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!