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Why no twin scrolls?

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  • Why no twin scrolls?

    I've been wondering this for a very long time: why pretty much every turbo commonly used in SyTys is single scroll?

    This doesn't make sense. The exhaust manifolds are divided to start with, all the way to turbo flange, making a twin scroll turbine housing an obvious choice for response and power. And I'm not talking about a small difference, typically at least 300-400rpm earlier boost curve and a good few horses at the top end. Why are so few people doing this? A matter of just choosing whatever is known to work without really thinking about it?

    I'm about to finally ditch the T04 I've been running since the 90's and go with a proper twin scroll, most likely BW EFR7670, IWG and divided T04 twin scroll housing to make the best of the existing manifold and its flow characteristics. Not really aiming at any insane power levels because maximizing streetability and area under the curve are high on my wish list. Just another Vortec head / Crane cam combo, to be mapped on stock ECU running code59.

    All this still baffles me... why are people still running single scroll turbos, or is there something I haven't noticed about these trucks? To many 1320' may be the king, of course, but giving up a crapload of real street response for a few hundred bucks lower pricetag isn't necessarily a good trade in my book.
    Last edited by hq_; 08-28-2018, 02:33 PM. Reason: Typo...

  • #2
    I think you need to consider the firing order with twin scroll turbo. How many twin scrolls have you seen on a V style engine? Just asking.


    • #3
      Thereís very little happening in the SyTy community in the way of groundbreaking performance builds. Those who are pushing the envelope and using BW twin scroll turbos in the Buick world are building mostly 7 and 8 second street cars. Thereís literally nobody here doing that. A handful of Bowtie or LS builds at this point and thatís it.

      Realistically why bother with all the changes necessary to run a twin scroll turbo when youíre running an off the shelf cam? Surely thereís more power to be had in a custom cam than there is between a twin scroll turbo and single scroll at about 1/10th the price.
      Justin "RealFastV6"
      1991 GMC Syclone
      1993 GMC Typhoon


      • #4
        As I understand it. In order to achieve the maximum benefit of a twin scroll turbo you would need to arrange companion cylinders based on the firing order to supply a sequential flow of exhaust to each half of the twin scroll. To do this one would have to combine two cylinders from the left bank combined with one cylinder of the right bank and so on.


        • #5
          Even if the cylinders arent arrange to take full advantage, there is better spool up just having each bank isolated to one side of the scroll. The theory is it has more velocity, and carries the engine pulses all the way to wheel. With an open flange the engine pulses get dampened. On average 300-500 earlier spool just separating sides.
          89Shortbox S-10
          406 cid 10.90@124 N/A
          New build : 406 Twin Turbo e85 BLOW THRU CARB


          • #6
            Twin scrolls are also suppose to have equal length headers and equal exhaust paths which would very difficult to do with our V6 set up. It may be worth the expense and extra work on a build with a bigger turbo that are typically require a bow tie build to take advantage of the air flow, but almost no one is done those. It is not that hard to build a syty with good boost below 3000rpm that makes decent power, good flowing heads, cam, ball bearing turbo and not turbo that is oversize for the build can do it.
            Sy #2661


            • #7
              Mine is a twin scroll and works great.