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Wierd Bov Flutter

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nek

10+ Year Contributor
832
9
Apr 17, 2009
Pittsburgh, Pennsylvania
So when ever I let off the boost (not while boosting) I get a bov fluttering. (could be called compression surge whatever I dont care). I read up and many say that its not damaging if its only happening when you let off the throttle but its a big problem if it happens under boost. Anyways, I have a 1g bov and the fluttering started happening after I did a 3in gm maf install on my upper ic piping. At first I thought it was because I moved the whole ic piping forward a little bit causing a kink in my vacuum line but when I replaced it with a long line the issue still occurs. What do you guys think the problem is and how do I fix it?
 
My cars always had bov flutter and its NOT compressor surge if its when you let off the gas, as you stated its only damaging when your WOT and building boost.

Watch the full video, its a good one...
http://www.youtube.com/watch?v=nCj6Spwl1CU

:dsm:
 
This will happen, you can try loosing the nut a little bit, but you will get a little bit of flutter. And like above stated, compression surge happens in boost, so this is just the normal surge and let off.

Its ok if its not during boost or hard driving. It should happen a little...more people will charm in with different opinions im sure. But my car does it as well and i dont see any problems.
 
thanks guys, the thing that really worried me was when you let off the gas, you can hear a very dim "ding" like when your microwave is done like 3 rooms away. Im almost positive its the turbine coming in contact with the housing which obviously isnt good but at least the surge isnt doing too much damage.
Thanks for the input
 
thanks guys, the thing that really worried me was when you let off the gas, you can hear a very dim "ding" like when your microwave is done like 3 rooms away. Im almost positive its the turbine coming in contact with the housing which obviously isnt good but at least the surge isnt doing too much damage.
Thanks for the input

Not sure that the microwave sound is what you think it is but if you believe that there is something turbo related check it out and reslove the issue - I wouldn't drive it and risk more damage. I know what you mean about the bov fluttering I was running a healthy 400 plus hp setup and when I got out of boost it would flutter like a freak'n turkey - the car sounded 100 percent cool in every way except when I let off the throttle coming out of boost:ohdamn:
 
My cars always had bov flutter and its NOT compressor surge if its when you let off the gas, as you stated its only damaging when your WOT and building boost.
If that's the case our cars would never need blow off valves / compressor bypass valves.

It may not be damaging if you run very low boost, but if you cut the throttle from 20psi and your blow off valve isn't opening (causing flutter / surge), you're loading the journal bearings and possibly twisting the shaft.

How do I know this? A good buddy of mine forgot to install the vacuum line back on his BOV after a motor mount install late one evening. He drove the car daily like that for about two weeks, and the next time he went to the track his Frank 5 20G started howling because the wheel had started to brush the housing due to the journal bearing wear from the BOV not functioning properly. Of course this wouldn't happen on ball-bearing turbos because the bearings fully support the shaft at all times and only require oil for lubrication, not a bearing buffer.


For the record, flutter is absolutely considered surge. The turbo is producing airflow the engine is not using (because the throttle plate is closed) and the airflow is backing up in the piping and causing the noise you're hearing. It doesn't matter if the engine is under load at low RPM, high RPM, or revving down with the throttle plate closed, surge can exist in any instance if the blow off valve is not functioning or is adjusted too tight.
 
If that's the case our cars would never need blow off valves / compressor bypass valves.

It may not be damaging if you run very low boost, but if you cut the throttle from 20psi and your blow off valve isn't opening (causing flutter / surge), you're loading the journal bearings and possibly twisting the shaft.

How do I know this? A good buddy of mine forgot to install the vacuum line back on his BOV after a motor mount install late one evening. He drove the car daily like that for about two weeks, and the next time he went to the track his Frank 5 20G started howling because the wheel had started to brush the housing due to the journal bearing wear from the BOV not functioning properly. Of course this wouldn't happen on ball-bearing turbos because the bearings fully support the shaft at all times and only require oil for lubrication, not a bearing buffer.


For the record, flutter is absolutely considered surge. The turbo is producing airflow the engine is not using (because the throttle plate is closed) and the airflow is backing up in the piping and causing the noise you're hearing. It doesn't matter if the engine is under load at low RPM, high RPM, or revving down with the throttle plate closed, surge can exist in any instance if the blow off valve is not functioning or is adjusted too tight.
I'm not trying to debate this with you Justin but this is what I understand about the topic, please correct me if I'm wrong...
Technically speaking "flutter" only occurs with a BOV and "surge" only occurs with the turbo.

A BOV fluttering is from too tight of a spring, the air starts to be released but the pressure on the spring drops so the BOV closes. Pressure builds up enough to open up the BOV spring again and the same happens, its just not as long because its even less pressure and this process repeats generating the "flutter" until it stops. This type of "flutter" isn't bad and to get rid of the noise just reducing the amount of pressure on your BOV should clear it up, however you might have trouble holding boost depending how much you run.

If the flutter is more of a metallic sound its not BOV flutter at all, its actually compressor surge and that metallic sound is the compressor blade hitting the air. The sound that compressor surge creates is continous, there is no less and less air slowing down like you hear when the BOV flutters. It doesn't actually spin the blades backwards but like Justin said it puts a load on the journal bearings and the shaft.

Seems to me that if the above is true the "surging/fluttering" air thats heard at the BOV isn't smacking the blades of the turbo, just actuating the bov open and closed... thus not rendering any harmful damage. My HKS SSQV has always fluttered, it did it when I was on stock boost from my t25 and it does it now and I'm pushing 28psi with a HTA68. Now that we're on this topic I wonder if I need to get a different BOV since my HKS doesn't have the adjustment screw on the back like some.

:dsm:
 
In any case, why would it only happen after I installed my gm maf and 2 I believe in either case, if the bov isnt opening at the right time it creates turbulence in the air which comes back and hits the compressor wheel. But if its such an issue, why do some cars not run any bov at all?
 
I'm not trying to debate this with you Justin but this is what I understand about the topic, please correct me if I'm wrong...
Technically speaking "flutter" only occurs with a BOV and "surge" only occurs with the turbo.

A BOV fluttering is from too tight of a spring, the air starts to be released but the pressure on the spring drops so the BOV closes. Pressure builds up enough to open up the BOV spring again and the same happens, its just not as long because its even less pressure and this process repeats generating the "flutter" until it stops. This type of "flutter" isn't bad and to get rid of the noise just reducing the amount of pressure on your BOV should clear it up, however you might have trouble holding boost depending how much you run.

If the flutter is more of a metallic sound its not BOV flutter at all, its actually compressor surge and that metallic sound is the compressor blade hitting the air. The sound that compressor surge creates is continous, there is no less and less air slowing down like you hear when the BOV flutters. It doesn't actually spin the blades backwards but like Justin said it puts a load on the journal bearings and the shaft.

Seems to me that if the above is true the "surging/fluttering" air thats heard at the BOV isn't smacking the blades of the turbo, just actuating the bov open and closed... thus not rendering any harmful damage. My HKS SSQV has always fluttered, it did it when I was on stock boost from my t25 and it does it now and I'm pushing 28psi with a HTA68. Now that we're on this topic I wonder if I need to get a different BOV since my HKS doesn't have the adjustment screw on the back like some.

:dsm:


I believe when it "flutters" not ALL the air is able to escape due to the stiffness of the spring, and when the throttle plate closes, the pressure in the piping flows back to the turbo, and spins the turbine wheel the opposite direction, thus being harmful. Now correct me if im wrong..?

In any case, why would it only happen after I installed my gm maf and 2 I believe in either case, if the bov isnt opening at the right time it creates turbulence in the air which comes back and hits the compressor wheel. But if its such an issue, why do some cars not run any bov at all?

The only cars that do not run bovs are deisels because there is no vacuum pressure.
 
...and spins the turbine wheel the opposite direction, thus being harmful. Now correct me if im wrong?
The other might be 100% accurate, I do know that this isn't correct though. When the surge occurs the compressor wheel hits the surging air putting pressure on the bearings and shaft. The exhaust side of the turbo is still spooling up the turbo forcing the compressor wheel to spin whether its surging or not, this is when you can damage the turbo.

:dsm:
 
I'm not trying to debate this with you Justin but this is what I understand about the topic, please correct me if I'm wrong...
Technically speaking "flutter" only occurs with a BOV and "surge" only occurs with the turbo.
I should have made it a little more clearly understood. Here's an attempt to make this sound better:
  • IF the noise is being emitted solely from the blow-off valve, it is not compressor surge.
  • IF the noise is coming from anywhere near the turbo's compressor cover (think "Dose Pipe"), then it is compressor surge.

There is a very fine line between the BOV being a little too stiff so the sound you're hearing is actually the valve opening and shutting rapidly or actual surge coming from the turbo.

In the case of my buddy's car mentioned above, there was no line or vacuum to draw the valve open so every time he cut the throttle while driving the car daily for a week or two, the flutter he was hearing was solely the turbocharger.

I believe when it "flutters" not ALL the air is able to escape due to the stiffness of the spring, and when the throttle plate closes, the pressure in the piping flows back to the turbo, and spins the turbine wheel the opposite direction, thus being harmful. Now correct me if im wrong..?
It's not actually spinning the other way, the air trapped in the piping is attempting to spin it the other direction. The exhaust flow while the engine is revving down is still driving the turbine wheel forward while the air in the piping is trying to back-spin the compressor wheel. This causes the rotating assembly to jump up and down rapidly, shearing through the protective layer of oil on the journal bearings and causing the bearings to contact the housing. This leads to accelerated bearing wear, and eventually a turbo failure.

The only cars that do not run bovs are deisels because there is no vacuum pressure.
This is false...there are plenty of gasoline-powered turbo cars out there without blow off valves. Look at all the 2.2 and 2.5 turbo Chryslers made in the 80's and 90's. The need to bypass the compressor to prevent surge is gauged by the factory boost level as well as the compressor cove design...a turbo with an anti-surge groove would not require the same valving that one without the groove would.

Diesels do not have vacuum, you are correct there...but where you are false is the REASON they do not have vacuum- there is no throttle plate on the intake manifold, so there is no vacuum between the manifold and throttle plate.

No throttle plate = no need for a blow off valve.
 
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