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Stock valvetrain vs high backpressure

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  • Stock valvetrain vs high backpressure

    Hi, i curious if somebody notieced any problems with high rpm boost on mid sized turbo/hybrid - small sized turbine side and 7A or similar higher lift cams. I feel problems over 6000rpm when my GT2871r, .63 T3 housing boosting over 1.3bar (400bhp+ power on 2.5 stroker, 10mm lift solid lifter cams). I notieced power curve starts to drop here on cca half off dyno pulls during tuning , also on streets is instantly notiecable. I have similar problem on another non audi car, and its caused due high backpressure in manifold and exhaust valves starts to float. I reduced boost around 1.2bar and feels smooth eg more power on top now.
    Sounds real?


  • #2
    What size impeller on cold and hotside? Most likely hotside is too small for high rpm on higher boost.
    1990 2.2 20VT Audi Coupe Quattro
    *Hx35/k26, MaxxECU, Wagner EM/IC *RS2 Recaro *4pot-Brembo *3" turboback *Treser *Koni/H&R
    1990 Audi 200 20v Avant 2.2 20VT *7 Seater *Full electric gizmos

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    • #3
      Isn't a GT2871 with a 0.63 hot side a little small for a stroker engine??
      sigpic
      1991 Audi S2 3B - 2.5 Stroker engine
      1997 A6 2.5TDi quattro avant - C4
      1976 RD250E / 350LC cafe racer

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      • #4
        It classic GT28 turbine so 53.8/47mm. On paper it can sound small, but also garrett note on paper that is for 475ps(56trim). I reused it from past 2.2t engine.
        Asking about real life, turbo/turbine can flow more, but on this engine makes so much backpressure which OEM valvetrain can withstant, probably. Just my clue

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        • #5
          What are you trying to achieve as you have stroker engine and 7a cams but your using gt2871 turbo .63 housing

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          • #6
            Im trying nothing, i know that engine potential is wasted, but expected widest possible powercurve peaking around 450bhp.

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            • #7
              Based on the calculations I did for my engine, you're off the chart with a GT28! Get a bigger turbo
              sigpic
              1991 Audi S2 3B - 2.5 Stroker engine
              1997 A6 2.5TDi quattro avant - C4
              1976 RD250E / 350LC cafe racer

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              • #8
                change housing size

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                • #9
                  Would Allso say go the biggest ar you Can get a 63 seems very small for a 2.5

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                  • #10
                    Originally posted by AndyP View Post
                    Based on the calculations I did for my engine, you're off the chart with a GT28! Get a bigger turbo
                    Do some calcs and see where you are on the chart, as Andy says you will find you are off past the choke line.
                    Garrett's quoted figures are ambitious. If you want 475ps you really need a bigger turbo.
                    There are of course many (too many?) options!
                    Panthero Coupé quattro 20vt
                    Indigo ABY coupé
                    Imola B6 S4 Avant

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                    • #11
                      Hi, i feel it teoreticaly the same, i will never buy GT28 turbo for over 400bhp on 2.5ltr engine, but its all teoretical talk as always on forums. This is my practical experience and i search others practical findings.
                      Andy posted compressor map calc (which i dont trust anyway), but there is no problem on this side in real. Check the GT3071r map, its same, because its same wheel even in same housing. And GT3071r is proven to make over 450hp on S2, isnt?
                      What i want to point, if i will put stronger valve springs, i can reach more power with still unchanged turbo, yes i know it sounds nonsense.
                      And repeating im going to do nothing, im just after others experience, if its same as mine.

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                      • #12
                        The compressor has more headroom, but you are getting excessive exhaust manifold pressure because your hotside/turbine is too small for the air mass you are trying to move
                        Panthero Coupé quattro 20vt
                        Indigo ABY coupé
                        Imola B6 S4 Avant

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                        • #13
                          I will answer improprietly by a question, how do you know it that is too small?
                          If we look on turbineflow chart, my turbine housing is T3-KKK .63 which can compare to standart GT28 .086 , that flows19.5lb at PR2.3 . GT3071 with .63 flow 20.5lb at PR2.6 . I dont quite understand this chart, but peak flow seems similar.

                          I did not measured manifold pressure(i noted before, its my feeling, based limited real life experience and some teoretics), maybe i have different problem somewhere ...

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                          • #14
                            I would bet your engine setup eats your turbo

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                            • #15
                              Originally posted by curamrdan View Post
                              Hi, i feel it teoreticaly the same, i will never buy GT28 turbo for over 400bhp on 2.5ltr engine, but its all teoretical talk as always on forums. This is my practical experience and i search others practical findings.
                              Andy posted compressor map calc (which i dont trust anyway), but there is no problem on this side in real. Check the GT3071r map, its same, because its same wheel even in same housing. And GT3071r is proven to make over 450hp on S2, isnt?
                              What i want to point, if i will put stronger valve springs, i can reach more power with still unchanged turbo, yes i know it sounds nonsense.
                              And repeating im going to do nothing, im just after others experience, if its same as mine.
                              The GT3071r makes 420 real hp on the S2 and nothing more.
                              Go Holset or Go RS4

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