You'll love this all ya Nerds:

Revenge of the Nerds

Lessons in Turbine Housings

By Mike Kojima

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There are basically two types of turbine housing configurations, tangential and on center. Tangential housings have the turbine inlet coming into the housing at a tangent to the turbine wheels. On center housings have the turbine inlet coming directly into the center of the turbine wheels axis. A tangential turbine housing is greatly more efficient as the gas path feeding the turbine has a straighter shot into the housing and a better impingement angle on the wheel. The gas entering an on center housing must go around a kink in the housing, which causes turbine efficiency to suffer by up to 10%. There are studies that show that a 2% gain in turbine efficiency can offset gains of up to 25% of the turbines inertia so a 10% loss in efficiency is quite significant. However the on center mounting can often give a bit of mounting flexibility to solve a difficult fit problem but because of the efficiency losses it is better to try to find almost any other solution to a packaging problem.


Another interesting aspect of turbine housing design is divided and undivided housings. A divided housing is exactly how it sounds, the scroll of the turbine housing is split in two. A savvy tuner can use a divided housing to his advantage on an engine with few numbers of cylinders. A divided housing works best on a 4-cylinder engine with some advantages on a 6 cylinder with a properly designed manifold. When a divided housing is used, usually cylinders 1 and 4 are fed into one side of the scroll and cylinders 2 and 3 are fed into the other side. The cylinders fed into each side of the scroll are as far apart in the firing order as possible. This allows the turbine to be hit with 4 distinct pulses as the engine goes through its firing order. This improves turbine efficiency, sometimes to the point where up to one size larger A/R housing with it’s attendant lower backpressure can be used, either that or less turbo lag can be enjoyed with the same size A/R housing. The divided housing can also improve volumetric efficiency by making reversion from adjacent in firing order cylinders much more difficult. This is because there is a great deal of separation in degrees of crankshaft rotation between the valve opening events of the adjacent cylinders. In order for a reversion pulse to contaminate an adjacent firing cylinder, it has to travel back through the spinning turbine blades and up the other side of the divided turbine-housing scroll to get to the adjacent cylinder. This is pretty difficult and the pulse will tend to take the path of least resistance, past the turbine to the area of lower pressure.

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http://www.nissanperformancemag.com/march06/nerds/