I presume you are fitting a 3 bar sensor to a car that has a 2.5 bar sensor . The 3 bar sensor would under read by 20% .
No point in increasing the boost if you aren't increasing the fuelling. All you are likely to do is wear out the turbo , increase fuel consumption and get underboost fault codes.
From what I've read up on it increases by 20% not decrease, due to the lower voltage reading.
Not trying to increase boost. I'm currently building my engine and want to put a few miles on it in stock tune before I start mapping it. Due to the location of the map sensor, its easier for me to fit the 3 bar map sensor now than take the front of the car off again.
All I need to know is if there will be any big problems.
Yes that is what I said the 3 bar sensor under reads by 20%, so effectively the ECU will be trying to provide 20% more boost than it is already.
A typical phase 1 map may increase boost by 5-10% max. A 20% increase is big.
To increase boost the ECU will keep the VNT vanes closed up more, this increases back pressure in the exhaust a lot and impacts fuel economy, the turbine will be spinning a lot faster than it needs to and you are likely to get limp mode due to underboost or possible CEL for out of range as the sensor will be reporting a vacuum at idle and low speed which is never the case in a diesel.
Typically a 20% overboost will cause the car to go into limp mode to protect the turbo.
Better to change the sensor over at the point where it's mapped. Or have the absolute minimum amount of running on the incorrect sensor by changing it shortly before it is mapped.