Exhaust Noise Like Gas Cars!

Hello.

Today, just out of curiosity, I removed the air filter and the airbox and put a nylon stocking over it. Now, when I accelerate, you can hear the turbo whistling very loudly, and when I release the gas, there's a really loud hissing sound, like a gasoline turbo with an open blow-off valve. It sounds really good.
Apparently, this only works with engines that have a VTG turbocharger. I tried it once on my 1Z engine, and nothing changed in the intake sound.

I'm going to order an open air filter now and build a box to isolate it from the engine compartment, supplying it with fresh air from an external source.

My question is: Why does the engine even need to 'blow off' (wastegate)? Shouldn't the pressure be relieved by the engine itself taking up the remaining pressure, since there's no throttle valve in a diesel engine? Isn't it just the variable turbine geometry (VTG) that's adjusted and the injection amount reduced?
I don't understand how so much pressure can dissipate like that.

'Has anyone ever done something like that to their car before, and if so, how good or bad was the result?'
If the air filter is completely isolated from the engine compartment and draws air from the right fender, then an open air filter might not be so bad, right?
What do you think???

Best regards,
Ralf

I've already tried that. The sound quality was okay, but I noticed that the performance was noticeably lower on my system.
I've tried an open filter ('mushroom' type) and even the BMC carbon-dynamic airbox.
This is a closed system that is used in place of the air filter housing.
It looks great, but I also experienced a performance loss here.
Now I'm using the original airbox with the air intake snorkel removed, and with a K&N panel filter (replacement filter). The sound and performance are now okay.

Hello.

Quote:

My question. Why is the engine blowing smoke in the first place?

Nothing will happen either.

Best regards, Rainer.

Hi,

"This hissing or puffing sound comes from the VTG (Variable Turbine Geometry). When you're running at full load with high boost pressure and release the gas, the VTG blades switch to "bypass" mode after 1-2 seconds. This diverts the exhaust flow around the turbine. This process involves turbulence, which should explain the hissing sounds."

Bertil wrote:

When operating at full load with high boost pressure, if you reduce the throttle, the VTG (Variable Turbine Geometry) blades will switch to "bypass" mode after 1-2 seconds. This diverts the exhaust flow past the turbine. This process is associated with turbulence. This should explain the hissing sounds.

Hi Bertil,

You are describing processes on the exhaust side of the turbocharger.

Ralf wrote that he only hears the noise when he drives (essentially) without the air filter - so he makes changes to the fresh air intake at the front, where the noises apparently have to "come out."

I can't quite imagine how exhaust noise from the exhaust side could penetrate so clearly into the intake path.

Hi Ulf,

As you may have read in my WIV thread, I've also observed the "puffing" sound with the dirty air intakes. I believe it's an exhaust noise, as it occurs with a delay of about one second.
Since nothing has been changed on the intake side when the gas is released, I have no other explanation than the VTG (Variable Turbine Geometry) vanes. Based on my observations, the noise sounds like air being forced through a narrow passage, which usually only occurs on the exhaust side at the VTG.
I can't say why this only becomes noticeable when there are changes to the intake (as it is in my case).


What happens to the EGR valve? Could it potentially open briefly when the accelerator is released?
That would be a different explanation!

Hello again.

Okay, the noise definitely isn't coming from the exhaust side. I only removed the entire air filter housing and put a nylon stocking over the mass airflow sensor (MAF sensor). (I've already put it back the way it was - it was just for testing!).
I think there might be a bar of pressure on the pressure side, starting from the turbo, that escapes into the intake hose when the VTG opens. This is because there's less resistance there than in the engine, due to the excess air – about 1 bar!



What happens to the EGR valve? Could it be opened briefly when releasing the accelerator?
That would be a different explanation!

My EGR valve has broken down for some reason. I can't explain it.
That can't be the reason.

During the drive, you can clearly hear the turbocharger whistling. I think it's actually air turbulence because a large amount of air has to be sucked into the mass airflow sensor (MAF) when the engine is running at full load. And the section of pipe leading down to the turbo gets really narrow.

Best regards,
Ralf

Oh yeah:

@Stefan.

What caused the performance loss?
In the warm intake air. It can only be.
Or was the airflow capacity of this airbox not large enough?

Today, after my testing yesterday, I suddenly only had 30 horsepower (approximately). I completely disassembled all the hoses and checked each one for leaks, which took me 1.5 hours.
And that's what happened. The ballast unit for the aftermarket xenon headlights got stuck in front of the air filter box inlet, which restricted the airflow to my TDI engine. And for such a small problem, it took so much work to fix.
What I mean is that the air filter or the airbox might have been too small.

Best regards,
Ralf

Hi everyone,

Could this hissing sound perhaps be explained as follows:
If you suddenly release the accelerator pedal while the turbocharger is under full boost, the wastegate is opened. At that moment, the turbocharger is no longer being driven. The remaining boost pressure can only be released relatively slowly through the engine. Some of it may flow back through the turbocharger in the opposite direction at the moment of the 'whoosh,' causing the noises.

Could it be?

From the exhaust manifold outlet to the intake manifold, there's a considerable volume of space that can store some pressure.

Regards,

Eike.

Hi,

Why can't the boost pressure decrease?

For every two revolutions, the engine "consumes" 1.9 liters of air without any turbocharger boost. When the turbocharger boost is at 1 bar, it's approximately double that amount.

Without any power input (not so much due to the VTG, but more because of the lack of exhaust gases), the turbo spools up very quickly, as it doesn't have much rotational inertia, and the engine easily handles the resulting boost pressure.

Don't forget, this is a diesel engine, which doesn't have a throttle valve! When you take your foot off the accelerator, the intake port sizes are exactly the same as when you're accelerating.

To force air in the *opposite* direction through a failing blower, the pressure would need to be greater than the pressure the blower is still providing, but where would that pressure come from? Everything seems to be getting sucked in through the engine.

Best regards,

Jan.

Hey everyone!

Perhaps some of you still remember me!

I've had this noise for a long time, actually, since I bought my 4-series.
Whether with or without an EGR system, it's always there! And in my case, the air filter doesn't make a difference; with the original filter, it's very noticeable, but with an open K&N filter, it's just a bit louder.

All I can say is that it has something to do with the exhaust system.

I once had a Remus exhaust (with less back pressure), and that solved the problem!
Even if the KAT is not involved, it's gone!
I'm just doing this to test what changes are being made on the server-side.
Now I'm using the original ESD+KAT system again, and everything is back to normal.

@golfman:

'No, I had the BMC carbon-dynamic-airbox installed. It's designed for turbo engines up to 2.5 liters of displacement, and it works great even in a 2.7 bi-turbo setup. It's a really big unit with a very large airflow capacity. It draws in cold air from the outside through a flexible hose, preventing warm engine exhaust from being drawn in.'
Furthermore, I also had a completely open air filter installed (a BMC Power-Venturi), also with a cold air intake. I experienced the same problem with that as well. The best result I achieved was with the original air filter housing, a K&N replacement filter, and the intake snorkel removed (although it still draws in warm air in that configuration).
For me, the sound changes significantly when I switch between a K&N filter and a paper filter.
It definitely isn't the exhaust, as mine is original.
My suspicion also points towards a VVT (Variable Valve Timing) adjustment. Nothing else really seems likely. The noise (a whooshing sound) occurs about 0.5 seconds after releasing the accelerator pedal, but only if there was sufficient boost pressure built up beforehand.

I'm just bumping this thread again...

I just happened to hear that noise as well... I have a Seat Leon with an 81 kW ASV engine, and it's been chipped (with approximately 1.2 bar of boost pressure, and 1.3 bar of overboost).
It happens to me when I give full throttle while running at low RPMs, just letting the turbo spool up, and then abruptly releasing the gas... especially in 4th gear, it sounds very much like a gasoline engine with a turbo and an open pop-off valve... I want to make this sound louder .

From the outside, you can't really hear it. When I drive with the window open, I don't hear anything. But with the window closed, it's noticeable again, and you can clearly hear it coming from the direction of the engine.
I'm going to try out the thing with the tights tomorrow, I'm curious.

sticki wrote:

I just happened to hear that noise as well... I have a Seat Leon with an 81 kW ASV engine, and a chip that boosts the turbo pressure to about 1.2 bar, with an overboost of 1.3 bar. It happens to me when I give full throttle while running at low RPMs, just letting the turbo spool up, and then abruptly releasing the gas... especially in 4th gear, it sounds very much like a gasoline turbo with an open pop-off valve...

I just had the following thought:

The loader is building up a lot of pressure, and then suddenly the power is cut off (the accelerator is released).
However, there are several liters of compressed air in the pipes between the turbocharger and the engine, which want to expand. Due to its low RPM, the engine only consumes a small amount of this air.

So, the "remaining" portion of the air will flow backward through the charger towards the air filter – but only for a brief moment, until the engine, which is continuously drawing air, reverses the flow again.

The backflow of air past the compressor impeller, which is still rotating "forward," could, in my opinion, cause a hissing sound.

Wastegate turbos probably can't do that as well because they don't provide as much boost at low RPMs.
And if the boost pressure is high enough to cause a whistling sound, the engine already sucks in so much air due to the higher RPM that there is hardly any stored pressure left to reverse the airflow.

Those who want to increase the whistling sound should primarily increase the "suction" pipe volume between the turbocharger and the engine, and/or install a larger intercooler.

That sounds very logical and realistic to me...

Yes, for me too... that's the only logical explanation.
'I just find it problematic to increase the volume between the turbo and the engine, because that will definitely affect the turbo's response time. It will have to work harder to pump in more air before it can build up the same boost pressure as before... and whether that larger volume actually brings a significant improvement needs to be tested.'

I'm going to experiment a bit with things like pantyhose and air ducts, but I don't actually want to experience any loss of performance because of the added sound.