Originally Posted by Heypacha

I've just put a map sensor from my spare Ghibli engine. It works perfectly, the idle is fine, there's no more smell of gasoline and the stuttering above 3500 is gone.

Originally Posted by foreigneRS

the only difference as stated is that one will measure to 3 bar (absolute pressure = approx. 2 bar above atmospheric pressure) and the other up to 5 bar.

generally, map sensors are supplied with a voltage (say 5V) and then their resistance changes with pressure so that the voltage measured across them varies, say from near enough 0V (0.25V) (very high resistance) at near enough 0 bar absolute (which is a vacuum, so created by the sucking of the engine against a closed throttle) to 5V (4.75V) at 3 or 5 bar (very low resistance).

so for a 3 bar sensor, you have a scale of 5/3 = 1.67 V / bar
and for a 5 bar sensor, you have a scale of 5/5 = 1 V / bar

your ecu is a digital device and needs to convert this device into a digital signal made up of 1's and 0's. this is done by a digital to analogue (d/a) converter, and depending on the ecu, this converter has a certain resolution, e.g. 16 bit. this resolution means how many 1's and 0's the digital number can be made up of. 1 bit can be either 1 or 0, 2 bit can be 00, 01, 10 or 11, etc.

therefore for a 2 bit d/a converter, you can see that the voltage signal can only be divided into 4 numbers so it cannot really see the difference between the 1V (approximately 1 bar on a 5 bar / 5V) and 1.2 bar / V or whatever.

therefore if you are only running 2 bar boost on a 5 bar sensor, the resolution of the d/a converter means what you are trying to measure will not be as accurate as if you were using a 3 bar sensor.

so it should be clear that you should use a sensor appropriate to the boost range that you are running.

having said that, i don't know what the resolution of the d/a converter is in a ford or marelli ecu. if it's 16 bit, you can see that the range can be divided into 2^16 (over 65,000) so it shouldn't make a lot of difference what sensor you use. if it's just 8 bit, this goes down to just 256 (still just 0.02 V or 0.02 bar on a 5 bar sensor which is still good enough). 4 bit is just 16, so 0.3 bar which is not really good enough.

hope this helps. if any mistakes spotted by anyone, please feel free to correct me (values used are not totally accurate, just to give an idea), and voltage values do not necessarily apply to any particular car, just an otline of the principle.