exhaust restricting turbo.
Apr 2, 2008 | 06:26 PM
#11
Hmmm...that may be true. I do remember seeing something in the tune about barometer reading on engine start-up. So I guess the absolute pressure that the MAP sensor reads against changes depending on the conditions when the vehicle is started. Way to over-complicate things, Erik
Apr 2, 2008 | 06:28 PM
#12
Your MAP sensor is also ref'd against the baro. estimate as I recall.
At one time, my PCM, my guage and my meth controller all worked off the same digital signal from my GM MAP. All worked read the same values at the same time. All of the digital guages I've had ran off a MAP sensor.
At one time, my PCM, my guage and my meth controller all worked off the same digital signal from my GM MAP. All worked read the same values at the same time. All of the digital guages I've had ran off a MAP sensor.
thats why when changing elevations alot it is good to stop occasionally to get a new baro reading
Apr 2, 2008 | 06:32 PM
#13
Yeah, I was talking about how a mechanical boost gauge would reference differently than the MAP sensor, but I forgot that the MAP sensor got a new barometer reading at key-on. The only time that the stuff I posted would happen is through significant elevation changes without stopping and restarting the engine, like David said.
Apr 2, 2008 | 06:34 PM
#14
Yeah, I was talking about how a mechanical boost gauge would reference differently than the MAP sensor, but I forgot that the MAP sensor got a new barometer reading at key-on. The only time that the stuff I posted would happen is through significant elevation changes without stopping and restarting the engine, like David said.
Apr 2, 2008 | 10:18 PM
#15
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From: colorado above 6000 ft.
I just added the meth no change in the tune, It seemed to be leaning out 12.-12.5 in 3rd at wot. Added the meth to come on at 8 psi and now a/f seems to be where it needs to be. Guage is mechanical off the manifold.
Not sure whats going on with the boost going higher.
Not sure whats going on with the boost going higher.
Apr 4, 2008 | 06:28 AM
#16
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From: Behind the TIG welder
Are you seeing the boost increase on the gauge or the map sensor?
I also see a 2 psi boost increase with my cut-out open...
Back to the off topic of boost vs. atmospheric pressure.
Your electric boost gauge doesn't care about the pressure at "key-on" it just want the voltage. It will auto zero from that point. From want I understand about the PCM, It does not "zero". It' expects to see a certain voltage for a set pressure. A mechanical gauge will always report correct boost pressure no mater the change in elevation do to the fact the bourdon tubes are always subjected to atmospheric pressure.
I wish everyone would drop the "boost psi" and just report PSI or KPA, life would be much easier....
Apr 4, 2008 | 07:56 AM
#17
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From: Colorado Springs, Co/ Central, Ca
The gauge references manifold pressure to atmospheric pressure. So for a given manifold pressure, you can vary atmospheric pressure and the reading on the boost gauge will change because the difference between the manifold pressure and atmospheric pressure will change. The boost gauge is relative.
The MAP sensor is absolute, meaning that it compares manifold pressure to some absolute pressure, which means that you can vary the atmospheric pressure all you want while keeping the manifold pressure the same and it will always read the same pressure.
Example....if I'm here in Phoenix at 1200 ft elevation and making enough boost to give me a reading of 195kpa on the MAP sensor, let's say it shows 12 psi on the boost gauge. Now I drive to the coast, where I'm at sea level. I make the same run and have the same MAP value of 195kpa, only now my boost gauge reads 10 psi because it is referenced to atmospheric pressure. When the atmospheric pressure is greater, the difference between manifold and atmosphere is lower, and the boost gauge reading will be lower.
http://en.wikipedia.org/wiki/Vacuum_gauge
The MAP sensor is absolute, meaning that it compares manifold pressure to some absolute pressure, which means that you can vary the atmospheric pressure all you want while keeping the manifold pressure the same and it will always read the same pressure.
Example....if I'm here in Phoenix at 1200 ft elevation and making enough boost to give me a reading of 195kpa on the MAP sensor, let's say it shows 12 psi on the boost gauge. Now I drive to the coast, where I'm at sea level. I make the same run and have the same MAP value of 195kpa, only now my boost gauge reads 10 psi because it is referenced to atmospheric pressure. When the atmospheric pressure is greater, the difference between manifold and atmosphere is lower, and the boost gauge reading will be lower.
http://en.wikipedia.org/wiki/Vacuum_gauge
Apr 4, 2008 | 08:02 AM
#18
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[QUOTE=Wilde Racing;3890281]Are you seeing the boost increase on the gauge or the map sensor?
I also see a 2 psi boost increase with my cut-out open...
QUOTE] Hearing this is giving me hope for a little more power
I have ben running through a single 3" muffler with no cuttout. If I could pick up even only 1 PSI un caping the exhust that would be sweet.
I also see a 2 psi boost increase with my cut-out open...
QUOTE] Hearing this is giving me hope for a little more power
I have ben running through a single 3" muffler with no cuttout. If I could pick up even only 1 PSI un caping the exhust that would be sweet.