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You cannot be comparing (turbine) A/R's from different family or turbos.
A 1.00 A/R T3 is going to be smaller than a 0.8x A/R T4 (numbers are not necessarily real), even though numerically the T3 is "bigger".
The Hitachi does not directly compare to any of the Garrett family, so you really cannot be comparing them.
Also, the HT-18 2S designed for the FC3S 13BT uses a different type of turbine wheel which will change it's efficiency.
Almost all the other (aftermarket) turbos are designed for piston engines with greater than or equal to at least 4 cylinders of combustion, but the Hitachi turbos are designed for the stronger pulse of a rotary engine.
If you're looking for a starting point for an aftermarket turbo, try around 0.9x for a good overall power band.
Up to 1.00 (or more) for more top end.
Drop to 0.8x for more response on the low end (but sacraficing top end).
Keep in mind that these numbers are strictly for (Garrett) T4 family size of turbos.
As a rule, compressor wheel dictates maximum power potential; turbine (A/R) section dictates whether the power will be low, middle, or high in the RPM band.
-Ted
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