Everyone seems to grasp that a larger exhaust on a turbocharger vehicle will gain faster spool up, increased power and thus faster acceleration. Here is the theory behind the phenomenon: The turbine housing is merely a volume for which exhaust gas (energy) is transmitted from the engine, to the turbine blade, then dispelled into the atmosphere. In order to better understand how this works we need to take a look at pressure. Turbines in general work off of a pressure differential. A pressure differential in layman's terms is the ratio of pressure before the turbine blade, and after the turbine blade. The greater the pre-turbine pressure compared to the post-turbine pressure, the greater the amount of work can be transmitted through the turbine/compressor shaft. This is where some engineering comes into play. Work is defined as the integration of force and displacement, keeping force constant. Again in layman's terms this merely means work is the force exerted on an object while taking into account the change in displacement, or position of the object. In relation to turbines, the greater the pressure differential, the greater the amount of work is created. The greater the amount of work created, the greater the amount of energy transmitted through the turbine, into the compressor through the connecting shaft. To break this down into another small explanation, compressors work off of rpm. The amount of air the compressor is able to “flow”, or merely put the lbs/minute the compressor can flow is dictated by the amount compressor blades, angles of the blades, etc. What the compressor uses to “compress” the air through the inlet is based off of how fast the compressor wheel is spun. The rpm at which the compressor blade has to be spun to “compress” the air varies from turbo to turbo from the different compressor blade characteristics. Another critical aspect of the compressor is the physical size and weight of the blade. The larger the blade, more amount of energy must be transmitted to allow the shaft to spin to the rpm at which the compressor can compress the air. In engineering terms rotating mass is called inertia, so smaller turbos have smaller inertia demands, larger compressors have larger inertia demands. To bring the focus back on the turbine side of things, the increased inertia demands more energy to be supplied from the turbine. The greater the pressure differential discussed above, the greater the amount of work that be be supplied to overcome the inertia effects of the compressor. Now looking at the exhaust, or post turbine the larger the exhaust, the larger the pressure differential can become. The increase in area of the exhaust, gives the exhaust gas much more room to expand. Hot gas has only one goal, to expand as quickly as possible. The goal pre-turbine is to focus the energy into the turbine to carry as much energy as possible. As the exhaust expands the energy dissipates, so the goal post turbine (i.e exhaust) is to have the largest area possible for the gas to expand. Looking at the immediate exit of the turbine housing, the downpipe, the exhaust gas is traveling at a very high rate of speed. The exhaust gas is expanding rapidly, and is in a very turbulence state from being flung from the turbine. At this point having a 3″ down-pipe becomes critical since the exhaust gas is both in a turbulent state, and is expanding. In turbulence, the expansion of an area the turbulence is forced to become more laminar (although this doesn't happen very quickly). Also the increased area allows the gas to expand rapidly, allowing the energy in the exhaust gas to dissipate quickly and letting the pressure created by the exit from the turbine housing to drop. Essentially you are creating a greater pressure drop. Looking at the turbine housing the sizing becomes a critical part in how the pressure differential is created. Take for instance the .48 A/R housing. Changing turbine A/R has many effects. By going to a larger turbine A/R, the turbo comes up on boost at a higher engine speed, the flow capacity of the turbine is increased and less flow is wastegated, there is less engine back pressure, and engine volumetric efficiency is increased resulting in more overall power. The .48 A/R is able to create the pressure differential at a much lower engine rpm, giving the compressor ability to make its maximum rpm speed sooner. As the engine rpm climbs, the pressure differential is lowered due to the physical volume of the housing size becoming a restriction on the post turbine side. As the housing size is increased, it take greater engine rpm speed (greater exhaust energy) to spool up the turbine, but the pressure differential is less effected by the physical volume of the housing. If you are after maximum midrange gains smaller housings are essentially, if top end gains are essential larger housing are essential. Selecting the power band of the engine is essentially dictated through the housing size, and the turbine physical characteristics.
