Weird noise from the intake. Possibly compressor surge, but doesn't sound like it. - Miata Turbo Forum - Boost cars, acquire cats.

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Old 05-17-2012, 08:46 PM   #1
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Default Weird noise from the intake. Possibly compressor surge, but doesn't sound like it.

Background info:
My setup really isn't anything unconventional...T25 turbo(minor shaft play), Tial Q bov(stiffest spring and shimmed just enough to stay closed at idle), manual boost controller, stock engine, megasquirt, etc.

I have been hearing very mild compressor surge for a while--not after letting off the gas after being in boost, but after letting off while not in boost. It was pretty mild so I didn't worry about it.

The problem:
I recently made an intake that draws air from the front bumper rather than the hot engine bay.

If it matters, here are pictures of the new addition.


There's an aluminium box that goes around this to keep wheel grime out, but that's not really relevant.


I still hear the very mild compressor surge that I have been hearing for a while, but now there's a new noise--one that's particularly unpleasant. Upon letting off the gas, if I'm at above 5psi or so, there's a noise which is the kind of sound that you may expect somebody with bronchitis to make upon being punched in the chest. It's quick, harsh, and easily louder than the bov. This started immediately after I made that intake. Before this, there was a K&N filter stuck directly on the turbo. I took a video of the bov--it appears to be functioning normally.

My best guess as to why this might be happening is that drawing air from the new location has changed the pressure ratio across the turbo, promoting compressor surge. Although conceivable if that area of the front bumper is at a lower pressure than the engine bay, which it probably is, this idea still seems unlikely.

Can anybody shed some light on my woes?
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Old 05-18-2012, 12:34 PM   #2
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I've decided to roll with the increased pressure differential across the compressor theory. All of the 3" piping may act as a low-pressure plenum as well, and may be enough to briefly reverse the flow through the compressor under certain conditions. I ordered a magnehelic gauge to quantify this, and in the mean time, I'll try to increase the air pressure around the filter.
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Old 05-24-2012, 12:00 AM   #3
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My pressure differential gauge came in today and the suspected compressor surge is definitely compressor surge, and is caused by the foam air filter.

The data:
-The engine bay is slightly higher in pressure than the new draw point for the filter. .25" H2O. That's pretty negligible but I'll still try to reverse it.
-The needle slammed into the stop (2" H2O) when I got into boost with the foam filter. With the old K&N filter, I never registered over 1.75" H2O. I got these numbers by putting a tube inside the filters and using a consistent spot in the engine bay as a reference point.

The compressor surge showed up, very clearly, on the gauge when I was using the foam filter. The needle would be pinned at 2, I'd let off the gas, the needle would jump back to 1.25 or so, and then return to about 1.75 as I coasted down. The drop to 1.25 is due to air flowing the wrong way through the turbo. This didn't happen at all with the K&N filter.

I'm trading filters with a friend who doesn't mind having a junk filter because he's selling the car that it's on. If that filter doesn't solve the problem, I'll just boss up and buy the biggest K&N that will fit in there.

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Old 05-24-2012, 12:57 AM   #4
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so basically the filter is crap?
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Old 05-24-2012, 01:24 AM   #5
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Does it sound like this video at 40-48 seconds?

That was with a short ~6" pipe for an intake with a filter on the end.
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Old 05-24-2012, 08:52 AM   #6
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Quote:
Originally Posted by Faeflora View Post
so basically the filter is crap?
Yep. It was a cheapie that I bought from the same web site that I bought the aluminium tubing and couplers from. I figured that a foam filter would be hard to screw up, but they certainly found a way.

Quote:
Originally Posted by Bryce View Post


Does it sound like this video at 40-48 seconds?

That was with a short ~6" pipe for an intake with a filter on the end.
Maybe I'm missing something, but that just sounds like blowoff valve. I would get a much harsher sound that overpower the bov sound.
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Old 05-24-2012, 09:41 AM   #7
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A video of YOUR sound would solve this pretty quickly...
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Old 05-24-2012, 10:35 AM   #8
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My problem has been diagnosed and a new filter should solve it. The gauge provided some convincing evidence. If not, I'll get a video and post it.
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Old 05-24-2012, 11:18 AM   #9
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just remove the filter and find out...
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Old 05-24-2012, 09:40 PM   #10
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The new filter didn't fix the problem. No filter didn't fix the problem.

My new thought: I'm getting compressor surge regardless of the intake setup. The difference is that the compressor surge sounds similar to the tial when the filter is mounted directly to the turbo, and like compressor surge through a vuvuzela when the new intake piping is attached. If I try to listen closely to where the noises are coming from when no filter or tubing is attached to the turbo, I initially hear a noise from the turbo(the sharp compressor surge that sounds deceptively similar to the bov) followed by release from the bov.

So I guess this means that the bov is the problem. Perhaps there's a delay in opening? That's consistent with the sound, now that I have a better idea of what's going on. I took it apart and inspected everything and removed one of the washers. I'm down to one washer, which is the bare minimum for keeping it shut at idle, and everything looked fine. I put it all back together and the problem persists. It's installed properly and the vacuum source is good. What do I does?

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Old 05-24-2012, 10:23 PM   #11
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Are those other two nipples always open like that?


When u going to post a vid?
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Old 05-24-2012, 10:38 PM   #12
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This thread is useless without a video with good quality audio.
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Old 05-24-2012, 11:08 PM   #13
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Quote:
Originally Posted by Braineack View Post
Are those other two nipples always open like that?


When u going to post a vid?
The other two nipples have pop rivets in them. They're sealed.

As for the video, right meow!


It sounds worse in person, and you can't hear the more mild compressor surge that actually sounds like compressor surge that the thing makes when I drive around lethargically. The noise that you can hear in the video isn't the bov.
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Old 05-27-2012, 01:39 PM   #14
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Any ideas as to why my baller Tial Q might be letting me down? Or perhaps another idea that hasn't been brought to the table yet?
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Old 05-27-2012, 01:54 PM   #15
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I think the valve is shimmed too tight, and that sound is just quick surge-pop. Try loosening the valve up a bunch and see what it does.
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Old 05-27-2012, 04:30 PM   #16
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I only have one washer in there, and that just barely keeps it shut at idle. Loosening the valve more isn't really an option.
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Old 05-27-2012, 04:41 PM   #17
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Firstly, the tial BOVs are very finicky. They will seal nicely under boost but it can be difficult to get them to open instantly when you hit vacuum.

Use a short signal line (yours looks good), and also a large inner diameter signal line. Yes, this will impact the responsiveness of your Tial BOV a huge amount.

Secondly, your car sounds totally fine. Stop being a vag.
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Old 05-30-2012, 08:16 AM   #18
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They really are. I'm using a large diameter line and I tapped a large diameter fitting directly into the manifold, and I'm still having problems.

The video doesn't do the sound justice. Also, note that the sound that you're hearing isn't from the bov.
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Old 05-30-2012, 09:03 AM   #19
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I dont hear anything wrong; I hear the BOV release pressure everytime you lift.

does this tial not have an adjustable spring pressure? does this tial have the spring that you need to source the correct "color" of? if so, what color is your spring?

some BOVs have to leak at idle in order to work and blow off correctly in vacuum without shutter, as the spring pressure is too great.
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Old 05-30-2012, 09:19 AM   #20
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Except, as best as I can tell, that noise is coming from the turbo. I might disconnect the bov for a minute to see if the sound is roughly the same.

The valve doesn't have any means of adjustment other than shims and different springs. I have the yellow spring, which is the stiffest that they make. My car draws enough vacuum to keep it cracked open at idle. I contacted Tial and they recommended an 1/8" or 1/4" shim. The single washer that I have in there now is about 1/16", but because of the design of the bov, pushes the spring out further than any additional washers would. My impression was that a bov that leaks at idle needs adjustment. If that's not necessarily the case, I'll try running shimless. This problem wasn't present when I first got the valve, and it didn't surface immediately after I shimmed it. I'll try just about anything at this point, though.
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