How does it "know" elevation/density?
Thread Starter
Team Owner
Joined: Aug 2004
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From: Park City Utah
How does it "know" elevation/density?
In the speed density cars, (mine specifically is a '92) how does it know that the elevation changes while driving?
I understand (was taught in automotive school 20 years ago) that the MAP sensor takes a quick "snap shot" of manifold pressure (ambient air pressure) as the key rotates past "on" to "start" and gets it's Baro reading in that instant. I was further taught that any WOT event would cause the ECM to reset the BARO. Maybe.
But what happens when you cruise from 6000' elevation to 10000' elevation, at part throttle? There is no WAY that the MAP would ever see atmospheric pressure during that event, so how can it "know" that the air pressure has dropped? Or increase, when going down?
I understand (was taught in automotive school 20 years ago) that the MAP sensor takes a quick "snap shot" of manifold pressure (ambient air pressure) as the key rotates past "on" to "start" and gets it's Baro reading in that instant. I was further taught that any WOT event would cause the ECM to reset the BARO. Maybe.
But what happens when you cruise from 6000' elevation to 10000' elevation, at part throttle? There is no WAY that the MAP would ever see atmospheric pressure during that event, so how can it "know" that the air pressure has dropped? Or increase, when going down?
Safety Car
Joined: Sep 2010
Posts: 3,752
Likes: 190
From: Indiana PA
In the speed density cars, (mine specifically is a '92) how does it know that the elevation changes while driving?
I understand (was taught in automotive school 20 years ago) that the MAP sensor takes a quick "snap shot" of manifold pressure (ambient air pressure) as the key rotates past "on" to "start" and gets it's Baro reading in that instant. I was further taught that any WOT event would cause the ECM to reset the BARO. Maybe.
But what happens when you cruise from 6000' elevation to 10000' elevation, at part throttle? There is no WAY that the MAP would ever see atmospheric pressure during that event, so how can it "know" that the air pressure has dropped? Or increase, when going down?
I understand (was taught in automotive school 20 years ago) that the MAP sensor takes a quick "snap shot" of manifold pressure (ambient air pressure) as the key rotates past "on" to "start" and gets it's Baro reading in that instant. I was further taught that any WOT event would cause the ECM to reset the BARO. Maybe.
But what happens when you cruise from 6000' elevation to 10000' elevation, at part throttle? There is no WAY that the MAP would ever see atmospheric pressure during that event, so how can it "know" that the air pressure has dropped? Or increase, when going down?
Thread Starter
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
Huh. Could be. I've been thinking about this since I moved. In my daily commute, I go from ~7000' to 9000' in about 6-7 minutes. Then back down to 7000' over about 15 minutes, then up to about 10,000' in about 20 minutes. Pretty quick and dramatic changes. Made me wonder, w/o a dedicated BARO sensor, or a MAF, how it could manage that.
Runs fine, no issues...just wondered.
Runs fine, no issues...just wondered.
Safety Car
Joined: Sep 2010
Posts: 3,752
Likes: 190
From: Indiana PA
Huh. Could be. I've been thinking about this since I moved. In my daily commute, I go from ~7000' to 9000' in about 6-7 minutes. Then back down to 7000' over about 15 minutes, then up to about 10,000' in about 20 minutes. Pretty quick and dramatic changes. Made me wonder, w/o a dedicated BARO sensor, or a MAF, how it could manage that.
Runs fine, no issues...just wondered.
Runs fine, no issues...just wondered.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
The pressure in the plenum changes whenever you change the throttle opening or the engine speed changes. The ECM reads the pressure several times per second to adapt the fuel injector pulses to the changing conditions. The older ECMs took readings 10 times per second, so I'm sure the newer ones do it even more often.
Safety Car
Joined: Sep 2001
Posts: 4,150
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From: Glenbeulah Wi
The computer does not care what the atmospheric air pressure is, all the engine cares about is what the pressure is in the intake manifold while the engine is running. The fuel and spark are adjusted based on the running MAP readings. At cruise your MAP readings are lower then atmospheric readings and the computer adjusts accordingly.
Safety Car
Joined: Sep 2010
Posts: 3,752
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From: Indiana PA
The computer does not care what the atmospheric air pressure is, all the engine cares about is what the pressure is in the intake manifold while the engine is running. The fuel and spark are adjusted based on the running MAP readings. At cruise your MAP readings are lower then atmospheric readings and the computer adjusts accordingly.
Safety Car
Joined: Sep 2001
Posts: 4,150
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From: Glenbeulah Wi
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Race Director
Joined: Apr 2002
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From: Anaheim CA
Just a side note. There are two ways to measure pressure/vacuum.
One is "gauge" pressure. One side of the gauge is open to atmosphere and the pressure reading is the difference between what is being measured and atmospheric pressure. Vacuum gauges work this way.
The second is "absolute" pressure (as in Manifold ABSOLUTE Pressure). This method does not use a reference and is just pressure.
One is "gauge" pressure. One side of the gauge is open to atmosphere and the pressure reading is the difference between what is being measured and atmospheric pressure. Vacuum gauges work this way.
The second is "absolute" pressure (as in Manifold ABSOLUTE Pressure). This method does not use a reference and is just pressure.
Safety Car
Joined: Sep 2010
Posts: 3,752
Likes: 190
From: Indiana PA
Safety Car
Joined: Sep 2010
Posts: 3,752
Likes: 190
From: Indiana PA
Just a side note. There are two ways to measure pressure/vacuum.
One is "gauge" pressure. One side of the gauge is open to atmosphere and the pressure reading is the difference between what is being measured and atmospheric pressure. Vacuum gauges work this way.
The second is "absolute" pressure (as in Manifold ABSOLUTE Pressure). This method does not use a reference and is just pressure.
One is "gauge" pressure. One side of the gauge is open to atmosphere and the pressure reading is the difference between what is being measured and atmospheric pressure. Vacuum gauges work this way.
The second is "absolute" pressure (as in Manifold ABSOLUTE Pressure). This method does not use a reference and is just pressure.
I've measured vacuum at the manifold with a gauge and read it using a scan tool via MAP. Two different numbers. Kinda thru me off for a second, then I realized it.
Race Director
Joined: Apr 2002
Posts: 10,036
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From: Anaheim CA
I think it remembers the atmospheric pressure to compare to the current reading to get a difference. The computer is calculating volumetric efficiency to determine the air flow through the engine. From that it determines what the injector pulse width should be to get the correct air/fuel ratio.
Thread Starter
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
I think it remembers the atmospheric pressure to compare to the current reading to get a difference. The computer is calculating volumetric efficiency to determine the air flow through the engine. From that it determines what the injector pulse width should be to get the correct air/fuel ratio.
I get that all that REALLY matters is the pressure inside the plenum -as long as it doesn't get so far out of whack w/the TPS to MAP to RPM ratio.
Instructor
Joined: Jul 2008
Posts: 218
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From: Utah County Utah
I was kind of under the impression that cars could measure the pressure two ways, one way is the way you described, the second way is to have two sensors. Do the speed density engines have two sensors?