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3-bar_resolution [2015/03/10 11:30]
shelgame [3-Bar (and larger) MAP sensor resolution issues]
3-bar_resolution [2015/03/10 11:31] (current)
shelgame
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 It's often stated that 3-bar (and higher) MAP sensors can have issues with '​resolution'​. Maybe you're wondering what that is all about. It's often stated that 3-bar (and higher) MAP sensors can have issues with '​resolution'​. Maybe you're wondering what that is all about.
  
-The resolution issue is related to the limitations of an 8-bit computer. The analog 0-5v MAP signal is digitized by the ECU into 8-bits. That gives 256 '​steps'​ for each 5v signal. So, if you have a 2-bar MAP, that is 29.4psi (total) divided into 256 steps, gives 0.115psi per step. This is the smallest increment of MAP change the computer can '​see'​.+The resolution issue is related to the limitations of an 8-bit computer. The analog 0-5v MAP signal is digitized by the ECU into 8-bits. That gives 256 '​steps'​ for each 5v signal. So, if you have a 2-bar MAP, that is 29.4psi (total) divided into 256 steps, gives //**0.115psi per step**//. This is the smallest increment of MAP change the computer can '​see'​.
  
-With a 3-bar MAP, that smallest step is larger. 44.1psi / 256 = 0.172psi per step. The concern is really in vacuum, not boost. As this change in step size (or resolution),​ means that small vacuum changes can be missed; resulting in fueling errors and drive-ability problems. This effect can be worse for engines with '​big'​ cams and porting work. The resolution issue can usually be '​fixed'​ by making small adjustments in the O2 feedback values so that the cell limits are not near the idle vacuum for your engine.+With a 3-bar MAP, that smallest step is larger. 44.1psi / 256 = //**0.172psi per step**//. The concern is really in vacuum, not boost. As this change in step size (or resolution),​ means that small vacuum changes can be missed; resulting in fueling errors and drive-ability problems. This effect can be worse for engines with '​big'​ cams and porting work. The resolution issue can usually be '​fixed'​ by making small adjustments in the O2 feedback values so that the cell limits are not near the idle vacuum for your engine.