I believe the PT61 uses the same compressor wheel as the SC61 used in the Precision Sport compact line. (According to Precision's website, they are the same compressor).
When I inquired to Precision over a year ago for a map of the S61, they informed me it was the Garrett GT35R (a.k.a. GT40) wheel and faxed me this map.
This same map has since been publsihed as the following (much clearer map!)