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Tech Tip: Twin Scroll Turbo Housing

When it comes to the turbine housing, you have two options: Volute Open Flow (VOF) and Volute Twin Scroll Flow (VTF). The BorgWarner tech team regularly fields questions about the differences between VOF and VTF as well as if single and twin designs can be crossed. This tech tip focuses on the benefits of VTF, or twin scroll turbo housing.

The Benefits of VTF

With the VTF design, the manifold and turbine housing are divided into two flow paths. This allows the engine firing order to alternate the flow and send the pulse energy directly to the turbine wheel.

VTF-VOF Comparison

Here’s how it works:

  • When the exhaust valve opens, this generates an engine blow-down pulse.
  • During the blow-down, the engine power cylinder is still at a very high pressure as a residual of combustion and the power stroke. This initial “pop” of energy travels at very high speeds down the manifold runner, through the volute, to the wheel.
  • The stream is a “pulsed flow,” and the divided nature of the system amplifies and arranges those pulses.
  • The engine firing order creates a “one-two” (alternating) punch on the wheel, keeping the pulses evenly spaced and in rapid succession.
  • As the engine speed increases, this becomes a blur and the alternating nature loses its value. That’s why twin-scroll housings only benefit the low and medium speed operation of an engine. Within this range, it is a very effective way to improve turbine effectiveness and low RPM response.
VOT vs VTF

The turbine wheel likes the high velocity, evenly-spaced pulses of gas; and as long as they are slow enough in succession and duration, the wheel can make good reaction usage of the energy. What results is an improvement in effective efficiency, and at lower engine speeds more turbine power can be generated. The obvious result is quicker spool and better low-end boost response.

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