Manifold Absolute Pressure (MAP) vs barometer when N/A

[FNBADAZ06]

When looking at the MAP sensor readings, can an unmodifed N/A LS motor ever see a MAP reading that is identical to the local barometer reading ? What about one with a modified air path that includes a CAI ? If at sea level with a barometer of one atmospheric unit ( 101 kPa, or 14.7 psi, or 29.92 inHg ), could you ever expect to see a MAP reading of 101 kPa at WOT on a N/A engine ?

[Ted Y]

Simple! When the engine is off...

[Two Fangs]

I believe that they can read remarkably close not only when the engine is off, but also at Wide Open Throttle. Vacuum, ideally, should be the difference between barometric pressure and manifold absolute pressure.

[FNBADAZ06]

I believe that they can read remarkably close not only when the engine is off, but also at Wide Open Throttle. Vacuum, ideally, should be the difference between barometric pressure and manifold absolute pressure.

OK, I follow that logic. So the next question would be, if one were to see a MAP reading of 97 kPa at WOT when local conditions are 101 kPa, where do you surmise that the vacuum is coming from.....a retrictive air intake ?

[Two Fangs]

A restriction somewhere, for certain. But it may not be the intake itself, it could be poor intake manifold design, poor cylinder head ports, or incorrect valve sizes. A truly optimized engine should be able to achieve very close to zero vacuum. What type of engine are we discussing, the LS7?

[FNBADAZ06]

A restriction somewhere, for certain. But it may not be the intake itself, it could be poor intake manifold design, poor cylinder head ports, or incorrect valve sizes. A truly optimized engine should be able to achieve very close to zero vacuum.

What type of engine are we discussing, the LS7?

Yep

Looking at some of my data logs this past weekend when I ran at Saturday morning's event at Speedworld, I see MAP readings of around 95-96 kPa at WOT during my runs...that translates to approximately 28.35 inHg.

While not exact, Dragtimes reported 30.00 to 30.06 inHg during the entire morning.

Maybe a dirty/clogged K&N filter ??? I guess I'll have to pull it and check.

[Two Fangs]

I would be surprised, they usually take a ton of dirt to drop pressure. It should be able to get to 100 kPa, though. Since I am boosted, I can't run mine to compare.

[Ted Y]

Are you able to get a MAP reading with the engine off? That way you can use it as a reference to eliminate gage error.

[Two Fangs]

Yep, you can.

[FNBADAZ06]

Are you able to get a MAP reading with the engine off? That way you can use it as a reference to eliminate gage error.

You read my mind

I used both my iPhone app and HP Tuners and just went into the garage, and with the ignition on without the engine running, I'm reading 97 kPa while sitting in the garage which converts to 28.64 inHg.

My Lexus ECU suprisingly doesn't offer MAP or barometer readings from the ECU...guess a MAP sensor isn't used for air/fuel metering in this car ?

Current barometer reading at Deer Valley Airport is 30.06 inHg or 101.7 kPa.

[Ted Y]

Are you able to get a MAP reading with the engine off? That way you can use it as a reference to eliminate gage error.

You read my mind

I used both my iPhone app and HP Tuners and just went into the garage, and with the ignition on without the engine running, I'm reading 97 kPa while sitting in the garage.

My Lexus ECU suprisingly doesn't offer MAP or barometer readings from the ECU...guess a MAP sensor isn't used for air/fuel metering in this car ?

Well, don't just sit there, go out and hammer it. That way you'll know if you're restricted.

[Two Fangs]

If it has a MAF, it doesn't need a MAP.

[Two Fangs]

Well, don't just sit there, go out and hammer it. That way you'll know if you're restricted.

Ted! I'm shocked!

[FNBADAZ06]

If it has a MAF, it doesn't need a MAP.

gotcha.

So the LS motors use both MAF and MAP...which one carries more weight when the ECU is determining fuel metering ?

[FNBADAZ06]

Well, don't just sit there, go out and hammer it. That way you'll know if you're restricted.

Ted! I'm shocked!

If I can get a Ted escort, I'll take it out on the I-17 north and satisfy his request

That should keep me on the right side of the law, correct ?

[Two Fangs]

It depends on the RPM. Low RPM it is the MAP, high RPM it is the MAF. These RPM switch points can be tuned.

[FNBADAZ06]

It depends on the RPM. Low RPM it is the MAP, high RPM it is the MAF.

These RPM switch points can be tuned.

So at WOT and above 5000 RPM, should I be concerned with a MAP reading of 97 kPa or lower when local conditions suggest a much higher barometric pressure ?

[Two Fangs]

As a measure of engine efficiency, yes. As a monitor of proper load calculation and fueling, no. ***EDIT*** Looking at the tune, the car is all MAF after 4,000 RPM. The PCM, however, will always compare MAP and MAF, regardless of which one the PCM is using for primary information. If they get too far apart, it will pitch a code and turn off Traction Control. If they get wayyy far apart, the car will go into REP.

[FNBADAZ06]

As a measure of engine efficiency, yes. As a monitor of proper load calculation and fueling, no.

I'm interested in where this ineffiecency is coming from.....or is it simply a defective sensor, as 97 kPa is being read from the sensor with the key on and engine off. Can oil ingestion into the intake manifold via the PVC system cause a MAP sensor to read incorrectly (that would sound weird to me) ?

[Two Fangs]

MAPs, like resistors, capacitors, or any other electronic device have an acceptable variance. It may just be what the sensor peaks at. We can fix it in the tune by changing the MAP Linear and MAP Offset values.

[FNBADAZ06]

MAPs, like resistors, capacitors, or any other electronic device have an acceptable variance.

It may just be what the sensor peaks at. We can fix it in the tune by changing the MAP Linear and MAP Offset values.

Cool

And for shits and giggles, I went over to Tom's (HOXXOH) to see what his MAP reads with ignition on and engine off........96 kPa.

So it looks like that is standard operating conditions for the MAP sensor.

[Jon]

It's not a big deal. It's just a sensor. Don't compare the weather station data to your car data. Apples to oranges. You need to compare your non-running MAP pressure to WOT MAP pressure to get an idea for any restriction. My LS3 car will see up to 99kpa so far depending on conditions and even with my new Callaway CAI I am seeing a 2kpa difference at the top of the RPM range at WOT. I thinking about having my intake manifold ported and seeing what that does. Some motorcycles are designed for a pressurized air box and will actually see boost at high speeds. You can do this for a car also. You should also be able to log barometric pressure and MAP pressure to compare the two on a data log.

[FNBADAZ06]

It's not a big deal. It's just a sensor. Don't compare the weather station data to your car data. Apples to oranges. You need to compare your non-running MAP pressure to WOT MAP pressure to get an idea for any restriction. My LS3 car will see up to 99kpa so far depending on conditions and even with my new Callaway CAI I am seeing a 2kpa difference at the top of the RPM range at WOT. I thinking about having my intake manifold ported and seeing what that does.

Some motorcycles are designed for a pressurized air box and will actually see boost at high speeds. You can do this for a car also.

You should also be able to log barometric pressure and MAP pressure to compare the two on a data log.

I believe the barometric pressure readings ARE coming from the MAP sensor.....I don't think there is a seperate sensor.

The question really was, can the MAP sensor readings (as read and displayed by a stock ECU tune) really be accurate as compared to a weather station barometric reading....and if it was an accurate measurement and display from the sensor, why the disparity in readings versus a true weather station reading.

If you were to take the MAP reading at face value and believed that it WAS showing true absolute manifold pressure, the MAP reading would indicate a significant restriction/ineffiecency somewhere in the induction system...as the value would indicate a vacuum if compared to the true kPa readings from a weather station. I agree with you that the comparison between a non-running state to a WOT reading would contain the same bias built into the reading, and as long as both values are close in the reported kPa values...you should be OK.

FYI - I believe it was demonstrated scientifically in the windtunnel testing that ram-air effect would not come into play on a Corvette until somewhere above 120+ MPH, and then, only with a sealed air inlet path straight to the throttlebody

[Jon]

On my old Hayabusa 120mph to 200mph didn't hardly take any time at all. One of the reasons I sold that bike. It would have gotten me in too much trouble. Now I have a Speed Triple that limits me to around 160mph for short bursts since I have no wind protection.

[Desertdawg]

Doesnt the computer adapt to the MAP reading and adjust the parameters to run at it's programed optimium running condition? In other words if you installed a perfect MAP that was reading true, the computer would just adapt to that new reading and adjust the parameters to run at optimium again... Better or worse, your motor should run the same based on the current pressure... I think your chasing a fictitious number here.