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Exhaust manifold sizing for VQ turbo application

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  • Exhaust manifold sizing for VQ turbo application

    OK folks, here we go..., VQ30DET, EFR-7163 twin scroll turbo in a front mount configuration. My preferences is to use equal-length headers, connected to 2.5" down pipes from each bank, tying into a CXRacing twin scroll adapter fitting that also serves as the Turbo mounting function . Typical NA manifolds are 16 gauge (.065") which is a bit thin. I found some 2mm thick 304ss headers for a V35DE that were 2mm thick 304ss (.080"). I purchased them, cut the flanges off and turned them around. After cutting one of the collectors off to shorten that side, I could see that the welds were made without purging and there are several joints with severely sugared welds inside that I can't get to with a grinder so I won't be using them.

    Next step (making headers) -

    The VQ30 exhaust valves are 2x31mm. The exhaust ports are 31mmx44mm, which back-calculates to the equivalent of 38mm(1.5") ID round tube.

    1-1/4" 304ss iron pipe size fittings are 1.65"(42mm) OD, .105"(2.67mm) wall and 1.442(36.6mm) ID.

    1.75" 304ss 11 gauge tubing is .120"(3mm) wall and 1.51" (38.3mm) ID.

    Which of these choices (or other choices) would you recommend?
    Don Johnson (really!)
    Just so you know.

  • #2
    I wouldn't think you'd hurt power using 1.25" sch 10, but that would need everything really well supported. Even then I might think about 1.5" pipe sch 40 for the extra strength.

    2.5" upstream of the turbo on each bank sounds really large, and I'm not sure how you're going to really squish that down into a T4 flange which probably has less area than 2x 2" tubes. Maybe 2.25" pipes trying into the adapter? That's where most your volume difference will be, not in the shorty manifold part.
    '18 Chevrolet Volt - Electric fun hatch for DD duty!


    DefSport Koni Sleeve and Spring Perch Buy!!!
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    • #3
      Gotcha. I've been having second thoughts about the 2.5" already. It's equal area of (3) 1.5" pipes, but it doesn't need that because the 3 runners aren't flowing continuously. The schedule 10 is much thicker than typical 16 gauge and isn't supporting the turbo. I've got a pair of VQ37VHR OEM headers that are about the same ID as the 1.25" schedule 10, but they can get away with it due to the variable lift and timing and I'm shooting for more power than the VQ37, which means more mass flow.
      Don Johnson (really!)
      Just so you know.

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      • #4
        You don't need more area though - the pressure in the turbo manifold is higher than in an NA header, so it will flow more massflow at the same pressure drop.

        Look at the size of the ports in any stock turbo manifold - it illustrates this perfectly.
        '18 Chevrolet Volt - Electric fun hatch for DD duty!


        DefSport Koni Sleeve and Spring Perch Buy!!!
        http://www.nissanroadracing.com/showthread.php?t=5902

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        • #5
          Gotcha. the VQ firing order is 1/2/3/4/5/6, 120 crank degrees apart so each side will see exhaust events at 240 degree intervals. I'll tighten up the design.
          Don Johnson (really!)
          Just so you know.

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