TDI Exhaust: Unknown Performance Limiter?

I hope Rainer's delete button doesn't come back to haunt me now... *hides face*

As is commonly known, the typical exhaust modifications (e.g., installing aftermarket mufflers, removing the catalytic converter) do not result in any significant increase in power for TDI engines.
Therefore, this topic will not (no longer) be discussed here.

I have now read about the flex pipe as the worst bottleneck in the exhaust system in several sources, which are presumably above the general level of idle chatter.
Behind it, you can practically do whatever you want; there's simply no way to achieve more performance (at most, you could experience a loss of performance if you further restrict the cross-section beyond what's already present in the flexible hose).

The turbocharger already increases the back pressure in the exhaust system, so an additional restriction, in my opinion, can only lead to power losses due to increased pumping work by the engine.

Following the principle that the weakest link determines the strength of the chain, to potentially increase performance through the exhaust system, one would first need to eliminate any bottlenecks at the transition point between the pipe and the flexible section.

I would now like to know if anyone here can provide more details about this approach.
Perhaps someone knows the detailed construction of the flexible components, including their internal dimensions?

Unfortunately, it's not possible to easily inspect or measure the area due to the 90-degree bend on the loader flange. And, unfortunately, I don't have an endoscope available.

Hello Ulf,

You seem to be very focused on finding ways to improve performance (or reduce power loss) lately. Aren't 130 horsepower enough for you?
I don't know the cross-section either.

I think you can't just arbitrarily increase its size, because otherwise there won't be enough exhaust pressure available to power the turbocharger.
I can't say exactly how close we are to the border.

Holger247 wrote:

You seem to be very focused on finding performance (loss) lately. Are 130 horsepower not enough for you?

You never have enough power.
But that has nothing to do with this topic, my Polo is already fine .

Quote:

I think you can't just arbitrarily increase its size, because otherwise there won't be enough exhaust pressure available for the turbocharger.

Cognitive biases?
The exhaust back pressure required by the turbocharger is automatically determined by the requested boost pressure, provided that the exhaust flow is sufficient.

Simply put, the loader "consumes" a pressure of, for example, 3 bar at the operating point xy.
If the exhaust gases go directly from the turbocharger into the open air, the engine must pump the exhaust gases with a pressure of 3 bar into the turbocharger.

If there is another restriction/silencer/catalytic converter downstream of the turbocharger, requiring an additional 2 bar of pressure drop (with the current volumetric flow rate) to pass through it, then that adds to the 3 bar boost pressure of the turbocharger.
Therefore, the engine must pump the exhaust gases into the turbocharger with 5 bar instead of 3 bar, which is generally not beneficial for the actual performance.
(all numbers are arbitrarily chosen!)

EDIT:
Especially in the higher RPM range, where the wastegate is already largely open or the variable turbine geometry (VTG) is mostly bypassing the turbine, the effect of the restriction should become increasingly prominent.

Or should our TDIs also be designed in such a way that the flushing losses increase when the back pressure is reduced?
Initially, this would only result in a loss of fresh air, which, however, would appear as a loss of boost pressure: less excess air -> potentially slower combustion = loss of efficiency = loss of power, higher exhaust temperatures, more soot, etc.

Hi Ulf,

Here are some pictures of those things:
http://cgi.ebay.de/ws/eBayISAPI.dll?ViewItem&category=61433&item=7909135899&rd=1

You should have asked a year ago; I would have had a 1Z flexible pipe left over .

m;

I remember buying an exhaust pipe in one piece once. The inner diameter was 55mm. It was located after the catalytic converter, towards the rear silencer. However, the section from the turbo to the catalytic converter should have the same diameter.

joergs wrote:

I remember buying an exhaust pipe in one piece once. The inner diameter was 55mm. It was located after the catalytic converter, in the direction of the rear silencer. However, the section from the turbo to the catalytic converter should have the same diameter.

Yes, in my case, the pipe appears to have the same thickness both in front of and behind the flexible section.

The alleged restrictions are supposedly located in the flexible pipe sections, or are created somehow during the manufacturing process at the transition between the pipe and the flexible section – following the principle of a welded-in washer used to restrict the exhaust flow of some motorcycles, such as the Honda XL 500 (?).

The photos from the auction don't necessarily look like those kinds of bugs...

Hmmm... I'm not sure if this can or should be applied to a TDI engine.

'I recently replaced the exhaust pipe on a gasoline-powered Passat with a 90hp 1.8L engine and examined the part carefully due to its rather high price. I couldn't detect any narrowing of the cross-section. But maybe things are different with exhaust systems on gasoline engines? At least the height of the exhaust system is the same...'

joergs wrote:

I recently replaced the exhaust pipe on a Passat with a 90hp 1.8L gasoline engine, and I carefully examined the part due to its rather high price. I couldn't detect any cross-sectional narrowing.

Inside or outside (-> no visible narrowing)?

Did that thing also have a flexible part?

'Similarly, there were no visible signs of narrowing or constriction on the outer surface of the flexible part, nor on the inner surfaces at the connections. In the case of the old, defective part, the inner diameter of the flexible section was approximately the same as that of the pipe.'

joergs wrote:

Similarly, there was no visible narrowing or constriction on the flexible outer part, and neither was there any on the inner transitions. In the case of the old, defective part, the inner diameter of the flexible section was approximately the same as that of the pipe.

Ah, that gives some hope for the TDIs... I just checked on mine:
The flexible section is significantly thicker on the outside than the pipe, so there likely isn't any narrowing or constriction built into the inside.

What was actually visible as the interior lining?
Is this also a type of stretch fabric, like the one shown in the diagram in the auction link?

Hey, you two idiots...

I'm also just coming from .

I can only confirm your observation. It has a beautiful, thick diameter... and it's the same with the A4 with the AFN engine!

Inside, there was also braiding, but it was much finer and denser than the braiding on the outside.

If I look at the prices on the website of that guy, his prices are really very cheap, but the quality will also reflect the price. However, you should definitely remember those flex pieces. The new trouser pipe cost 186 marks at VW back then , and here it would have been done for a maximum of 30 + fixed price .