| I'm talking about the effective throat area into the turbine wheel's periphery, not the area of the opening just aft of the flange. I'm guilty of mixing/matching some terminlogy here - in the gas turbine world, the throat area is the effective area for the gases to pass through just before entering the turbine. It specifies what the density and velocity of a known volume and temperature of gas will have as it enters into the turbine. Important information to have when calculating the design of the nozzle guide vane system in a gas turbine, which is responsible for controlling the gases into the turbine to optimize energy extraction to power the compressor within its most efficient range of operation. A lot of my understanding of the thermo involved in turbomachinery has come from my tinkering with building gas turbine engines so I apologize for mixing some of these terms up. :) In a scroll housing, the impingement angle of the gasflow into the turbine varies as the A/R changes, which is what affects the flowrate of the housing (and ultimately the density and velocity of the gases that impinge on the turbine). A small A/R housing decreases in area much quicker than a large A/R housing and this causes the gases to impinge at a lesser angle into the turbine wheel blade tips (closer to tangent) while a larger A/R housing maintains a larger area per radius, which causes the gases to enter at a greater angle (from tangent). What really changes the flowrate is the fact you are changing the angle the air is entering into the turbine blade tips. In reference to the .86 A/R housing on the GT2860RS turbo, the easiest way to say it is the housing is not guiding the gases into the turbine at an angle that creates the required velocity and density to transfer the energy into the turbine that the compressor requires to reach higher boost pressures. Even the .64 A/R housing is too large for the 60mm compressor, which is why that housing doesn't allow the compressor to produce max boost. If they made a .54 A/R housing of for this turbo, it would likely be a great match for our engines - allowing the best of boost response, boost peak, and power/torque. I'm sure you have seen the variable area turbine housings that use pivoting vanes to change the impingement angle as well as the throat area (throat area being the area the gases pass through right at the turbine's periphery). Something like that on our cars would certainly be of great interest. :)
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