 with 25k on the clock for $9, for my future setup, cheap piece of mind.

If somone were to get a nice new expensive turbo, i think its definatly worth the slight extra cost, to get a bov, so your not trying to surge air back through it. Treat it right and give it the best possible working conditions.

The only reasons i can see to skimp on this part, is because your doing a setup on the cheap, and are using a cheap 2nd hand turbo.

EDIT: Or cause you love dose^^^

I'm going to buy one for these just to be on the safe side plus I want my setup completely legal
http://cgi.ebay.com.au/ws/eBayISAPI....ayphotohosting

[aimre]

Aimre, you seem dead set trying to convince everyone theres no benifit, do you understand how the surging air effects the turbines mechanicals? Im guessing not, which really dosnt make what your saying any more than an uneducated opinion, the worst kind there is.

Ok so does this guy you quoted have this super fluid dynamic knowledge u speak of?

How can u say im uneducated? i have also shown articles saying that the use of a BOV is pointless. You must know it all cos u quoted some guy off some forum, and i just quoted the guy that owns nizpro and is responisble for this.

http://youtube.com/watch?v=6RpGKP_FsTc

Im sorry


Btw, luke, if ur gonna buy a bov, dont buy them cheapies, they leak.

[Lukezen27]

Ok so does this guy you quoted have this super fluid dynamic knowledge u speak of?

How can u say im uneducated? i have also shown articles saying that the use of a BOV is pointless. You must know it all cos u quoted some guy off some forum, and i just quoted the guy that owns nizpro and is responisble for this.

http://youtube.com/watch?v=6RpGKP_FsTc

Im sorry


Btw, luke, if ur gonna buy a bov, dont buy them cheapies, they leak.

I don't so much care if they work or not but want my setup to pass whatever tests needed to make it legal?

They are required are they not?

Man that's red hot

[aimre]

O, lets not forget the people on calaisturbo who have had their cars dosing for 4 years without rebuilding turbos.

[Lukezen27]

Is it or is it not legal to run without a BOV?

Can anyone just answer that?

[aimre]

VL turbos run NO bov from factory. As do CA18DET's too, supposedly

[aimre]

but hell, what do i know, im uneducated

[Muzz]

What do i know, im uneducated

So you feel you can explain to us how surge effects the turbocharger?
Im not saying your uneducated, your not, im saying that you have nothing to show that surging wont effect turbo life, while there is plenty of SOLID PROOF out the from the top manurfactures that it is infact bad for the products they are engineering.

Here, directly from turbobygarrett.com

Surge Line
Surge is the left hand boundary of the compressor map. Operation to the left of this line represents a region of flow instability. This region is characterized by mild flutter to wildly fluctuating boost and “barking” from the compressor. Continued operation within this region can lead to premature turbo failure due to heavy thrust loading.
Surge is most commonly experienced when one of two situations exist. The first and most damaging is surge under load. It can be an indication that your compressor is too large. Surge is also commonly experienced when the throttle is quickly closed after boosting. This occurs because mass flow is drastically reduced as the throttle is closed, but the turbo is still spinning and generating boost. This immediately drives the operating point to the far left of the compressor map, right into surge.



Surge will decay once the turbo speed finally slows enough to reduce the boost and move the operating point back into the stable region. This situation is commonly addressed by using a Blow-Off Valves (BOV) or bypass valve. A BOV functions to vent intake pressure to atmosphere so that the mass flow ramps down smoothly, keeping the compressor out of surge. In the case of a recirculating bypass valve, the airflow is recirculated back to the compressor inlet.

And..

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

[Muzz]

You must know it all

No, not at all.
Infact, id even go as far as to say that out of us, im the only one who understands how very extreemly little I actually know.

The engineering involved when talking about turbochargers, is WAY WAY beyond crazy, none of us here can even BEGIN to understand how that surge of high pressure is going to effect everything within. What extra forces are being introduced, where are they distributed, what are the effects?

This is the exact reason i would rather listen to what manurfactures say is bad for their turbos. Garret states that compressor surge is bad for their turbos, what have you got to show them to be wrong?