A lesson on stop screw and turbo actuator rod length please.....

[bhodgkiss]

As I understand it, the stop screw sets the starting position of the vanes, so when the engine is switched off, the actuator moves back (no vacuum) and hits the stop screw. So thats the starting point of the vane position at low rpm.

Is that correct? So if I want to improve low end boost as the boost is lagging the request, I adjust the stop screw slightly?

The rod length affects the rate at which the vanes move? is that correct?

And then on top of this there's the N75 solenoid duty cycle map on the ECU which can also be changed......

So what do I need to do if my low end boost is lagging the requested boost please?

Thanks!

 

[A5INKY]

These are not really "TDI 101" questions. You would have more and better feedback in the Performance section.

That said, the stop screw sets the VNT vanes closed position that corresponds to full vacuum from N75 valve. Vanes closed position is when the VNT is wasting minimal exhaust energy and therefore trying to create maximum spool. This is a tricky thing to set and the better versed will tell you it is a balance between EMP and boost pressure that should be used to make adjustments. You need to have the VNT open some at full closed position to not only prevent a massive EMP spike but also to allow enough flow past the turbine to get it moving. In your case, I would rig an EMP gauge and have and very clear plan worked out before making any changes to the stop screw. It is really easy to screw that adjustment up.

From what I have gathered and experienced with VNT rod length, it is a calibration between ECU N75 intent and resultant VNT action. Since (I believe) you have been doing some of your own mapping you may have a better understanding of how the ECU controls N75 than I. But my understanding is that the N75 map is more of a "suggestion" than anything. At some point complex algorithms in the ECU take over. This is why I am of the opinion that VNT rod length should be calibrated prior to tuning changes.

VNT rod length calibration protocol that seems most accepted amongst the tuners around here is as follows:

-Log Group 11 while doing a 3rd gear WOT run from 1500 RPM to 4500 RPM (or higher if you are willing)
-Look at log for N75 duty cycle at 4000 RPM, should be in the 70-80% range (or "reversed" 20-30% range for some ECUs)
-Assuming a older VNT "push with vac applied" actuator: If duty cycle is too high, lengthen rod to open vanes more for a given duty cycle. If too low shorten the rod.

The logic is to have a rod length that allows maximum VNT control over the useful range required by your set-up, with ~20% left over to give the ECU a chance to control over boost conditions.

 

[A5INKY]

I realized I didn't directly answer your questions with my post, here goes:

As I understand it, the stop screw sets the starting position of the vanes, so when the engine is switched off, the actuator moves back (no vacuum) and hits the stop screw. Nope, no vacuum opens vanes away from stop screw So thats the starting point of the vane position at low rpm. Yes, once running with N75 applying maximum vacuum at idle

Is that correct? So if I want to improve low end boost as the boost is lagging the request, I adjust the stop screw slightly? Maybe, see post above

The rod length affects the rate at which the vanes move? is that correct? No, rate does not change from mechanical adjustment. Rod length changes shifts range of opening as in the calibration I attempted to describe above.

And then on top of this there's the N75 solenoid duty cycle map on the ECU which can also be changed......

So what do I need to do if my low end boost is lagging the requested boost please?

Thanks!

I think Gurus and Tuners here would agree: get the hardware set right, then adjust tune. For any given hardware set up there is an optimal VNT stop screw and rod length adjustment that is independent of tune. Get those right first, then optimize tune and it will perform as well as possible.

 

[BD76]

These are not really "TDI 101" questions. You would have more and better feedback in the Performance section.

That said, the stop screw sets the VNT vanes closed position that corresponds to full vacuum from N75 valve. Vanes closed position is when the VNT is wasting minimal exhaust energy and therefore trying to create maximum spool. This is a tricky thing to set and the better versed will tell you it is a balance between EMP and boost pressure that should be used to make adjustments. You need to have the VNT open some at full closed position to not only prevent a massive EMP spike but also to allow enough flow past the turbine to get it moving. In your case, I would rig an EMP gauge and have and very clear plan worked out before making any changes to the stop screw. It is really easy to screw that adjustment up.

From what I have gathered and experienced with VNT rod length, it is a calibration between ECU N75 intent and resultant VNT action. Since (I believe) you have been doing some of your own mapping you may have a better understanding of how the ECU controls N75 than I. But my understanding is that the N75 map is more of a "suggestion" than anything. At some point complex algorithms in the ECU take over. This is why I am of the opinion that VNT rod length should be calibrated prior to tuning changes.

VNT rod length calibration protocol that seems most accepted amongst the tuners around here is as follows:

-Log Group 11 while doing a 3rd gear WOT run from 1500 RPM to 4500 RPM (or higher if you are willing)
-Look at log for N75 duty cycle at 4000 RPM, should be in the 70-80% range (or "reversed" 20-30% range for some ECUs)
-Assuming a older VNT "push with vac applied" actuator: If duty cycle is too high, lengthen rod to open vanes more for a given duty cycle. If too low shorten the rod.

The logic is to have a rod length that allows maximum VNT control over the useful range required by your set-up, with ~20% left over to give the ECU a chance to control over boost conditions.

Hi mate, I just want to know N75 duty cycle, should I looking for or MAP duty cycle,? as the group 011 show MAP %!!!

Thanks Dani

 

[kiva822]

In your case, I would rig an EMP gauge and have and very clear plan worked out before making any changes to the stop screw.

I have tried to find a post describing what is needed to rig a gauge...if you have a link or the proper search string, that would be grand.

thanks

 

[DanG144]

Yes, N75 duty cycle is MAP duty cycle. Typically shown in MVB 011, field 4 on many VWs.

An emp gauge needs to be rigged onto the exhaust manifold before the turbo. So the EGR take-off flange is usually a good point to start. Drill and tap a temporary block off plate, or you can drill and tap directly into the exhaust manifold.

Be very careful if you screw a steel pipe fitting onto the exhaust manifold, as it is cast iron, you can crack it if you tighten too much. I would use steel fittings, as the temperature is too high for copper or brass to be the first choice.

You can use an electrical gauge or a mechanical one. Copper tubing would work to the gauge, as there is no flow in the line and temperature drops very quickly as you leave the large components bolted to the exhaust manifold.

If my turbo was laggy, and the boost deviation was negative (running lower than requested), I would adjust the rod length so that it started to move with a lower vacuum. This would be to shorten the rod on an ALH or BEW style turbo.

 

[BD76]

Thanks a lot

 

[DanG144]

You are welcome.

I should add that vacuum system problems seem more likely to me. Ensure you get more than 20" of vacuum out of the vacuum pump at idle. Do engine module basic settings test 011 and read the vacuum out of your N75, it should vary from less than 2" to about 18" or more.

Unless it was installed or misadjusted improperly earlier, it is unusual to have make the rod shorter, vacuum movement point earlier. Usually it is the opposite.