Boost Pressure 2nd Gear AFN

I've noticed that in second gear, when I floor the accelerator from a low RPM to just before the rev limiter, the boost pressure only reaches 0.9 bar initially, then settles down to 0.7-0.8 bar. In third gear, it's a bit more, and in fourth and fifth gears, I get the normal 1.2 bar initial boost, settling down to around 1.0 bar.

Is this normal in 2nd and 3rd gear? Or are the AFN engines a bit more responsive where you are?

I was thinking that we might be able to get a little more out of it by, for example, minimizing the VTG control.

Or is this intended as protection against overpressure and excessive wave speed, because the short acceleration time of the second gear prevents proper engagement? (I'm putting forward a theory here }).

Sorry, I forgot:
Volkswagen Golf 3 AFN, 166,000 km, engine completely stock except for the large vacuum pump on the turbocharger!

Hello Marcus,

Normally, the boost pressure overshoots are higher in the lower gears.
For me, also with AFN, it's around 1.3-1.45 bar. In the higher gears, it's about 1.2 bar.
Possibly, you didn't keep your foot on the accelerator for long enough for the turbocharger to build up the correct boost pressure (in the lower gears).
More dangerous for the turbocharger are large pressure surges at high speeds (e.g., on the highway), as the mass airflow through the engine is much higher in these situations, making it easier for the turbocharger to overspin.
Therefore, caution is advised when shortening the actuator rod.

Best regards,
Albrecht.

Thank you for the response.

I've had the LDA (Lane Departure Assist) system for half a year now, and it happens every time (with constant acceleration, pedal fully pressed, even up to the speed limiter).
The only way I can get a pressure overshoot of around 1.2 bar in second gear is when I'm at full throttle at a higher RPM, I slightly lift my foot off the accelerator and then press it back down (and even then, it drops back down to 0.8-0.9 bar)! Otherwise, there's no chance, whether going uphill, downhill, on a flat surface, or whether the engine is warm or cold.

So, as I said, you can shorten the rod a bit if your boost pressure behaves like that. But after that, be sure to test it carefully to see if it was too much.

How was the rod adjusted after the vacuum sensor was replaced?

Best regards,
Albrecht.

To be honest, I have no idea.

It's been a while since my car had the entire turbocharger, control unit, and control valve replaced because the people at VW back then were too clueless to recognize the problem with the vacuum sensor. While diesel mechanics were generally aware of the issue, the vacuum sensor itself was still a bit of a mystery to them.

I can say that the car doesn't seem to be performing noticeably worse than before, although it's quite leisurely in its acceleration! You can almost count to two before you feel the full power (which is understandable, as it's not a racing engine).

I'll try it carefully. I don't want it to go into emergency mode again. I've dealt with that enough times!

Könnte es evtl auch sein, dass deine LDA überdämpft und damit zu träge ist? Dann würdest du die Druckspitzen natürlich auch nicht so ausgeprägt sehen. Nur so as an idea...

Regards,
-Private

Okay, the LDA was a bit of a challenge, but I think it's running great now. It had a lot of vibration at first, which I managed to dampen with three small, offset hose clamps. I spent a long time adjusting it until it was properly set.
It only vibrates minimally under high load and at low (below 1900/min) RPM, and it reacts very precisely (overshoot and pressure regulation are clearly visible)! If I loosen one of the clamps, the vibration starts again!

Conclusion: Don't think it's related to the LDA!

I'm going to carefully feel around the VTG rod.

'Thank you.'

Hello,

I decided to postpone the adjustment of the VTG linkage for now and, together with a colleague, ran a few test runs.

I drove each gear, second and third, at full throttle, and the road was more or less uphill (so don't be surprised by the times).

I would like to ask you to please review the values for boost pressure (LD) and air mass/stroke (LM) and compare them to your known values.

Link: http://www.the-WSS.de/Golf.htm

Okay, in my opinion, the boost pressure curve in second gear is a bit too aggressive! The target pressure is never actually reached.

I find it hard to believe the overpressure values of up to 1.4 that my colleague (Albrecht) mentioned earlier, given the pressure readings I obtained from my own engine.


Golf 3, AFN engine, 169,000 km, all engine components are original, large vacuum pump on the VTG (Variable Turbine Geometry).

Hello Markus,

You should work on improving the presentation of your measurement results (labeling).
Have you checked if the adjustment lever pulls the VTG (Variable Turbine Geometry) into the maximum boost pressure position when the engine is idling?
By the way, my LDA is undamped, with overshoots exceeding 1.4 bar.
However, I'm starting to worry that the problem might be due to some slight stiffness.
I might be able to record a test run in about two weeks. I'm quite busy at the moment. I'll post it here when I do!

Best regards,
Albrecht.

My system rarely reaches a boost pressure of over 80%. I suspect the variable turbine geometry (VTG) might not be closing completely. It only moves a small amount, around 100 degrees.
Then I would adjust the length of the shaft slightly to achieve a duty cycle of approximately 70%.