So here is my attempt to explain what a twin scroll turbo is, and what twin scroll headers are.
I won't go too much into the science behind it, because there are plenty of great youtube videos that explain it incredibly well. Here is a good one:
However I will go into some detail on the subject. For more info checkout these links:
Turbos have a housing that covers the compressor wheel (the compressor housing) and the turbine wheel (the turbine housing). A typical turbine housing has one inlet for the exhaust gases to enter the housing and then reach the turbine wheel. This is the "snail" looking part of the turbine housing, which can also be referred to as the "scroll". Just to be clear, you can refer to the turbine housing part that the exhaust gas travels through to reach the turbine wheel as the "scroll" part of the housing.
A twin scroll turbo is a turbo that has the turbine housing divided into two separate sections for the incoming exhaust gases to travel through (or you could say it has two separate "scrolls" for the exhaust gases to travel through to reach the turbine wheel). This means that there are two separate passageways for the exhaust gases to enter into the turbine housing. Please see the pic below that shows how the inlet to the housing is divided in the center:
This is what a twin scroll turbo looks like:
Once the air enters the turbine housing on each side, it then travels around the housing and the two passageways (or two scrolls) meet at the turbine wheel, and the exhaust gases from each side of the housing converge at the wheel and then spin the turbo. Here are some photos showing how the turbine housing is divided in the center:
In this photo you can see the tiny light pointing to a raised "lip" inside the housing. This is the divider that separates each scroll, or passageway, that the exhaust gases travel through once they enter the turbine housing:
You can see that raised "lip" in this photo as well. If you look closely you can see that below the lip there is a shadow from the light. This is the passageway / scroll, for one part of the exhaust gases to travel through. Above that lip is the passageway / scroll for the other exhaust gas to travel through.
Now onto Twin Scroll Headers...
Twin Scroll headers split up the exhaust gases coming from the engine so that each side, or bank, of the engine has the exhaust gases coming from it completely separated from the other. Meaning that the driver sides exhaust gases are completely separate from the passenger side exhaust gases. This means that the crossover pipe coming from the driver's side of the engine never
meets up with the passenger side header and it remains completely separate. The crossover pipe coming from the driver's side of the engine goes all the way up to the turbo mounting flange, which is divided into two areas, just like the turbo's twin scroll housing. Here are some pics to show you how the crossover pipe from the driver's side goes all the way to the turbo flange. The pipe that goes to the top part of the flange is the pipe that the crossover pipe from the driver's side connects to:
In this photo (sorry it is blurry) the pipe on the right side is the pipe that connects to the crossover pipe coming from the driver's side of the engine:
A clear shot of how the pipes go into each side of the divided turbo mount flange:
Picture showing how the header piping is completely separate and the two pipes are independent of each other :
So why go twin scroll?
I will only briefly touch on this because as I stated above, there are many videos on youtube or links that describe the benefits very well.
The main benefits of twin scroll turbos/headers can be understood better by understanding how a non-twin scroll setup functions. In a header system that has the driver side exhaust gases meeting up with the passenger side exhaust gases (like the stock GN headers) you have all the exhaust gases mixing together and traveling in the same pathway. Now our engines fire the cylinders in a manner that one side of the engine is on the power stroke, where the other side of the engine is on the exhaust stroke, pushing out the exhaust gases into the header system.
Let's say for the sake of this example, that cylinder 1 (driver's side front cylinder) is on the exhaust stroke and pushes out the air into the headers. Now right after that let's say cylinder number 4 is just finishing up the exhaust stroke and right about to start the intake stroke, and the cylinder and camshaft have a small amount of overlap happening. (I understand this may not be the actual firing order, but this is simply to illustrate a point). So right now the exhaust gases from cylinder 1 are traveling through the headers and going through the crossover and meeting up with the passenger side collector and going up towards the turbo mount flange. At the same time cylinder number 4 is finishing the exhaust stroke and beginning the intake stroke. At this point for cylinder number 4 because it is at the moment of overlap and both the intake and exhaust valves are open, it creates a low-pressure system in the header piping.
Since high pressure always wants to find the path of least resistance, which would be a low-pressure system in this case, the exhaust gases from cylinder 1 will want to travel up the header primary and actually travel into cylinder 4's cylinder. Yes, you read that correctly - the exhaust gases coming from cylinder 1, as they travel by cylinder 4 (passenger side middle cylinder) in its period of overlap, will actually want to travel up the primary tube and go into
the cylinder for cylinder 4. This means that you are getting exhaust gases going into the cylinder, taking up space inside the cylinder which means you have less space inside the cylinder to fill it with fresh cold intake air. This means now your engine displacement has just been reduced because the cylinder isn't completely full of cold air-it now is partially full of hot exhaust gases. You also lose part of your effective compression ratio since you aren't compressing only cold air.
So basically you are losing displacement, compression ratio, power, and increasing the risk of detonation because of the hot exhaust gases entering the cylinder and raising the effective air temperature inside the cylinder.
With a twin scroll configuration this is completely avoided because the exhaust gases from each side of the engine never
meet. You will have no issues with exhaust scavenging or exhaust gases from a cylinder contaminating the intake charge of another cylinder that is along the pathway the exhaust gases are traveling.
I would go into more detail, but I would really recommend watching that youtube video I posted at the beginning of the post because the video has great drawings and he goes into more technical details describing the benefits.
Hopefully this helps you understand a basic understanding of what it means when we say "twin scroll" turbos or "twin scroll" headers!