A guide to twin scroll turbos

"The twin scroll is one of the latest turbocharger innovations."

Twin scroll turbos are relatively new innovations on the performance car tuning scene.

People have used twin turbos in the past to address many of the problems you hit with a single turbo and the twin scroll turbo offers a neat drop in upgrade and provides many advantages.

So, what is a turbo? You can read our comprehensive guide if you want more information than this simple definition.

The turbo takes the exhaust flow and uses this to pull fresh air into the engine, using two turbines connected via a central shaft.

The faster the engine spins, the greater the flow of exhaust gases and the more air can be compressed and pulled into the engine.

What is a twin scroll turbo

Instead of taking the entire exhaust flow, a twin scroll turbo is fed by half of the engines cylinders.

Every other cylinder is fed into one half of the turbo intake. This means that the exhaust design should provide equal length runners, and be optimised for a matching flow rate.

Taking the common 4 cylinders 1-3-4-2 firing order, you'd pull 2 & 3 into one half of the scroll and 1 & 4 into the other. When you have more cylinders you continue with every other odd firing cylinder.

 

Our top tip on setting up a twin scroll turbo system is to make sure that the exhaust pipe lengths/flow rates are balanced. Equal length does not always equal flow.

In 5 cylinder engines or those with an odd number you'd hit a number of technical problems and negate many of the benefits of a twin scroll turbo.

Instead of the exhaust hitting the turbine in the same place, a twin scroll turbo divides the flow into each half* of the turbine, making it smoother and reducing the pulse effect you get with a typical exhaust design.

*One part, the smaller channel is angled directly towards the turbines inner surfaces at the leading edge, which helps it spool up more quickly. The other half is set at a more acute angle to the turbine pushing more force into the turbines rotation.

The two angles cause the turbine to spool up faster improving the low end response whilst also giving a boost to the top end power.

The narrow channels flow much faster than a single large channel and this adds to the overall efficiency and output of the turbocharger.

The advantages of twin scroll turbos

  • The boost response is faster coming on much sooner
  • Fuel consumption is better (upto 5%)
  • Anecdotal evidence suggests they are more reliable
  • Takes up less space than a comparable twin turbo system
  • More bottom end torque and faster smoother spooling up
  • Better power throughout the rev range
  • More efficient, throughout the rev range (upto 8% more efficient)
  • They overcome the unequal exhaust lengths typically from a single turbo setup

As each cylinders valves open and close you have a Scavenging effect, where the newly emptied cylinder creates a vacuum ready to pull in fresh air for the next cycle.

The twin scroll setup maximises the benefits of this effectively allowing more air to enter each cylinder causing more negative pressure between each intake phase.

In a twin scroll setup you minimise the losses you would get as the cylinders are separated into odd firing pairs.

Another way to achieve this would be by adding a second turbo, but this is costly and complex and requires that the power outputs of each turbo are matched if run in parallel or optimised in a series configuration.

Twin scroll turbos are a relatively new innovation, with modern machining and manufacturing methods it makes it possible to produce these cheaply and there are many drop in options where one is looking to upgrade a standard single turbo.

Please note that equal length exhaust headers may not flow at the same rate, you need to take into account the bends and hopefully avoid any seams or imperfections.

When adding a better turbo you could always fit a larger twin scroll turbo and maximise your performance gains.

Expect to replace your exhaust headers and rework the intake pipe routes even when you buy a "hybrid" drop in replacement twin scroll turbo.

To maximise your power gains you should get it mapped by a specialist, there is hidden power if you know how to get it out.

TorqueCars suggest that you join the friendly tuning forum and get some specific tips and advice for turbo charging your car model - we now have a dedicated turbo tuning forum.

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