Boost Pressure: Does 20% More Mean 20% More RPM?

Hello,

After shortening the VTG strut (to bypass the blocking at the lower stop), I observe a brief increase in boost pressure to approximately 1.5 bar when suddenly accelerating in lower gears. This boost pressure then quickly returns to the target value of 1 bar.
The question is, does an x% increase in boost pressure also result in an x% increase in the rotational speed of the turbocharger wheel (at the same engine speed)? In other words, is the relationship proportional?
Should I be worried about the charger, or can it handle these short-term power surges?
Thank you in advance.

Best regards,
René K.

Hello René,

I'm just trying to answer your question, and reading compressor maps is also new territory for me.
Here you can see a typical compressor characteristic curve:


On the left, the pressure ratio is shown before and after the compressor, while on the right, the volumetric flow rate or mass flow rate of air is plotted.
For low flow rates, your assumption that the charger speed is approximately proportional to the boost pressure is correct.
However, at high flow rates (and thus engine speeds), the turbocharger speed increases much more significantly than the pressure ratio.
This means that a general statement cannot be made because the relationship between the impeller speed and the boost pressure varies depending on the operating point.
Unfortunately, I haven't been able to find a characteristic curve for the turbocharger used in the AHF/AFN engines. However, 1.5 bar seems a bit high at first glance. Therefore, it might be beneficial to adjust the linkage slightly to increase the boost pressure.

Best regards, Rainer.

Hello Rene,

By what method did you shorten the connecting rod?

Regards,
Thorsten

Hi!

@Rainer: Thank you very much for your explanation, it was very informative . I will probably extend the strut again, especially since yesterday, while driving on the highway, I briefly reached 1.5 bar on the 5th gear at around 3000 rpm when I suddenly accelerated. This is probably not very beneficial for the longevity of the turbocharger.

@ttns4: It's probably around 3mm to a maximum of 4mm.

Best regards,
René K.

Hi,

I extended the pole again, and the over-swinging is now within limits, a maximum of 1.2 bar .
But the nice side effect is now gone, because the shortening resulted in a slightly increased boost pressure of about 1.1 bar from 3000 rpm -> practically no soot, even in the tuned operation. The pressure now settles at 0.9 bar . I thought the boost pressure was around 1 bar on the AFN, so I'm considering a moderate boost increase, as described in the '10 Cent Tuning' article.
Here are a few questions about it:

- If I understand correctly, you need to replace a resistor that is located in the control unit. Do I need any special tools to open it, or will a few regular screwdrivers suffice?
- Are standard SMD resistors sufficient?
- Is there anything specific I need to be aware of, or, in other words, how risky is the procedure?
- As far as I could tell from the posts, Rainer and Ulf are at least using this modification. How about you guys, what's the situation with the boost overshoots?

Questions upon questions... I hope for your expert answers .
Thank you in advance.

Best regards,
René K.

zzzZZ wrote:

... - If I understand correctly, you need to replace a resistor that is located in the control unit. Do I need any special tools to open it, or can I use a few regular screwdrivers? ...

...if a Torx screwdriver is a standard item in your toolbox, then you don't need any specialized tools...
... a good soldering iron (or soldering station).

Quote:

... - Do standard SMD resistors from Conrad work? ...

Yes... but pay attention to the size!!

Quote:

... - Is there anything specific I need to be aware of, or, in other words, how risky is the procedure? ...

... before the work on the exposed "heart" of the Earth!
... solder properly, don't "fry" the components!!
"SMD soldering isn't easy... If you don't install the resistor completely stress-free, it will break over time, which means it will stop working."
... You can essentially make any mistake, even to the point of damaging the storage tank. However, if you work carefully and diligently, you are unlikely to encounter any problems.

Quote:

... - As far as I could tell from the posts, Rainer and Ulf are using this modification. What about you guys, how are you handling the boost overshoots? Questions upon questions... I hope for your expert answers . Thank you in advance. Best regards, René K.

Thank you for the important information.


...if a Torx screwdriver is a standard item in your toolbox, then you don't need any specialized tools...
... a good soldering iron (or soldering station).


Okay, I got it.


Yes... but pay attention to the size!!


Do they vary in size depending on the control unit? In other words, do I need to check the control unit before buying them, or can you tell me the part number (including size) of the SMD resistors? My wife doesn't really understand my tinkering anymore; it takes a lot of time. I have almost a free morning on Saturday, but I just need to look after our child, who hopefully will take a nap between 10 and 12, so I can get started *hopefully*.

Best regards,
René K.

Hi.

Regarding my AFN (pressure sensor in the engine computer), the control behavior has not fundamentally changed.

Whether or not there is over-extension is, in my opinion, more a matter of mechanical adjustment (e.g., the threaded rod) than of resistance.

Regarding the size of the resistors in your car, I'd rather not say anything specific... in mine, the "larger" ones should technically fit, but the "smaller" ones can also be accommodated.
Don't ask me about the standard designations right now...

Hi everyone,

Thank you for the posts. I'll go to Conrad today and stock up on a handful of resistors in various sizes; they're not that expensive.
I will start the modification attempts on Saturday. Of course, I will keep you updated.

Best regards,
René K.

Hi,

I was just at Conrad... a ridiculous store. Even though it's not exactly small (it's in Cologne), they only had 10k and 27k Ohm SMD resistors, and they only had them in one size (microscopically small). There was nothing in between . Well, I immediately bought 5 of each type, you never know.

See you then... René K.

Hi,

As promised, here's my report from the soldering/assembly event on Saturday:
Removing the control unit -> no problem.
Opening the control unit -> no problem.
Then I got a big surprise , because now I knew why the invoice for my so-called 'chip tuning' didn't say 'chip tuning' but rather 'engine control unit modification.' I had never heard of such a modification, let alone seen one. Various cables were installed, including one from the sensor output of the turbocharger pressure sensor, all leading to a small circuit board that was mounted upside down inside the engine control unit. This circuit board has four potentiometers and a spindle trimmer, as well as a few small chips. However, the labeling is no longer visible because everything has been covered with sealant. You can't turn the potentiometers anymore because of this.
The traces on the underside of the circuit board can no longer be followed. To me, it looks like tuning with a box integrated into the control unit, so it's not chip tuning. If I had a digital camera, I would love to show you all.
Okay, but this doesn't stop me from my original plan. I've now replaced the SMD resistor with a 27k resistor, and the boost pressure has actually increased from approximately 0.9 to 1.1 bar. A nice side effect: the soot is now barely visible.
Allerdings kämpfe ich jetzt wieder mit den extremen Überschwingern, die kurzzeitig (unter 0,5 s) bis zu 1,6 bar erreichen. Damit drehn die Ränder (215 40R16) nun bei feuchter Straße im 3. Gang through
It seems I have no other choice but to extend the VTG boom again . I don't really have anything else to do... when I can start working on it is still uncertain. The current weather conditions aren't exactly motivating for outdoor crafting.
Of course, I will keep you updated on whether I manage to get things under control.

See you then... René K.

... it sounds like an Authal box that has been integrated into the engine control unit (ECU). The Authal box adjusts the tuning based on the boost pressure.

Hi,

I've now extended the VTG rod again, and the over-swinging seems to be contained - 1.3 bar at peak .
The actual problem that made me shorten the rod seems to have been resolved. The copper paste seems to have worked wonders. Hopefully, this will last *knocks on wood*.

I have one more question: If the resistor I soldered in should come loose, would I have serious problems, or would the control unit recognize an implausible value from the boost pressure sensor and go into emergency mode?

Thank you everyone for the helpful tips.

Best regards,
René K.

zzzZZ wrote:

... I have one more question: If the resistor I soldered in should come loose, would I then have serious problems, or would the control unit recognize an implausible value from the boost pressure sensor and go into emergency mode? ...

Hi René,

Do you trust your solder joints?
Yes, the motor controller would enter emergency mode (due to an implausible reading).