Maybe you're getting those readings while the motor is cranking.
It's because I'm now running a BMW 2.5 bar MAP sensor instead of the stock Triumph 1 bar
As it turns out, it's more like a 2.6 bar BMW MAP sensor and the stock Triumph a 1.2 bar I think.
I was seekng 15 - 16 psi on the stock one so even when going into boost the stock one was able to read low level of boost not just 0 - 1000hPa (1 bar) max.
Say for example here the barometer which is almost identical afaik to the stock MAP sensor, is seeing 1023hPa but transmitting only 4.12V.
Based on this, one could calculate the stock MAP to be good for over 1200hPa or 1.2 bar at 5.0V.
Also in the above you can see the new MAP is transmitting 2.00V to the ECU, so if the barometric is accurate at 1023hPa, the BMW MAP will read up near to 2.6 bar at 5.0V.
Anywho if you do the calcs, idling at around 640hPa with a stock MAP will read 640 on screen because the ECU parameters that we cannot see make it so (its programmed for a ~1.2 bar MAP so 4.12V means 1023hPa), and when you switch it out to a ~2.5 bar MAP the voltage drops and the ECU thinks the MAP is seeing 340hPa instead, and displays that.
So without a voltage correction gizmo my displayed MAP values will be way off actual but i reckon that's ok as long as i compensate for it in the map tables.