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.

They have been around for a while, but we are starting to see mainstream adaptation and twin scroll turbo upgrades available for many popular cars.

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 engine's 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.

In the turbo above you only have one intake channel (top right) and all exhaust pipes feed into this one zones.

In the twin scroll below you'll see that the engine exhaust are halved and separately channelled into the exhaust side of the turbo.

This gives a faster spool up, partly because the exhaust gases will be moving faster in this narrow chamber.

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 shape and design of the turbine blades will be setup for maximum advantage of this.)

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

The way a twin scroll turbo flows from alternating cylinders helps with scavenging in the engine and is another benefit to fitting one.

Please watch our video which explains turbos and all you need to know about them. Be sure to subscribe and support our new channel.

How do the exhausts flow into a twin scroll Turbo?

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.

So as the first cylinder finishes it's expansion stroke the valve opens on the exhaust ports, the second cylinder will have it's exhaust valves open as they overlap and it comes to the end of it's exhaust flow, and this gas pressure will interfere with the exhaust flow rates from the engine with two different speeds/pulses meeting.

A twin scroll completely negates this and gives the maximum flow rates from the engine.

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.

(Although we should note that on some engines the power output is smoother when you use unequal length exhaust headers.)

Can a twin scroll turbo be used in all engines?

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.

Engines that use a cylinder on demand where part of the engine shuts down will also create a set of challenges to installing a twin scroll turbo.

The RPM range of the engine concerned and your power output needs will determine if a twin scroll turbo gives the best solution.

How does a twin scroll turbo compare with a twin turbo?

Interestingly on the 335i BMW ditched a twin turbo setup for a twin scroll single turbo, and power output is very similar.

The twin turbo setup generally gives more top end power and when tuned will give better gains, but both turbos work really well with the twin scroll turbo giving a much faster spool up and better low end power.

Are Twin Scroll Turbos Better

They take the pulses from the engine and turn the turbine efficiently, whereas the single input turbo mixes these streams together so the flow speeds are reduced. This reduction in flow rate will lead to a slower spool up time generating lag.

So for a fast spool up and greater range of power the twin scroll turbos are better, but for large power gains where the bottom end is not important a big turbo with single inlet is preferable in most cases.

The Advantages of Twin Scroll Turbos

  1. The boost response is faster coming on much sooner
  2. Fuel consumption is better ( up to 5%)
  3. Anecdotal evidence suggests they are more reliable
  4. Takes up less space than a comparable twin turbo system
  5. More bottom end torque and faster smoother spooling up
  6. Better power throughout the rev range
  7. More efficient, throughout the rev range ( up to 8% more efficient)
  8. 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 maximizes 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 minimize 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 optimized in a series configuration.

FAQ Guide to tuning a car with a twin scroll turbo

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.

You can run more valve overlap with a twin scroll turbo system which further improves the scavenging effect.

Fuel economy gains of 5% and turbine efficiency of 7% are common when switching to a twin scroll turbo.

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

Twin scroll turbos will need to be mapped when fitted to your car. Even if they are similar to your OEM turbo they work quite differently and this needs adjustment to the ECU to fully realise your power gains and benefits from this mod.

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

You can use a twin scroll turbo manifold on a single inlet turbo in most cases but why you'd do this is a mystery, unless you were planning to fit a twin scroll at a later date.

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.

Further reading - in depth articles about turbos.

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.

Check out my YouTube channel, we're regularly adding new content...


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3 Responses to “Twin scroll turbos”

  1. TCJBOLDIE says:

    IMO there is almost no point to fitting a TS Turbo on a non equal length manifold as the exhaust pulses may end up hitting where they meet instead of following smoothly as a equal length manifold is designed to keep the pulses from hitting/interfering/colliding with with each other preventing the smooth flow of exhaust gasses to the turbine wheel

  2. Craig Harroff says:

    So, its a “relatively new innovation”? This “innovation” was described in Corky Bells’ book, Maximum Boost copywright 1997. It was not “new” then either. LOL. Good article though, Thanks.

  3. TorqueCars says:

    Yes I doubt there is anything actually NEW when you look into it. They are now available for most cars, just a few years ago it was a specialist custom build upgrade. Now most models of car have a twin scroll option in the upgrade library.

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