Some info from Geoff @ Full Race that I thought was useful.
If you go bigger on the compressor that is the same thing as going smaller on the turbine. I put up a bunch of posts explaining the U/C ratio of Compressor OD / Turbine Wheel OD in that big road racing turbo thread... basically divide the compressor OD by the turbine OD - an optimal match will be in the neighborhood of 1.09-1.18 whereas a mismatch turbo will be 1.18+... Also keep in mind that a small a/r will have low turbine efficiency at low flow rates, and higher turbine efficiency at higher flow rates. Conversely the large a/r will have high turbine efficiency at low flow rates and lower at high flow rates...
6blade compressor wheel -- ideal match ratio of ~1.14-1.24
7blade compressor wheel -- ideal match ratio of ~1.09-1.18
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