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Command vs Actual boost ??

Discussion in 'A3/S3 Forum (8L Chassis)' started by Essflee, Feb 20, 2011.

  1. Sandip
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    Sandip Well-Known Member

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    [Mar 17, 2011]
    Interesting, once mine is mapped i'll do some logs of the N75 connected at the chargepipe and another connected at the inlet


    Dave i know what you mean, i wonder if it makes thing more accurate or less accurate???
    #41
  2. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Mar 17, 2011]
    Very interesting. This is the first ive heard of taking the reading from the inlet.
    #42
  3. Prawn
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    Prawn My other car is a MINI!!!!

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    [Mar 18, 2011]
    doesn't sound like a good idea to me. For more accurate boost control you want to N75 take off as close to the turbo as possible. You'll get different spool and boost characteristics on part throttle I'd have thought, but I wouldn't expect too much difference when you're flat out.

    What's the idea behind it? I can see the theory if the N75 worked like an MBC just cracking at a given pressure, you'd be negating the pressure drop within the system. But seeing as the N75 is simply controlled by what it sees at the map sensor and airflow, I don't see how it'll make any difference
    #43
  4. <tuffty/>
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    <tuffty/> Badger 5 Edition Staff Member Moderator

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    [Mar 18, 2011]
    Makes no sense to me... the ECU applies a duty cycle to the N75 based on the reading from the MAP sensor to meet the requested boost in the map... got shag all to do where you take the N75 feed from...

    <tuffty/>
    #44
  5. gez
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    gez S3 hunter

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    [Mar 18, 2011]
    Of course it does! So why does it take a boost feed from the system atall then??? Its basicallu an electronic bleed valve. Gets to set boost then opens and bleeds the rest back to the tip insted of to atmosphere.
    If you have n75 connected to charge pipe, why have your boost gauge on the manifold, there both getting different readings.
    #45
  6. Welly
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    Welly VX220 SC Driver :)

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    [Mar 18, 2011]
    You need pressurised air to open the actuator.... that's what the N75 actually regulates. There is no pressure sensor in the N75 valve at all - it simply opens and closes, and allows pressurised air into either the actuator pipe to open the actuator, or vents it back into the TIP as it's not needed.

    The MAP sensor is what sees the pressure, and the ECU opens the valve to accomodate. The physical pressure that the N75 valve sees is neither here nor there!

    The actuator pressure is about 6 PSI. It doesn't matter whether you get the boost source from the inlet manifold, or the charge pipe, it is going to be enough to do the job of opening the actuator.

    I'd suggest doing some reading on the subject before dictating to people what things do and don't do, when you obviously don't know yourself.
    Last edited: Mar 18, 2011
    #46
  7. Welly
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    Welly VX220 SC Driver :)

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    [Mar 18, 2011]
    Sorry for any who are now confused by all this - and the complete and utter waffle that has been posted.
    #47
  8. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Mar 18, 2011]
    Not confused, but interested.
    #48
  9. Welly
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    Welly VX220 SC Driver :)

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    [Mar 18, 2011]
    If anyone wants to give it a try, then go for it.

    The only difference it will make is a bad one. The less pressure the N75 gives the actuator (i.e like taking the feed from a source with less pressure - going from charge pipe (higher pressure) to inlet manifold (less pressure)) at the point of there being a need to open the wastegate will result in more of a boost spike.

    If you are changing the setup so that the N75 feeds the actuator 21 PSI at the manifold, instead of (for example) 28 PSI at the charge pipe, what is going to be the result?

    It doesn't take much to figure out that the result is more sluggish actuator (and thus boost) control, due to the reduction in pressure to force the actuator open. The more pressure available, the quicker the actuator will open the wastegate.

    And that is pretty much it in a nutshell. End of.
    #49
  10. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Mar 18, 2011]
    I was wondering why In 2 and a half years i'd not heard one person mention anything about plumbing in the N75 differently.
    #50
  11. <tuffty/>
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    <tuffty/> Badger 5 Edition Staff Member Moderator

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    [Mar 18, 2011]
    Probably because VAG spent eleventy billion quid in R&D putting it where it currently is... :)

    <tuffty/>
    #51
  12. superkarl
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    superkarl MAN OF STEEL

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    [Mar 18, 2011]
    I'm going plug my n75 into my anus, thus giving a better reading of what's going on in the engine when I fart. Similar theory
    #52
  13. Welly
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    Welly VX220 SC Driver :)

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    [Mar 18, 2011]
    :applaus:

    The more you fart, the less boost you'll have....
    #53
  14. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Mar 18, 2011]
    haha, pics!!!!
    #54
  15. Welly
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    Welly VX220 SC Driver :)

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    [Mar 18, 2011]
    Ummmm, I'm unsubscribing from this thread..... :undwech:
    #55
  16. Sandip
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    Sandip Well-Known Member

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    [Mar 18, 2011]
    We'll have to do some logs to compare:fart:
    #56
  17. superkarl
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    superkarl MAN OF STEEL

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    [Mar 18, 2011]
    Westle I had up most respect for you until now, wishing to see a hose going from my anus to my n75 valve. That's perverted car porn, like necrofilia is to conventional porn! Dirrrrty dirrrty bastar......
    #57
  18. superkarl
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    superkarl MAN OF STEEL

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    [Mar 18, 2011]
    Sandip if we were to do logs it would block the pipe!
    #58
  19. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Mar 18, 2011]
    haha, this thread has taken a big turn for the worse me thinks!
    #59
  20. superkarl
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    superkarl MAN OF STEEL

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    [Mar 18, 2011]
    Sooooooo, leggy, your car is ok in the end. Excellent! Happy motoring.
    #60
  21. Essflee
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    Essflee Well-Known Member

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  22. s3dave
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    s3dave TFSI Hybrid Supporter

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    [Mar 18, 2011]
    Let him get on with it, i say......
    #62
  23. Biggied
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    Biggied They called him mellow yellow.

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    [Mar 18, 2011]
    Well done Welly, a good explination there, if anyone still has any doubts over this then I suggest reading up on the subject a bit more & don't post anymore arguments towards the statment as you really don't understand whats being said.

    Ewan.
    #63
  24. S3 Nattie
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    S3 Nattie Quattro ist für sissies

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    [Mar 18, 2011]
    haha you guys slowing go off the topic, Karl and Westle..... funny guys lol

    I was talking to a guy I know about this few weeks back about taking the feed for the N75 from the manifold, was interesting when you think about it.

    Nice info though Welly, although it would still be interesting if sandip still did some logging on this.
    Last edited: Mar 18, 2011
    #64
  25. Essflee
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    Essflee Well-Known Member

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    [Mar 18, 2011]
    Right...

    Was confused as to why liquid gauge appeared to be showing different results to vcds, so tonight I've tried some more runs in 3rd 4th using the liquid S3 to see what request vs actual I was getting.

    Strangely now everything seems to be fine, the liquid gauge was showing the same results as yesterday with the vcds, a good thing obviously :)

    I've read that if you put a new N75 on a car it can take a while for it to adjust to the car, how true this is I don't know but the day after having the new N75 fitted I tried the liquid gauge and it was showing to much actual boost...

    In short really, whatever the reason I'd like to withdraw my comment on taking the liquid gauge results with a pinch of salt because it's now reading spot on.
    #65
  26. Broken Byzan
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    Broken Byzan Photographic Moderator Staff Member Moderator VCDS Map User

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    [Mar 18, 2011]
    Because there is no gain to be had. The N75 just regulates the pressure that is sent to the wastegate to open it. It has no idea what the pressure is, it doesn't care either,lol
    #66
  27. badger5
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    badger5 www.badger5.co.uk Site Sponsor

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    [Mar 19, 2011]

    Its *ALWAYS* been reading it as it is...... Its feeding off the same ecu data

    the inconsistency is your logging and comparisons, different gears, different throttle, different N75, different days etc etc etc etc etc ..

    What does it read on the 5th/6th gear 2krpm full throttle application? >2550mb
    #67
  28. Essflee
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    Essflee Well-Known Member

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    [Mar 19, 2011]
    Bill, just tried 5th and 6th from 2000rpm and was seeing a gradual increase in boost as the revs built up peaking at aprox 2440mb so all seems well, no harsh spike or readings exceeding the map sensor...
    #68
  29. badger5
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    badger5 www.badger5.co.uk Site Sponsor

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    [Mar 20, 2011]
    Excellent
    Excellent news
    #69

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