do automatics need to run a bov?

[midnight_purple_GA70]

Trdee explained it better , you gotta take your foot of the accellerator sometime. Haven't you seen those fully sik vl commos with automatics and no bovs.

i am fully aware of what a blow off valve does... But if you dont run one on a manual it will also flutter... The fact its auto wont matter. Thats the point i was getting at.

The noise you hear is the air forcing its way back past the turbine wheel trying to force it to spin the opposite direction. All the bov does is stop it from trying to force the turbo back the other way as it gives it an opening to escape out of.

[Z2TT]

Whether auto or manual, the engine still has a throttle plate that closes when you take your foot off the gas, you still need some sort of Bypass valve or Blow off valve if you like to vent the pressure when the throttle closes.

Doesn't rob it off power unless the valve is leaking.

[Cuts]

lots of differing opinions, im sure im not the only person here who drives an auto turbo car that is sans bov and doesnt hear dose on a regular basis because throttle imputs are gental and when driving normally not aggressive enough to cause flutter.

[trdee]

bollocks

There's no 'stalling'. All a BOV does is reduce noise, end of story! They are there factory for this reason only. The performance difference from having or not is two parts of fark all (in favour of not and tested).

So, not needed but a plumb back will most likely have you more under the radar if that's what you want.

what proof do you have that compressor stalling does not damage the compressor in the long run? what tests?

as for power, no it doesnt affect power but it does affect response. no bov = compressor stalling = needs more time to spool back up. quite a simple concept really

[woggin]

Whether auto or manual, the engine still has a throttle plate that closes when you take your foot off the gas, you still need some sort of Bypass valve or Blow off valve if you like to vent the pressure when the throttle closes.

Doesn't rob it off power unless the valve is leaking.

only true on flap-type AFM'd motors, like a 1g.

with a MAP/MAF it just exits via the turbo, the same way it came in, a flap tpe afm usually shuts, or is closer to shutting due to the movement of the air, thus the pressure stays in!

[Z2TT]

Flap or non flap, still required to stop the reversion of air that could cause damage to the turbo over time.

[JustenGT8]

what proof do you have that compressor stalling does not damage the compressor in the long run? what tests?

as for power, no it doesnt affect power but it does affect response. no bov = compressor stalling = needs more time to spool back up. quite a simple concept really

Bulllllshit

There's NO stalling. The only reason the compressor slows at all is because your foot is off the throttle, ergo minimal exhaust gas.

FYI Turbo 101. The compressor is a turbine. When air ceases to flow thru a turbine it SPEEDS UP. When you close the throttle there's no pressure build up caused by the turbo trying to 'pump' more air in...they aren't pumps. All turbines do is speed air up. If there's nowhere for the air to go, the airflow stops and the turbine speeds up as it is no longer doing work

Now, how this all happens in a highly dynamic situation with throttle on/off etc is very complicated with a number of different variables interacting at the same time.

2 things that clearly happens are 1) 'surge'. This is part of the dose sound you hear but you need to understand what surge is. Surge is essentially where the compressor is forced to produce boost but below an efficient airflow (yet more evidence that the compressor hasn't stalled ). It's about the efficiency of the compressor moving air and happens to the left of the 'surge' line on a compressor wheels efficiency map.

the 2nd thing is the pressure reflection pulse off the throttle plate when it snaps shut. This does travel back thru the intake pipework and 'hit' the still spinning compressor, also making goofy noises.

Now with big boost and old skool race turbos operating at much higher rpm that street stuff will ever see, this might damage the compressor. I have only ever seen this once in all my years of playing with turbos. It's not a consideration for OEM, they use BOVs ONLY for noise suppression.

Extra wear from no BOV? nup, the difference is wear might mean a few thousand km shortening of the life of the turbo but something else will happen before you'd ever notice the difference

So, there you have it



As has been said

[trdee]

FYI Turbo 101. The compressor is a turbine. When air ceases to flow thru a turbine it SPEEDS UP. When you close the throttle there's no pressure build up caused by the turbo trying to 'pump' more air in...they aren't pumps. All turbines do is speed air up. If there's nowhere for the air to go, the airflow stops and the turbine speeds up as it is no longer doing work

But the compressor wheel is not a turbine??? It is a centrifugal compressor. The exhaust wheel is a turbine, the compressor wheel is not. They are mutually exlusive definitions. A turbine has energy put into it from the stream, a compressor imparts energy to the stream. Centrifugal compressors work by converting velocity to pressure. they initially speed the gas up yes, but then in the outlet the velocity drops significantly, thus increasing the pressure.

So the way I see it is, when the throttle closes, the compressor wheel is still drawing air in, still pressurising it. As a result, this increases the pressure difference across the compressor wheel, which increases the work the wheel has to impart to the gas, which slows the wheel down.

Am I missing something here? I'm always eager to learn, but from what I understand of turbines and compressors I cant see how your theory makes sense???

Also I just had a read about compressor surge and it is exactly what I am trying to explain. Surge is when the boosted charge is at a higher pressure than the compressor wheel is able to impart on the air, and thus the boosted charge pushes backwards through the turbo. The surge you hear when you overload a turbo is exactly the same phenomenon as what you hear from "dose". The cause is different, but the result is the same. As for the damage. Yes obviously the damage will be more pronounced on a drag car boosting 35psi. But over 100,000kms the damage could be significant.

[river]

Hi,

I got a BOV on Trini at the recommendation of Dave at SAS that "whatever the stupid sound it makes when I back off the throttle" will eventually damage the turbo.

It wasn't about noise for me, it was about being told (by someone who I assume knows what they're talking about) a BOV would increase the lifespan of the turbo.

seeyuzz
river

[JustenGT8]

Hi,

I got a BOV on Trini at the recommendation of Dave at SAS that "whatever the stupid sound it makes when I back off the throttle" will eventually damage the turbo.

It wasn't about noise for me, it was about being told (by someone who I assume knows what they're talking about) a BOV would increase the lifespan of the turbo.

seeyuzz
river

Yea who normally has such little faith and much cynacism...you must be getting old and soft to be so compliant River Where's your evidence based decision??

Show me a turbo, any turbo that has failed from absence of BOV???? All respect to Dave but he's not the only guy who knows what he's talking about.

Many turbo cars run without a BOV. Lets start with the ST165/185's. BOV appeared regularly when NVH requirements and emissions basically forced them to appear. End of story. On a new style BB turbo the wear difference would be such that many other things would have happened before you ever needed to replace the turbo...just not a consideration.

trdee - It is a turbine wheel, just a more complex one. same deal as the hot side.

You have how they work spot on...air speeds up, hits diffuser plate and slows to produce increased pressure and therefore charge density.

As i said it's such a dynamic system that how it works is messy, but as an example.....Turn on an electric floor fan, now block the outlet...what happens? It speeds up. It's not moving air, so it's not really working, so more energy avaialble to make it spin. Same deal with the turbo but with a number of complicating factors....fast moving air has inertia so it keeps moving, there's a pressure reflection pulse, the driving force (exhaust gas) is changing etc

What i can say with decent certainty is there's no significant stalling of the compressor wheel sans BOV. I logged boost response on my 18RGT with/without BOV. Without BOV produced repeatable tenths of a second better boost response. Nothing you could notice by feel and 0-100 and 1/4 time differences were so small as to be unmeasurable, either +ve or -ve.

What would be good would be to measure wheel speed with/without and see what difference, if any, there is and which way it goes. That's obviously the most accurate way but a fairly difficult exercise.

In practical terms though i am certain, BOV or not makes no difference to anything apart from noise.

[Z2TT]

Well Garret's Turbo 101 Pages also state a Blowoff valve is required. I guess out of anyone, they would know what they are talking about?

The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo’s compressor from going into surge. The BOV should be installed between the compressor discharge and the throttle body, preferably downstream of the charge air cooler (if equipped). When the throttle is closed rapidly, the airflow is quickly reduced, causing flow instability and pressure fluctuations. These rapidly cycling pressure fluctuations are the audible evidence of surge. Surge can eventually lead to thrust bearing failure due to the high loads associated with it.

Blow-Off valves use a combination of manifold pressure signal and spring force to detect when the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in the intake tract to atmosphere to relieve the pressure; helping to eliminate the phenomenon of surge.

Still I'm wondering does this only apply if your running higher than stock boost, or have a larger aftermarket turbo.

I've seen lots of cars without Bypass valves from factory, could it be that some turbocharger designs are less prone to damage and don't need a bypass valve, like size of turbo, bearing design?

[Duk]

I've seen lots of cars without Bypass valves from factory, could it be that some turbocharger designs are less prone to damage and don't need a bypass valve, like size of turbo, bearing design?

Angular contact ball bearings have plenty of thrust load capacity, more than a plain bearing turbo.

[trdee]

if you arent running much boost then the damage is probably negligible. which is what justen is saying. the theory is sound, but in practice it may well be that you dont get any significant damage occuring till you get to high boost levels

justen is the only one here who has real world testing behind him so yeah.......

[Cuts]

did the old turbo f1 cars rn them?

what about older group B rally cars?

[Blair]

Did their turbo's get reused for long?