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N75J/N75C/MBC Comparison - with graphs

Discussion in 'A3/S3 Forum (8L Chassis)' started by Welly, Aug 2, 2010.

  1. Welly
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    Welly VX220 SC Driver :)

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    [Sep 3, 2010]
    Hmmmm, I've heard various rumours. I believe these are the N75H valve from the S4 relabelled with Awesome's logo. It's much the same as te J valve and encompasses many of the same problems. I.e limp mode caused by overboost on a standard map.
    #81
  2. Madeley
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    Madeley Member VCDS Map User

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    [Sep 3, 2010]
    What about a J valve with a mbc fitted between the tip and n75 to tweak the boost up abit before it over boosts. only a idea..
    #82
  3. Welly
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    Welly VX220 SC Driver :)

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    [Sep 3, 2010]
    Tried that in the past - will bring on limp mode. It will over boost. End of.
    #83
  4. Madeley
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    Madeley Member VCDS Map User

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    [Sep 3, 2010]
    Can you compare the n75c and J reducer sizes?
    #84
  5. S3 Nattie
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    S3 Nattie Quattro ist für sissies

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    [Sep 4, 2010]
    Ive been running a J valve for ages on a standard map and its not once put me in limp mood.

    Nathan
    #85
  6. Danny Cawsey
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    Danny Cawsey Cawsey

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    [Sep 7, 2010]
    my mbc turned up today, could somebody please post up some instructions on where and how to fit it ?
    cheers danny
    #86
  7. s3dave
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    s3dave TFSI Hybrid

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    [Sep 8, 2010]
    Not been funny or anything, but as you dont know where to put it,, do you actually no the risk of fitting this?
    most that fit them on here know a thing or two of how it works....just dont want it to end in disaster for you danny....:thumbsup:
    #87
  8. s3_kev
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    s3_kev Stage2'd!!!!!!

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    [Sep 8, 2010]
    Have to agree here, seen all to many cars over my time where people who dont no what there doing make a hash of it leaving it to myself and my work mates to fix..
    i took mine off as it was no good caused over boost and limp mode anyway so unless your patient enough to set it up or can have it setup/mapped accordigly it may not be suitable anyway.
    just dont go silly with boost as you may end up bending rods
    #88
  9. Speedy Steve
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    Speedy Steve Active Member

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    [Sep 8, 2010]
    Can i use my G/S to claim my BHP? hehe
    #89
  10. Welly
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    Welly VX220 SC Driver :)

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    [Sep 8, 2010]
    Yep, do you know the conversion equation?
    #90
  11. Danny Cawsey
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    Danny Cawsey Cawsey

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    [Sep 8, 2010]
    alright guys i fitted it today after work on its own and the n75 electronically connected, before i had fitted it i was getting 1.5 bar in 5th gear and droping to 1.25

    i wound it right out and went up 2 clicks at a time till i got to 41 clicks which went into limp mode was reading 1.3 bar in 5th gear and dropping to 1.1

    so ive now wound it back 2 clicks and it is reading 1.2 bar and droping to 1 bar but it holds all the way to redline and feels just as strong as the n75 running 1.5

    Any explanations to why it feels better running less boost?
    And what triggers limp mode?
    cheers Danny
    #91
  12. s3dave
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    s3dave TFSI Hybrid

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    [Sep 8, 2010]
    did you plug the ends of the n75?
    overboost will have caused the limp, if it deviates to far from requested boost you will hit limp.....
    you say it feels just as strong, it is better in what way ....smoother?
    Last edited: Sep 8, 2010
    #92
  13. Welly
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    Welly VX220 SC Driver :)

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    [Sep 9, 2010]
    Firstly, what N75 valve are we talking?

    Normally the N75 will give you a good spike, and then fade away quite quickly. The reason being that the map that's on the car controls it. You can see what's going on by looking at the N75 duty cycle on a log.

    You need to understand how the N75 actually works for openers. Boost controller - Wikipedia, the free encyclopedia & What the N75 valve does... - SEAT Cupra.net - SEAT Forum.

    When trying achieve max boost the duty cycle will be at 95% - this means the N75 is effectively closed. As the boost gets close to requested, this will drop down to the 70s and then down further. This means that the valve is opening and is allowing the boost to fade away as it opens the wastegate on the turbo via the actuator.

    An MBC doesn't suffer from this. The ball and spring mechanism on the MBC will not open untill the pre set boost level is reached, and will keep this boost level for as long as the turbo can produce it. As the pressure drops because the MBC is open, the ball & spring will close the valve again, allowing boost to come back to the preset level. Good info here: Intro to Boost Control

    Limp mode, as you know is a self protective safe mode effectively. Brought on by many things, boost being one of them. The engines have a MAP (Manifold Absolute Pressure) sensor - so the ECU knows what the boost pressure in the charge system is. The ECU therefore can compare what boost it asked for, and what boost it has. If there is a difference between the two of more than 250 millibars, it will assume there is a problem and hit limp mode.

    Also - if fitting a manual boost controller, it is usually worth doing the N249 bypass. This is another way that the ECU is able to vent boost from the charge system - it opens the DV at the ECU's request.

    Even with the N249 disconnected, the ECU can still have you though. I have no N75 and no N249. This means that boost is completely beyond the ECU's control. However, being the clever little sod it is, when (and I do occasionally on the MBC) hit limp mode, it cuts the spark to the coil on cylinder 1 to stop it being driven...

    Little git.
    #93
  14. Danny Cawsey
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    Danny Cawsey Cawsey

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    [Sep 9, 2010]
    Cheers for all of that juicy info its a great help, its the standard n75 that i was running.
    right ive been out for a long drive today testing.
    On the n75 as i stated on the last post it was boosting at 1.4 in 5th gear,
    now im running the MBC and can only get 1.2 untill it goes into limp mode but the dv is a lot louder and the the turbo sounds loads lauder then it did running 1.4 on the n75. which doesn't make sense, so the only thing i can come up with is
    that the turbo is actually producing 1.4+ and i have a boost leek in the inter-cooler pipe as i have relocated my dv to the cold side and my boost gauge is plumbed in to the dv vac pipe, so the pressure on the gauge would be the boost im achieving at the inlet mani hope that all made sense to you.

    Im going to change the MBC back to the n75 tomorrow and see if i still get 1.4 in 5th that should tell me if i have a leek.

    if anyone has anymore theory's please share them.

    danny
    #94

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