I have this option on my 1997 uk turbo but when it is selected, it just comes up blank, should this be reading anything?

 

There's not an ACT sensor fitted as std on the Impreza, so that pinout is not connected to anything, hence the <blank> on the Commander.

You can fit an aftermarket ACT sensor and wire it to the correct pinout in the loom, though....

 

I not long ago, got an ACT gauge from Harvey, which is in the TMIC, so I take it that, that would go in the inlet pipe?

 

You can position it anywhere in the inlet trunking, post-turbo.

For a more accurate readout, then as close to the TB/IM as possible. Some people strap it around/glue it into the IC-TB short hose...