Can anyone explain what the difference between these two exhaust housings? Is one better than the other? Is one a better setup for a particular combination?

 

Quoted from Maximun Boost- Turbocharger Systems:

Split-Inlet Exhaust Housing
A split-inlet exhaust housing permits the exhaust pulses to be grouped (or separated) by cylinder all the way to the turbine. The merit of doing this is in keeping the individual package of energy, an exhaust putt, intact and unmolested by other putts all the way to the turbine. This can give the turbine a little better kick to get it moving. When you consider the absolute barrage of pulses and energy coming down the tube from an eight-cylinder engine, the
Fig. 3-13. The effect of varying the A/R ratio, all other factors remaining constant.
Chapter 3, page 34


turbine will get more energy than it needs for almost any given situation. Thus, a split housing will make zip for improvement on a single-turbo V-8. A four-cylinder, by comparison, which sees only one putt every 180° of crank rotation, needs all the energy it can get from each pulse. Keeping them separate and undisturbed will therefore pay some dividends.

Fig. 3-14. The split-inlet exhaust housing theoretically offers a small performance advantage by keeping exhaust pulses in a tight bundle all the way to the turbine. This is more effective for engines with fewer cylinders, and thus fewer pulses, per engine cycle.
Chapter 3, page 34

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.