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Old 04-16-2016, 06:28 PM   #1 (permalink)
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Default OEM MAP vs boost gauge vs AEM FIC

Quick question guys,
I'm boosted running the AEM FIC (onboard MAP sensor).

Boost gauge reads 10 psi
OEM MAP output ~4.8v
AEM FIC sees 29 psia.


According to boost gauge and OEM MAP, i should be around 10 psi. According to FIC, I'm boosting about 14-15 psi.


Which should I trust? Has anyone experienced this before with the FIC?
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Old 04-16-2016, 07:50 PM   #2 (permalink)
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depends where the readings are coming from. manifold? turbo's housing? at some point on piping?

also, not everyone is at sea level.

check a baromater, or check your elevation. Perhaps something is causing a disturbance in the force
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Old 04-16-2016, 07:51 PM   #3 (permalink)
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also, aem fic ties into wiring. extra resistance means slightly different sensor reading
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Old 04-16-2016, 08:38 PM   #4 (permalink)
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Where I'm at (Melbourne, FL), we're pretty much sea level so I kind of expect 14.7 psia atmospheric.

The FIC's MAP sensor is taken from the intake mani, same tee used for BOV and boost gauge. It's also a vacuum hose to the FIC's sensor (no wiring).

Either way, wiring or vacuum, I would think any form of noise/interference/resistance would make the FIC read the lowest, right?

It wouldn't be a big deal for me to tune just based on what I "know" the boost really is. My real issue is my FIC map is tuned to pull a deg/pound of boost. So it might be pulling more timing than it should.
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Old 04-16-2016, 10:51 PM   #5 (permalink)
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have you looked at a voltage table for the map sensor?

lower voltage is more vacuum/less boost

higher voltage is increased boost/less vacuum


this is why oem map sensors are garbage to use past 7-8 psi and nothing to rely on at 10psi.

the oem map sensor is designed to be accurate in vacuum and not designed to be accurate under boost. it reading 10psi could be maxxed out, reading bad, or simply bugging out.


get a real tuning setup and use a proper MAP sensor if you wish to use the MAP sensor for boost reading.

else look up the operating range of the fic's sensor


oem map can be probed to see if its getting proper reference voltage. 5volts with key on engine off. make sure battery is outputting at least 12.0 volts, else reference voltage will be lower. if this is the case, of course just run the engine for a few minutes to give the battery a good surface charge
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Old 04-18-2016, 08:03 AM   #6 (permalink)
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4.8v is about maxed out for the OEM MAP, most 5v pressure sensors actually max out at 4.7. The firewall mounted MAP tends to max out sooner than the TB mounted MAP.

Gauges are usually off anywhere from half to a few PSI. Can you reduce boost to see what everything shows? Also, what does everything show with the key on but engine off? If they all 0psi with the engine off and the FIC at 50% more boost at any given point above that, then you know the FIC is reading wrong. The FIC should be grounded to the thermostat housing to avoid any voltage offsets.
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Old 04-18-2016, 09:20 AM   #7 (permalink)
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Taken from AEM FIC instructions..

Quote:
Absolute Pressure Explained
(Note: The F/IC reads and displays absolute pressure. Please read the
following section before tuning.)
When talking pressure, there are two common ways pressure is represented,
Gauge, and Absolute, Gauge being by far the more common way. Take a tire for
example, if the measuring gauge says 50 psi, we say the tire has 50 psi of air in it.
However, that is not 100% correct. The tire actually has air at 50 psi above atmospheric
pressure, which is known as psi gauge (psig). The total pressure, or absolute pressure
(psia), is actually 50 psi on the gauge plus the atmospheric pressure. So, the absolute
pressure is the gauge pressure plus the atmospheric pressure. What about boost
gauges? Most boost gauges display gauge pressure both above and below atmospheric
pressure. Pressure above atmospheric pressure is commonly referred to as “Boost”
pressure. Pressure below atmospheric is commonly referred to as “Vacuum”. So why
the lesson on absolute pressure? BECAUSE THE F/IC READS AND DISPLAYS
ABSOLUTE PRESSURE!!! For easy reference, the following formulas can be used to
determine “Boost” and “Vacuum” pressures. Atmospheric pressure can be determined
by reading the F/IC pressure with the key on and the engine off.
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Old 04-18-2016, 01:53 PM   #8 (permalink)
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Quote:
Originally Posted by mattliston View Post
have you looked at a voltage table for the map sensor?

lower voltage is more vacuum/less boost

higher voltage is increased boost/less vacuum


this is why oem map sensors are garbage to use past 7-8 psi and nothing to rely on at 10psi.

the oem map sensor is designed to be accurate in vacuum and not designed to be accurate under boost. it reading 10psi could be maxxed out, reading bad, or simply bugging out.


get a real tuning setup and use a proper MAP sensor if you wish to use the MAP sensor for boost reading.

else look up the operating range of the fic's sensor


oem map can be probed to see if its getting proper reference voltage. 5volts with key on engine off. make sure battery is outputting at least 12.0 volts, else reference voltage will be lower. if this is the case, of course just run the engine for a few minutes to give the battery a good surface charge
The FIC's map sensor reads up to 41 PSIA (~25 psi boost). You were right about the OEM being maxed out. A couple datalogs later I figured I'm boosting 11 psi. The gauge and FIC are right (FIC showing ~26 psia).

Quote:
Originally Posted by HiProfile View Post
4.8v is about maxed out for the OEM MAP, most 5v pressure sensors actually max out at 4.7. The firewall mounted MAP tends to max out sooner than the TB mounted MAP.

Gauges are usually off anywhere from half to a few PSI. Can you reduce boost to see what everything shows? Also, what does everything show with the key on but engine off? If they all 0psi with the engine off and the FIC at 50% more boost at any given point above that, then you know the FIC is reading wrong. The FIC should be grounded to the thermostat housing to avoid any voltage offsets.
Thank you; you're exactly right about the OEM MAP. It was being maxed out. FIC and boost gauge were correct.
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Old 04-18-2016, 02:28 PM   #9 (permalink)
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When i ran my fic. The sensor was correct.
Oem will stop reading boost pressure at that 4.8volt its maxing out.
Also some gauges have been known to be off a bit.
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Old 04-18-2016, 02:32 PM   #10 (permalink)
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Quote:
Originally Posted by btruong968 View Post
The FIC's map sensor reads up to 41 PSIA (~25 psi boost). You were right about the OEM being maxed out. A couple datalogs later I figured I'm boosting 11 psi. The gauge and FIC are right (FIC showing ~26 psia).
25-26psia is right for 11psi gauge, but the original post said 29psi for FIC. Reading 3-4psi higher would have been a cause for concern, but under 2 is typical error for gauges.
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Old 04-18-2016, 05:01 PM   #11 (permalink)
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Quote:
Originally Posted by HiProfile View Post
25-26psia is right for 11psi gauge, but the original post said 29psi for FIC. Reading 3-4psi higher would have been a cause for concern, but under 2 is typical error for gauges.
I was wrong in my OP, sorry about that. It was a 29 psia spike. The FIC reads 25-26 psia on when in full boost.
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