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Turbo exhaust back pressure

Hal Adams Aug 8, 2018

  1. Hal Adams

    Hal Adams Active Member Team Nardo VCDS Map User Audi RS3 Black Edition

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    Is anyone here well versed in turbo engine theory? Interesting topic this that I'd like some more background on, but there a multitude of articles across the web, each contradicting the other!

    My understanding has always been that n/a engines need a degree of exhaust back preesure, but install a turbo, the idea is to have as low a back pressure as possible to allow the turbine the least resitance to spinning. It is for this reason I spent a little extra in ordering the Scorpion down pipe that splits into separate pipes at the top as opposed to the Milltek that is a single pipe down to the secondary pipes.

    Had a long discussion on this last night (we had a storm that took the power down and it was pointless letting the beer get warm...) and the 'other' subject of pre turbo back pressure. Checking on google this morning confirmed why this is such a heated subject!
     
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  3. Gnasher

    Gnasher Member

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    From what I'd heard, it's as you say - the lower the backpressure on the exhaust the better but with mass produced OEM turbo's there is still an element of doubt.

    For instance Mk1 Astra VXRs would knacker the turbo seals with a decent exhaust, to the point where someone developed an oil restrictor bolt.
     
  4. Hal Adams

    Hal Adams Active Member Team Nardo VCDS Map User Audi RS3 Black Edition

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    Co-incidentally I was reading some of the woes of the Astra, especially from someone with a stage four conversion by Courtney. They simply put turbo failures down to 'shoddy equipment' although one person did say that people were concentrating on engine power (and manifold velocity) with little thought on getting rid of it.
    I am surprised this forum is not as interested in what makes the cars tick as much as say, the Imprezza guys.
    I certainly find the subject fascinating and recommend this article by a Garrett chap that pretty well sums up the exhaust side of things. Of especial note for those of us with 450+ upgrades - by his reckoning our exhausts are already too small..:blush:
    https://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html
     
    jungle650 likes this.
  5. DW81

    DW81 Well-Known Member

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    I have my exhaust valves unplugged, so would this have a negative effect on things lads?

    PFL car.
     
  6. T-800

    T-800 Well-Known Member Team Glacier TFSI Owners Group VCDS Map User Audi RS3 quattro Sportback S tronic MQB Platform

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    On the FL car unplugging exhaust valves causes a problem at 5000 rpm

    @Beebee-one experienced the ECU reigning the power back in.

    It is more than likely when you pull the connections and just tape up or if you fit the terminal delete plugs while the canbus is still live, then fault codes are generated and these cause the ECU to reign in things because it thinks there's an issue with the flaps.

    Best thing to do if you want no issues buy a AVC controller, this has been proven to not cause issues at higher rev ranges because no errors are generated.
     
    DW81 likes this.
  7. Hal Adams

    Hal Adams Active Member Team Nardo VCDS Map User Audi RS3 Black Edition

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    In theory and in the low to mid range, closed flaps are creating a backpressure which is my point - most of us are de-catting and plugging the flaps generally for sound, whereas there is a lot of technical reasons for doing these things. But I agree, whilst you appear to be helping the exhaust, I'd get a controler designed for the job.

    The problem is, if you read into the theory in depth, as T800 has pointed out, things change throughout the range and it might be necessary in some cases to add a little back pressure - I don't know exactly. It is complicated and all to do with the correct balance (or imbalance) before and after the turbine. That is why I'd be interested in a turbo design expert to chime in and explain the detail...
     
    DW81 likes this.
  8. vinnysS4

    vinnysS4 New Member

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    The theory behind back pressure is for exhaust pulse and resonant tuning. Your exhaust is exiting and creating exhaust pulses that can hit each other (OR restrictions such as cats, bends etc..) when they "bounce" back, it can disrupt exhaust flow. If done right it can help to evacuate the cylinder gasses after combustion and help get the exhaust moving.
    On a turbo car, you want as little restrictions as possible both in and out. One use for an x pipe on a v motor is to help with exhaust scavenging. One exhaust pulse helps pull the opposite banks exhaust gasses along, hence speeding up evacuation of exhaust pulses.
    Depending on your LSA (lobe separation angle) and how you have your cam timing set, you can help dial in the right amount of timing to compliment the exhaust pulses.


    Sent from my SM-N920V using Tapatalk
     
    penetrator and AudiNutta like this.
  9. Hal Adams

    Hal Adams Active Member Team Nardo VCDS Map User Audi RS3 Black Edition

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    A few interesting reads out there on LVA - one US Chevvy enthusiast even had bespoke cams made at 108 degrees to gain a little extra over the standard Chevvy 111 degrees...

    Now, catalysers. What makes a freeflow sports cat? I have the Scorpion sports cat secondary pipes, and looking down them through the cats, looks very similar to the standard Audi ones that they replaced! Is this all a big con, or is there a subtle difference..?
     
  10. Gnasher

    Gnasher Member

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    Sports cats are normally more free flowing by having a lower number of cells per inch - normally 100/200 CPI whereas standard are normally around 400 CPI.
     

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