You beat me to it. 
How does an LSx PCM measure MAP?
How does an LSx PCM measure MAP? I am becoming increasingly more interested in tuning, so I am poking around the tuning section and I see "MAP" mentioned alot. I thought that a vehicle with a MAF sensor did not use a MAP sensor and vice versa? I also see that FI cars are going to a MAP system, is this because the MAF is not as accurate under boost conditions?
Thanks!!!
Thanks!!!
TECH Senior Member
Joined: Nov 2001
Posts: 7,036
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From: Lake Zurich, IL
I'm not sure of the dynamics on how it measures it exactly... but basically it's just a sensor in the intake manifold that measures vacuum
at WOT with full engine fill at sea level 102kpa or so is considered you're cramming in the most air you possibly can into the engine w/out creating a boosted scenario
With boost you then run into 2 and 3 bar (1 bar is 100% vacuum, 0kpa , no cylinder fill at all, to the 102 or so value where it's 100% fill, so it's -14.7psi to 0psi)
THey measure above the 0psi (point at which you have full cylinder fill but no boost) ... so it is basically reading the pressure in the intake manifold.
Regarding FI and MAP, the biggest problem with the MAF is that in stock format, it can only read so much airflow... the PCM only goes up to a certain value of valid hz readings, so on an LS1 for example, around 7-8psi of boost (so around 150kpa) it can no longer compensate, the MAF is maxing out on how much air it can meter
With the MAP sensor and a custom OS (if using the stock PCM) you can then increase fueling based solely on MAP (so if you're running 14psi of boost, you'd want to have a 2 bar MAP sensor and a 2 bar OS) (not solely on the MAP, but the MAP in this case is used instead of MAF)
On the LS1, reverting to using the MAP is a failover mode (speed density) ... on 93 f-body's for example, they didn't come with a MAF and relied purely on the MAP... then in 94 GM added the MAF but set a failover (in case the MAF goes bad, don't want to blow the engine or anything) to using the MAP
at WOT with full engine fill at sea level 102kpa or so is considered you're cramming in the most air you possibly can into the engine w/out creating a boosted scenario
With boost you then run into 2 and 3 bar (1 bar is 100% vacuum, 0kpa , no cylinder fill at all, to the 102 or so value where it's 100% fill, so it's -14.7psi to 0psi)
THey measure above the 0psi (point at which you have full cylinder fill but no boost) ... so it is basically reading the pressure in the intake manifold.
Regarding FI and MAP, the biggest problem with the MAF is that in stock format, it can only read so much airflow... the PCM only goes up to a certain value of valid hz readings, so on an LS1 for example, around 7-8psi of boost (so around 150kpa) it can no longer compensate, the MAF is maxing out on how much air it can meter
With the MAP sensor and a custom OS (if using the stock PCM) you can then increase fueling based solely on MAP (so if you're running 14psi of boost, you'd want to have a 2 bar MAP sensor and a 2 bar OS) (not solely on the MAP, but the MAP in this case is used instead of MAF)
On the LS1, reverting to using the MAP is a failover mode (speed density) ... on 93 f-body's for example, they didn't come with a MAF and relied purely on the MAP... then in 94 GM added the MAF but set a failover (in case the MAF goes bad, don't want to blow the engine or anything) to using the MAP
So the stock MAF, can handle up to approx. 7-8Lbs of boost regardless of the tuning? At that point it reaches its limit and no amount of tuning can compensate correct?
I would assume that you would have to add a MAP sensor to the engine to convert over to that system as the stock MAP(which I still wanna find) would only be capable of measuring vacuum and not preasure
I would assume that you would have to add a MAP sensor to the engine to convert over to that system as the stock MAP(which I still wanna find) would only be capable of measuring vacuum and not preasure
Originally Posted by LiENUS
Map is also used in MAF mode for setting the timing, timing is typically set by rpm vs map.
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TECH Senior Member
Joined: Nov 2001
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From: Lake Zurich, IL
Originally Posted by zapp168
So the stock MAF, can handle up to approx. 7-8Lbs of boost regardless of the tuning? At that point it reaches its limit and no amount of tuning can compensate correct?
I would assume that you would have to add a MAP sensor to the engine to convert over to that system as the stock MAP(which I still wanna find) would only be capable of measuring vacuum and not preasure
I would assume that you would have to add a MAP sensor to the engine to convert over to that system as the stock MAP(which I still wanna find) would only be capable of measuring vacuum and not preasure
well... even stock it still measures pressure... just negative pressure to 0
7-8lbs is the approximate point at which the MAF maxes... however there are work arounds, but they involve adding lots of fuel at the max point, or running dual MAFs (Harlan's the only one I'm aware of running this setup)
once you start making serious boost it's easier to just switch over to a custom OS and be done with it... and yes you'd need to get a 2 or 3 bar MAP sensor and replace your current MAP sensor (the custom OS's for the PCMs take care of the conversions in the PCM)
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...and don't waste time with a 2bar if you ever see boosting to 15psi. A custom OS is not absolutely necessary but in my case running EFILive there are a lot of features that make yanking the MAF and running SD a no brainer.
In addition to being a fail-over, the MAP is used in addition to the MAF in airmass calculation, largely occuring in throttle transitions. For simple proof of this, disable dynamic airflow calculation (B0120 in EFILive) by reducing it to 1rpm. This will cause all airmass calculations (fuel and spark depend on this) to be pulled solely from the MAF Calibration. While idling, quickly stab the throttle. If you have a wideband hooked up, you'll see it go insanely rich and the engine will stumble. Re-enable dynamic airflow calculation and repeat the experiment. You'll see that the MAP has the effect of calming down the MAF's "Pump-shot" effect in throttle transitions. This pump shot can be seen by logging a cylair calculated by the MAF and by logging the actual GM PID DYNCYLAIR. You may see a max MAF derived cylair of 1.2+ on a stock LS1, yet DYNCYLAIR will be a much more expected .7xg/cyl level.
Originally Posted by dc_justin
In addition to being a fail-over, the MAP is used in addition to the MAF in airmass calculation, largely occuring in throttle transitions. For simple proof of this, disable dynamic airflow calculation (B0120 in EFILive) by reducing it to 1rpm. This will cause all airmass calculations (fuel and spark depend on this) to be pulled solely from the MAF Calibration. While idling, quickly stab the throttle. If you have a wideband hooked up, you'll see it go insanely rich and the engine will stumble. Re-enable dynamic airflow calculation and repeat the experiment. You'll see that the MAP has the effect of calming down the MAF's "Pump-shot" effect in throttle transitions. This pump shot can be seen by logging a cylair calculated by the MAF and by logging the actual GM PID DYNCYLAIR. You may see a max MAF derived cylair of 1.2+ on a stock LS1, yet DYNCYLAIR will be a much more expected .7xg/cyl level.
