Second off - The GM-8 is a definite improvement over the GM-4 on both ends.
Decreased EBP post-turbine, pre-turbine, and increased compressor efficiency are resulted.
The wastegate assy in the GM-4 turbine 'snail' was relocated to the exhaust elbow in the -8.
The reduced 'snail' length allowed opening up the exhaust adaptor elbow by removing the 180deg fold-back
Also opened up the wastegate exit path, allowed freer wastegated exhaust flow.
Result : lower EBP in the elbow, post-turbine, increased flow thru wastegate, and reduced EBP in the turbine.
Not insignificant, when combined with a freer flowing exhaust system and upgraded PCM.
A radial, or centrifugal, compressor housing usually consists of two parts - the scroll housing, and a plate to seal the open side, which is also attached to the center section containing bearings, wheels, and drive shaft..
The plate acts as part of a diffuser, which creates pressure by reducing the gas velocity after it slings out of the high-speed impeller wheel..
The GM-4 is of the parallel-wall diffuser type, meaning the scroll is cast in two pieces, with an integral vertical separator, which will be parallel to the main sealing plate, but spaced apart from it at a dimension depending on designed flow rate and pressure ratio
The -4 compressor has a multi-stepped inducer throat area, and is limited to a more specific flowrate range by the dimensions of and between the parallel walls.
The -8 is of the scroll-diffuser type, where the shape of the scroll acts as the diffuser, and which, with the inducer throat steps removed and smoothed, is more efficient over a broader range of engine flowrates.
The inducer throat area is where linear flow into the compressor begins to whirl, in anticipation of becoming radial flow into and out of the spinning wheel.
Any turbulence here, as caused by the stepped -4 surfaces, can mean easier 'stall', where the compressor wheel, spinning faster than inlet air flow rate, begins to cavitate.
Turbine rpm surges as load decreases, and - well....exploding turbochargers are not generally considered spectator sport.
A compressor wheel and a turbine wheel may be used in various sized housings for various flow rates and pressure ratios, where even the shape of the housing will raise or lower the expected output.
Or, various size housings may be used with same wheels to alter turbocharger output, as required, desired, and inspired..
So - it may be seen that the GM-8 is of improved efficiency over the -4, even with the same size and shape wheels.
Remember - science, without reason, makes no sense.
