Weird noise from the intake. Possibly compressor surge, but doesn't sound like it.
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.
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.
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.
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 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.
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?
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.
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.