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Discussion Starter · #1 ·
I've noticed the bird chirp noise or squeeking sound on my car ever since my TurboXS mods. I know there are plenty of other sounds (turbo spool, etc.) you hear once you add the downpipe and I like it but I'm trying to identify the bird chirp. Is it the wastegate?

It's funny but I hear the exact same noise whenever I watch the WRC Rally on TV especially on the Subi's but also on some other cars.

I thought I've seen this discussed before but can anyone elaborate so I can intelligently explain to passengers? The "I guess I sucked a bird through the STi scoop" line is getting old. :D
 

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Discussion Starter · #4 ·
WRC_Obsessed said:
compressor stall...
Can you elaborate? When would this stall show itself?

Thanks in advance!
 

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i just looked at my post and realized that it should be compressor surge not stall...

Compressor Surging
What to do about surging. Experts at Turbonetics and elsewhere can certainly add depth to what I have to say here, but you can't expect too much out of them for free. You're right, the obvious thing is to limit boost to stay out of the surge zone, by adjusting the wastegate. Preferred solution is to select the most suitable compressor for your application that allows you to run desired boost without surge. Since surge tends to be an issue mainly at lower rpms due to the nature of turbocharged engines (study a compressor map if you're not already familiar with this phenomenon), a possible solution can be to select a larger turbine housing to delay the increase of boost with rpm so that the WOT boost curve on the compressor map misses the surge zone. A sophisticated electronic control *could* be devised to guide the turbo past the surge zone, but I've never heard of this actually being done ever. It would certainly need to be carefully calibrated for the application, taking into account varying environmental and operating conditions. I work in a group that develops engine management systems for natural gas truck and bus engines. Our strategy is to size the turbo such that surge is impossible under any normal operating conditions. In addition, our electronics will be able to detect, from rapidly changing airflow and manifold pressure, if surge is inadvertently encountered. A severe intake restriction or operation at very low atmospheric pressure such as at very high altitude are two possible causes of surge in a engine that normally operates nowhere near the surge zone. In this unlikely event, our system will take action to quickly take the engine out of surge. This corrective action will effectively derate the engine, which is proper and necessary under such conditions in any event.

Manifestations of surge. There are four surge phenomena I have personally observed. One, which I am not completely sure was surge, occurred when I was test driving the used Merkur that I subsequently bought. Unbeknownst to me at the time, the wastegate hose was broken. It was the first Merkur (or turbocharged car of any kind) I had driven any distance, so I was completely unfamiliar with expected turbo behavior and boost levels. At full throttle on this test drive, boost rose quickly through the red zone, setting off the buzzer. As this happened very quickly and as explained above, I did not realize it was abnormal, I did not let off the gas immediately. At somewhere over 20 psi, the engine bucked noisily a couple of times before I finally let off the gas. All this took only a second or two from the moment the buzzer sounded. After I bought the car, I soon found and fixed the wastegate hose. Another incidence of surge that is quite familiar to me is the fluttering sound (several times per second) synchronized with the rapidly up and down changing volume of the turbo whistle made by a intercooled throttled engine with no compressor bypass valve, when the throttle is closed suddenly while the boost is high, e.g. during initiation of a manual shift. The third manifestation I have observed occurred during an engine development project. The engine was running at steady state WOT, when suddenly its torque just seemed to collapse and the resistance of the dyno stalled the engine. This occurred near peak torque rpm but not at higher rpms. We repeated this test many times while troubleshooting our fuel and ignition systems. These were eventually eliminated as potential causes. Our data from these tests showed the onset of rapidly fluctuating air flow, with the fluctuation increasing in magnitude prior to the engine stalling. I theorized that the turbo could be surging. Unfortunately, we did not have ready access to a compressor map to check this theory, so I just reduced the wastegate setting from 18 psi to 15 psi. The problem disappeared. A fourth I encounter occasionally with my modified Merk. I have the 0.36 A/R turbine housing, which increases low rpm boost and an intercooler which increases mass airflow rate, other things being equal. Both of these effects move the compressor operating point toward the surge line. It's no big surprise that I sometimes get a mild "rocking" sensation when at a fairly steady state condition climbing a hill at say 2000 rpm with 15 psi boost. I suppose this could be the wastegate or knock control, but I never get it at higher rpms, and if I lug the engine down lower than this it gets worse. As I've said, it occurs rarely and is pretty mild, so I am content to leave things as they are and drive around it.

What is happening. The compressor blades are analogous to a wing. As they move through the air, they direct the movement of the air according to their aerodynamic design. If an airfoil is operated at an angle of attack beyond its design limit, the flow on the low pressure side of the foil separates instead of following its contour. This condition is known as "stall". The surge line indicates where the pressure differential, air velocity, and compressor rpm conditions combine to define the aerodynamic limits of a particular compressor. If operated at or beyond this line, the compressor stalls and surges in and out of stall until external changes are imposed to keep the compressor away from this zone.
 

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Discussion Starter · #6 ·
Wow...I think I'll stick with the bird sucked up in the scoop story.

:p

Do you get the same noise on your Stage 4?
 

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i do...especially with the stiff bov spring that i am running right now.

makes me think i am petter for a second. ;)
 

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I don't know if it helps....

This may be completely off the mark...but with my stock engine if I rev it at a standstill in my garage, when the rpm range is coming down, I can sometimes hear a small chirp which seems like it is coming from the muffler area...I don't exactly know what it is but I just thought that I would post my experience with small bird-like noises. If this is a compressor surge in my car should I be worried?????

Thanks.
 

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The chirp from the muffler is normal on the stock exhaust, if you get a aftermarket exhaust the sound will happen at the same time but it will be a deeper sounding pop. For some reason though the stock muffler makes the high pitched chirp instead. Ask anyone who has a aftermarket exhaust and they will tell you about the pops it makes, its a nice sound. It almost sounds like a Anti lag system for a second when it pops.

Back on to the original topic, when does it make the noise and how often? I am guessing its just the wastegate echoing off that large TurboXS DP.
 

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Vrrrrrrrrrrmmmm, Ksshh!!!, Vrrrm vrm VRRMMM, Ksshh!!!

LOL, extreme A-BOV action is what it sounds like...
 

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The chirping noise is high velocity exhaust gases escaping from the wastegate. As I tell a lot of people, the TD04L is a noisy little turbo, adding the TXS DP only makes it more noticable.

-Jim
 

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Discussion Starter · #12 ·
Thanks Jim! That's what I was thinking. It seems to happen on deceleration both in my car and when watching the WRC. :D

The DP makes you here things that were muffled stock. My Stealthback/ScoobySport sounds great and often gives the lovely pop when downshifting.

P.S. I cleaned up my signature of the growing list of mods! ;)
 
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