starting from evo 4 turbo..
twin-scroll turbo kept exhaust pulse energy high all the way to the turbine wheel, improving boost response so effectively that the size of the turbine housing was stepped up (from 7 cm2 to 9 cm2). Coupled with a new cam grind, the larger turbine housing promoted better volumetric efficiency at higher engine speeds, while the twin-scroll took care of the low end
evo 6....
Though Mitsubishi did not claim an increase in engine output for Evolution VI, its power delivery improved in RS by using a titanium-aluminide turbine wheel, a world's first in a production car. With lower density than a traditional Inconel turbine wheel, this intermetallic alloy reduced rotational inertia, improving boost response. However, titanium-aluminide is more brittle than Inconel, and to avoid potential durability problems the titanium-aluminide turbo was not available in the street-biased GSR model.
evo 7...
a smaller twin-scroll turbine housing (reduced from 10.5 to 9.8 cm2). Paired with a 20mm larger intercooler and a lower-backpressure exhaust, the sauce was spread over a broader range of engine speeds than ever before. Overall, the VII was an even more potent performer-if a more civilized one-than the VI.
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.
The exhaust-gas-driven turbine is surrounded by a twin-scroll housing that helps direct and maintain exhaust gas velocity over the turbine wheel at lower speeds in order to improve response in the low-to-mid rpm range. In addition, a lubricated shaft connects the turbine to a compressor wheel, whose impeller helps build boost earlier, which reduces lag and improves response across the range.
