I've always wondered what a smaller compressor cover does to the compressor map. I've searched but haven't been able to find the answer (could be using the wrong terms). For instance take GT4718 (88) and put the smaller Precision cover on it (PT8847), or take a T76 and put a smaller cover on it (TC76). I know you will loose overall flow but does it shift the entire map to the left? I was thinking about picking up an older PT88 (not the PT8847) with the small compressor cover for a new project, but I'm worried about the surge line. It has a nasty dip on the left side of the map. If the smaller housing pushes the map left I would be fine.
Thanks Mike

 

There is a lot more to it than overall diameter of the housing but in general you can expect that the map will probably shift down reducing the maximum boost it can safely generate without overspeeding. It also will likely drop a bit of efficiency and change the surge line (perhaps for the better). A good portion of the pressure recovery is done in the diffuser (section between the tip of the wheel and the volute) so by reducing it's diameter you loose ability to slow the high velocity air leaving the wheel. It is not uncommon for as much as 50% of the pressure rise to occur in the diffuser. Some of this can be offset by managing the width of the diffuser and through any pinch it may or may not have.

Steve