Excellent.Dougal.... I think you know the answer! :beer: It is where the exhaust inlet opens into the full volute. The 16cm2 H1C housing has an A/R of 2.34 cm2/cm.. On that turbine housing the radial distance from the center of the housing to the center of the critical area of the volute opening is approximately 6.7cm
The 60mm exducer (small diameter) is a very common H1C turbine wheel for sure...there are others. That specific exducer size (small diameter) is generally machined with a cast inducer (large diameter) of 70mm for an H1C.Excellent.
Garrett measure their A/R in inches, so 2.34cm = 0.92".
If we can confirm (or deny) this, I'll try to sticky it.
Do H1C's have 60mm turbine wheels?