Accuracy of gauges?
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Accuracy of gauges?
I am at 4500 feet and have differing pressure readings from my autometer boost gauge and 3.5 bar MAP sensor. With the car off, the gauge reads 0 and the MAP sensor reads 4 in/hg vacuum. Frankly, I don't even know how much boost I am running because the boost gauge will say ~12-13 psi and the MAP sensor will say ~9-10 psi.
I am having a hell of a time tuning for the proper pressure when I am not sure which one is accurate. Can someone shed some light on this?
I am having a hell of a time tuning for the proper pressure when I am not sure which one is accurate. Can someone shed some light on this?
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Re: Accuracy of gauges? (nowtype)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nowtype »</TD></TR><TR><TD CLASS="quote">Um, gauge pressure != absolute pressure.</TD></TR></TABLE>
I know that. That wasn't my question.
I know that. That wasn't my question.
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Re: Accuracy of gauges? (Dartanian)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Dartanian »</TD></TR><TR><TD CLASS="quote">
I know that. That wasn't my question.</TD></TR></TABLE>
It's exactly your question dumb dumb. You are reading off a boost gauge. That reads gauge pressure. Your ECU reads from a MAP sensor which reads absolute pressure. The two readings will not be the same unless you are at sea level. Read up on gauge pressure and absolute pressure if you don't know what they mean. I believe wikipedia's info is pretty valid on the topic.
I know that. That wasn't my question.</TD></TR></TABLE>
It's exactly your question dumb dumb. You are reading off a boost gauge. That reads gauge pressure. Your ECU reads from a MAP sensor which reads absolute pressure. The two readings will not be the same unless you are at sea level. Read up on gauge pressure and absolute pressure if you don't know what they mean. I believe wikipedia's info is pretty valid on the topic.
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Re: Accuracy of gauges? (nowtype)
They are both gauges since my MAP sensor is integrated into my wideband display. They are within 3-4 psi of each other. All I want to know is which one is accurate or how do I calculate the number based on the readings I am getting?
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Re: Accuracy of gauges? (Dartanian)
Holy ****. Dude, one gauge is reading gauge pressure. The other one is reading absolute pressure. They are both reading accurately. If you don't know what gauge and/or absolute pressure is, look it up.
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Re: Accuracy of gauges? (nowtype)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nowtype »</TD></TR><TR><TD CLASS="quote">Holy ****. Dude, one gauge is reading gauge pressure. The other one is reading absolute pressure. They are both reading accurately. If you don't know what gauge and/or absolute pressure is, look it up.</TD></TR></TABLE>
Not quite...the autometer is calibrated to read 0 at atmospheric pressure (probably at sea level). If the sensor is accurate changing your altitude would absolutely move the needle. Just because it's measuring gauge pressure doesn't mean it is immune to changes in pressure due to altitude. I would trust the map sensor. My autometer boost gauge always sat a little below 0 when off unless you tapped it. Map sensors are alot more precise than many mechanical boost gauges out there.
Not quite...the autometer is calibrated to read 0 at atmospheric pressure (probably at sea level). If the sensor is accurate changing your altitude would absolutely move the needle. Just because it's measuring gauge pressure doesn't mean it is immune to changes in pressure due to altitude. I would trust the map sensor. My autometer boost gauge always sat a little below 0 when off unless you tapped it. Map sensors are alot more precise than many mechanical boost gauges out there.
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Re: Accuracy of gauges? (BlueSi2k)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by BlueSi2k »</TD></TR><TR><TD CLASS="quote">
Not quite...the autometer is calibrated to read 0 at atmospheric pressure (probably at sea level). If the sensor is accurate changing your altitude would absolutely move the needle. Just because it's measuring gauge pressure doesn't mean it is immune to changes in pressure due to altitude. I would trust the map sensor. My autometer boost gauge always sat a little below 0 when off unless you tapped it. Map sensors are alot more precise than many mechanical boost gauges out there.</TD></TR></TABLE>
Mechanical gauges read gauge pressure which means where ever you are, they read 0. 15000000 ft above sea level or 15000000ft below sea level, they still read 0psi. From there they read whether additional pressure(boost aka positive pressure) is applied or taken away(negative pressure ie vacuum). I could agree with you if his gauge was reading anything but 0 when the car was off, but since this is not the case, I'm going with the standard gauge pressure vs atmospheric pressure argument. Either way, take the two readings, average them and say thats the pressure you are running. It has no effect on tuning so who cares really.
For those too stupid to use google:
* Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
* Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.
* Differential pressure is the difference in pressure between two points.
http://www.autometer.com/productPDF/1115A.pdf
Modified by nowtype at 7:59 PM 8/30/2007
Modified by nowtype at 7:59 PM 8/30/2007
Not quite...the autometer is calibrated to read 0 at atmospheric pressure (probably at sea level). If the sensor is accurate changing your altitude would absolutely move the needle. Just because it's measuring gauge pressure doesn't mean it is immune to changes in pressure due to altitude. I would trust the map sensor. My autometer boost gauge always sat a little below 0 when off unless you tapped it. Map sensors are alot more precise than many mechanical boost gauges out there.</TD></TR></TABLE>
Mechanical gauges read gauge pressure which means where ever you are, they read 0. 15000000 ft above sea level or 15000000ft below sea level, they still read 0psi. From there they read whether additional pressure(boost aka positive pressure) is applied or taken away(negative pressure ie vacuum). I could agree with you if his gauge was reading anything but 0 when the car was off, but since this is not the case, I'm going with the standard gauge pressure vs atmospheric pressure argument. Either way, take the two readings, average them and say thats the pressure you are running. It has no effect on tuning so who cares really.
For those too stupid to use google:
* Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
* Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.
* Differential pressure is the difference in pressure between two points.
http://www.autometer.com/productPDF/1115A.pdf
Modified by nowtype at 7:59 PM 8/30/2007
Modified by nowtype at 7:59 PM 8/30/2007
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Re: Accuracy of gauges? (Dartanian)
remember to always tell your friends what the pressure on the map reads, not the pressure at the gauge. 
lol
It really doesnt matter.. if you feel comfortable with the hp, duty cycle, egt etc and how the motor runs @ 13psig than use 13psig and 10psia as your reference.
Is 3psi or accuracy a really a big deal on your setup? If so, use psia (map).
lolIt really doesnt matter.. if you feel comfortable with the hp, duty cycle, egt etc and how the motor runs @ 13psig than use 13psig and 10psia as your reference.
Is 3psi or accuracy a really a big deal on your setup? If so, use psia (map).
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Re: Accuracy of gauges? (nowtype)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nowtype »</TD></TR><TR><TD CLASS="quote">
Mechanical gauges read gauge pressure which means where ever you are, they read 0. 15000000 ft above sea level or 15000000ft below sea level, they still read 0psi. From there they read whether additional pressure(boost aka positive pressure) is applied or taken away(negative pressure ie vacuum). I could agree with you if his gauge was reading anything but 0 when the car was off, but since this is not the case, I'm going with the standard gauge pressure vs atmospheric pressure argument. Either way, take the two readings, average them and say thats the pressure you are running. It has no effect on tuning so who cares really.
For those too stupid to use google:
* Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
* Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.
* Differential pressure is the difference in pressure between two points.
http://www.autometer.com/productPDF/1115A.pdf
Modified by nowtype at 7:59 PM 8/30/2007
Modified by nowtype at 7:59 PM 8/30/2007</TD></TR></TABLE>
Thats neat that they have a vent like that I never knew that, however thats only for the liquid filled gauges. My autometer had no vent and would therefore change based on altitude.
Mechanical gauges read gauge pressure which means where ever you are, they read 0. 15000000 ft above sea level or 15000000ft below sea level, they still read 0psi. From there they read whether additional pressure(boost aka positive pressure) is applied or taken away(negative pressure ie vacuum). I could agree with you if his gauge was reading anything but 0 when the car was off, but since this is not the case, I'm going with the standard gauge pressure vs atmospheric pressure argument. Either way, take the two readings, average them and say thats the pressure you are running. It has no effect on tuning so who cares really.
For those too stupid to use google:
* Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
* Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.
* Differential pressure is the difference in pressure between two points.
http://www.autometer.com/productPDF/1115A.pdf
Modified by nowtype at 7:59 PM 8/30/2007
Modified by nowtype at 7:59 PM 8/30/2007</TD></TR></TABLE>
Thats neat that they have a vent like that I never knew that, however thats only for the liquid filled gauges. My autometer had no vent and would therefore change based on altitude.
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