whats a twin scroll
17 November 2005, 09:02 PM
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i found this...
What are twin scroll or divided inlet turbine housings? What are the benefits of using a twin scroll turbo?
A "twin scroll" or "divided inlet" means that there are two separate volutes within the turbine housing. The main reason for doing this is to isolate the pulses coming from each exhaust port and maintain more of the pulse energy from each cylinder all the way down to the turbine wheel. There are no differences between the turbine wheels used in open or single inlet turbines compared to those used in twin or divided inlet turbines.
Generally speaking, a divided inlet turbine setup will respond faster and produce boost quicker than single or open design of the same nozzle area, of course this is dependent upon proper execution. The simple fact that a divided housing is used does not guarantee these results.
While it does not cause any problems or harm to run a divided inlet turbine housing on a manifold that is an "open" design, none of the benefits of the twin inlet will be seen.
What are twin scroll or divided inlet turbine housings? What are the benefits of using a twin scroll turbo?
A "twin scroll" or "divided inlet" means that there are two separate volutes within the turbine housing. The main reason for doing this is to isolate the pulses coming from each exhaust port and maintain more of the pulse energy from each cylinder all the way down to the turbine wheel. There are no differences between the turbine wheels used in open or single inlet turbines compared to those used in twin or divided inlet turbines.
Generally speaking, a divided inlet turbine setup will respond faster and produce boost quicker than single or open design of the same nozzle area, of course this is dependent upon proper execution. The simple fact that a divided housing is used does not guarantee these results.
While it does not cause any problems or harm to run a divided inlet turbine housing on a manifold that is an "open" design, none of the benefits of the twin inlet will be seen.
17 November 2005, 10:49 PM
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The twin scroll turbo/equal length 4:2 header setup increases turbo efficiency over the single scroll/ unequal length 4:1 header used by most Subarus in a number of different ways.
The hotside of the turbo has two separate gas inlets. The exhaust manifolds, instead of being combined into a single up-pipe, are combined into two, with the twin up-pipes keeping the gas from adjacent (in firing order) cylinders separate all the way up into the turbo, where the twin scroll exhaust housing separates them all the way onto the turbine blades.
The first advantage is that the equal length design of the headers evens out the time intervals with which the exhaust gas pulses hit the turbine blades. This imparts more energy into the turbo, which helps promote quicker spool.
The second advantage is that the separation of adjacent gas pulses in the headers and turbo prevents something called reversal, which is where part of the gas pulse takes a "wrong turn" in the normal manifold, and starts travelling back towards the other cylinders instead of to the turbo, where it can collide with the next gas pulse, reducing the energy available to drive the turbine round in the process. By virtually eliminating this, you're again getting more energy into the turbine blades rather than wasting it.
The increases in turbine efficiency mean a smaller one can be used for any given size of compressor and desired flow rate, and the reduction in size leads to a reduction in rotating mass, which again helps spool and response.
Another happy advantage of the lack of reversal is that the knock-on effects can improve fuel efficiency, so it's an improvement all round.
The hotside of the turbo has two separate gas inlets. The exhaust manifolds, instead of being combined into a single up-pipe, are combined into two, with the twin up-pipes keeping the gas from adjacent (in firing order) cylinders separate all the way up into the turbo, where the twin scroll exhaust housing separates them all the way onto the turbine blades.
The first advantage is that the equal length design of the headers evens out the time intervals with which the exhaust gas pulses hit the turbine blades. This imparts more energy into the turbo, which helps promote quicker spool.
The second advantage is that the separation of adjacent gas pulses in the headers and turbo prevents something called reversal, which is where part of the gas pulse takes a "wrong turn" in the normal manifold, and starts travelling back towards the other cylinders instead of to the turbo, where it can collide with the next gas pulse, reducing the energy available to drive the turbine round in the process. By virtually eliminating this, you're again getting more energy into the turbine blades rather than wasting it.
The increases in turbine efficiency mean a smaller one can be used for any given size of compressor and desired flow rate, and the reduction in size leads to a reduction in rotating mass, which again helps spool and response.
Another happy advantage of the lack of reversal is that the knock-on effects can improve fuel efficiency, so it's an improvement all round.
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