Author Topic: VNT Turbo Vane Position at Various Stages of Boost  (Read 15487 times)

November 19, 2017, 03:59:34 pm

beichMTDI

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VNT Turbo Vane Position at Various Stages of Boost
« on: November 19, 2017, 03:59:34 pm »
So I am currently in the process of designing a "mechanical VNT control" for my MTDI build. There's a lot of great information on different mechanisms that people have built to do this here on the forum. My question though isn't so much about the mechanical mechanism I'm building but rather what I'm trying to "mirror" from the ECU ran (N75) VNT controlled turbos. So here's my 2 simple questions as it relates to a stock computer controlled TDI ALH:

1st: When at IDLE, are the vanes closed or open? My personal opinion is that they are closed (boosting) because as you apply vacuum to the actuator the vanes close and if you ever lost vacuum for some reason you would want the turbo to default to the vanes open position (for safety reasons). I have heard people say that manuals idle closed and automatics idle open, I have also heard people say they all idle closed (hence the nice high pitch whistle when idling) and other say they all idle open (why have the unnecessary back pressure when no boost required?). So which is it?

2nd: Say you 'floor it' from at rest (full throttle at 0 boost). The vanes close (or remain closed if the answer to the above question is 'closed at idle') and boost begins to build. Do the vanes stay COMPLETELY CLOSED until max boost is reached or do they gradually open as the boost climbs to prevent excess/unnecessary back pressure??? Said in another way: As the boost is building towards max boost, does the N75 proportionately start bleeding off vacuum or does it maintain full vacuum until max boost is achieved and then drops vacuum to open vanes?

I've done a ton of reading but can't seem to find a clear answer to these two questions. If anyone could lend some facts it would make me sleep a lot better at night.

Thanks fellas!
Brad


1982 Mk1 4 door 1.6 IDI 4 speed, all stock, daily driver.

1981 Mk1 2 door project car soon to be MTDI (ALH)
Build Blog: https://beichmtdi.wordpress.com/

Reply #1November 20, 2017, 12:43:49 pm

vanbcguy

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Re: VNT Turbo Vane Position at Various Stages of Boost
« Reply #1 on: November 20, 2017, 12:43:49 pm »
I have been doing a lot of tuning on my standalone VNT controller lately... Some things I've learned:

- the turbo will often spool much faster with the vanes part way open - fully closed vanes restrict exhaust flow preventing the engine from fully evacuating itself. Less is often more.

- at WOT you really don't need the vanes closed much at all to build lots of boost - I can easily make 30 PSI with the vanes between 50% closed around 2k down to less than 30% closed above 4k

- you virtually never want the vanes fully closed over 1600 RPM, especially not over 60% accelerator

- I personally have the vanes mostly open at idle, I like the way the car sounds better. There might be a slight increase in lag from a full stop but my car will just spin the tires if I try and go full pedal from idle anyhow. I definitely see my EGTs come down faster at idle with the vanes open vs closed.

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Bryn

1994 Jetta - AHU M-TDI - Jezebel Jetta
2004 Jetta Wagon - 1.8T - Blitzen

Reply #2November 21, 2017, 07:26:09 am

beichMTDI

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Re: VNT Turbo Vane Position at Various Stages of Boost
« Reply #2 on: November 21, 2017, 07:26:09 am »
Vanbcguy, thanks for the good thoughts. What your saying makes total sense. I'm assuming then that in your system your vanes never "fully close"? Good point about the EGT's coming down faster with open vanes, I had not considered this. I guess optimally the best scenario would be a vane controller that proportionately opened the vanes from a starting position (some form of 'closed') at the same rate as the climbing boost. This would alleviate unwanted back pressure and help balance this with the wanted fast spool time. My current design (from a 0psi boost position) closes the vanes and keeps them closed until the desired boost is reached (this being roughly determined by throttle position). I'm thinking I may want to change this so I don't get a situation where vanes completely close while above the mentioned 1600 RPM.

I'm still curious what a stock ECU does related to our discussion. I do happen to know someone that has a completely stock manual '03 Jetta TDI ALH. I may give him a ring and get him to drive over the pit and stomp on his accelerator a couple times while I'm up underneath with a good view of the turbo actuator. 
1982 Mk1 4 door 1.6 IDI 4 speed, all stock, daily driver.

1981 Mk1 2 door project car soon to be MTDI (ALH)
Build Blog: https://beichmtdi.wordpress.com/

Reply #3November 21, 2017, 07:13:53 pm

vanbcguy

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Re: VNT Turbo Vane Position at Various Stages of Boost
« Reply #3 on: November 21, 2017, 07:13:53 pm »
They fully close with my setup at low RPM and low accelerator input... I should grab a picture of my current N75 map, it would probably be the easiest... I have a copy of a stock map too, it has a very similar shape to mine though I didn't find it until long after I'd worked mine out through many iterations of testing.

I still am continuing to work on my map; I have the most common driving range (less than 50% accelerator, RPMs between 1600-3200) pretty solid now. I still have some work to do in the upper RPM ranges and at full accelerator pedal, weather has been too crappy to do much of that sort of driving lately.

Sent from my LG-H873 using Tapatalk

Bryn

1994 Jetta - AHU M-TDI - Jezebel Jetta
2004 Jetta Wagon - 1.8T - Blitzen

Reply #4November 22, 2017, 01:15:29 am

vanbcguy

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Re: VNT Turbo Vane Position at Various Stages of Boost
« Reply #4 on: November 22, 2017, 01:15:29 am »
My current N75 map is below - each band represents 10% more accelerator pedal than the previous one. I have the vanes quite open at zero go pedal or the turbo isn't very happy during shifts at high boost. While it looks fairly flat the vanes open as RPM increases even when all other factors are unchanged.

You can see how much the vanes open around 1600 RPM... They're virtually never fully closed except just off of idle.

Sent from my LG-H873 using Tapatalk

Bryn

1994 Jetta - AHU M-TDI - Jezebel Jetta
2004 Jetta Wagon - 1.8T - Blitzen

Reply #5November 22, 2017, 01:19:37 am

vanbcguy

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Re: VNT Turbo Vane Position at Various Stages of Boost
« Reply #5 on: November 22, 2017, 01:19:37 am »
Here's the actual numbers - I'm quite actively tuning at the moment but these are getting close... 204 = 80% duty cycle on the N75 valve = vanes fully closed. Any value below 60 is vanes fully open - they don't start to move until about a 20-25% duty cycle.

Sent from my LG-H873 using Tapatalk

Bryn

1994 Jetta - AHU M-TDI - Jezebel Jetta
2004 Jetta Wagon - 1.8T - Blitzen

Reply #6November 22, 2017, 08:40:13 am

beichMTDI

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Re: VNT Turbo Vane Position at Various Stages of Boost
« Reply #6 on: November 22, 2017, 08:40:13 am »
Well I'm not the most familiar with reading these MAPS but I can say from the limited experience I do have it looks like you are well on your way! Especially in your cruising range. I'll have to take some time and dwell on these a bit. Lots of good info crammed into those charts. Thanks for sharing! Is your manual VNT controller more of a "libby type" mechanical system (ie. springs, rods, and cams) or do you have more of vacuum/pressure system? I'm trying to design mine with as few adjustments as possible (without compromising ability to properly adjust of course). Otherwise I'll be tinkering with the darn thing for the rest of my life!!
1982 Mk1 4 door 1.6 IDI 4 speed, all stock, daily driver.

1981 Mk1 2 door project car soon to be MTDI (ALH)
Build Blog: https://beichmtdi.wordpress.com/

Reply #7February 10, 2019, 09:45:14 pm

Timothycrock

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VNT Turbo Vane Position at Various Stages of Boost
« Reply #7 on: February 10, 2019, 09:45:14 pm »
I still dont know what that turbo is. Trim numbers dont mean much without knowing what family of turbo it is because trim is just a ratio between the inducer and exducer diameters of a wheel.

If you know what family of turbo it is, trim numbers mean something because USUALLY the exducer diameter is the same and you can work backwards from that.

So is it something like a t3 or t4?  A lot of turbos are t3 turbine and t4 compressor "hybrids" so it could be something like that as well.

 

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