Xantia – manifold pressure

[lancia58]

Hi

I hooked a Lexia and observed the manifold pressure parameter. It showed the following

At Idle it was 390 mb
At 1300 RPM it was 294 mb
At 2200 RPM it was 285 mb
At 4200 RPM it was 302 mb
At 4500 RPM it was 280 mb

Are the above readings OK, if not does it mean that the MAP is faulty

Thanks
Zohar

[Mandrake]

With the key on but the engine not running is the reading equal to local barometric pressure ? If so its probably ok.

Another way to check it would be to put a vacuum gauge on the inlet manifold and compare the readings. Bear in mind that most vacuum gauges read relative pressure in inches of mercury (Hg) while the MAP sensor reads absolute pressure in millibars, so the reading must be converted with the following formula:

v = (a - m) / 33.86

Where:

v = vacuum in inches of mercury (" Hg)
a = ambient barometric pressure in millibars
m = MAP sensor pressure reading in millibars

So for example assuming your ambient barometric pressure was 1000mb, your idle reading of 390mb equals 18" of mercury which is a typical vacuum reading at idle.

[LFY]

v = (a - m) / 33.86

Where:

v = vacuum in inches of mercury (" Hg)
a = ambient barometric pressure in millibars
m = MAP sensor pressure reading in millibars

So for example assuming your ambient barometric pressure was 1000mb, your idle reading of 390mb equals 18" of mercury which is a typical vacuum reading at idle.

Excellent explanation, but there is something very strange!

We all know that when throttle opens more air rushes into the manifold and then Vacuum decreases.

For example @ idle the MAP reads 390 millibars and @ 2200 RPM it reads 285 millibars.

So @ 2200 RPM, vacuum should be lower than idle because the throttle is open.

According to your formula (assuming ambient pressure is 1000 mbar):

Vacuum @ 2200 RPM = (1000 - 285) / 33.86 = 21 inches of mercury
Vacuum @ Idle = (1000 - 390) / 33.86 = 18 inches of mercury

21 > 18

How can this be true? Vacuum should be lower @ 2200 RPM than idle.

Also we all know that when the throttle is closed and the car decelerates (high rpm with closed throttle), high amount of vacuum is created.

In this condition MAP reads a higher number like 400 - 500.

So assuming ambient pressure of 1000 mbar:

Vacuum of decelerating = (1000 - 500) / 33.86 = 14.76 inches of mercury

14.76 < 18

It is again contradictory because vacuum @ idle (800 - 850 RPM) should be much lower than vacuum at decelerating with closed throttle (3000 - 4000 RPM).

What is the idea behind these contradictory results?!

[Old-Guy]

I have no practical experience of modern fuel-injected petrol engines, however from experiences with carburettors I'd point out that 'Manifold pressure' will depend on where it's measured. Bernoulli's principle states that for a given flow of a fluid, an increase in the speed of the fluid results in a decrease in pressure.
Mass Air-Flow metering in throttle bodies generally exploits this principle by integrating throttle opening and pressure drop at the throttle. The pressure measured at the throttle will be lower than further downstream; even when the throttle butterfly or slide is wide open, the venturi shape of the throttle body will cause a pressure drop.

[Mandrake]

Interesting questions...

Excellent explanation, but there is something very strange!

We all know that when throttle opens more air rushes into the manifold and then Vacuum decreases.

For example @ idle the MAP reads 390 millibars and @ 2200 RPM it reads 285 millibars.

So @ 2200 RPM, vacuum should be lower than idle because the throttle is open.

According to your formula (assuming ambient pressure is 1000 mbar):

Vacuum @ 2200 RPM = (1000 - 285) / 33.86 = 21 inches of mercury
Vacuum @ Idle = (1000 - 390) / 33.86 = 18 inches of mercury

21 > 18

How can this be true? Vacuum should be lower @ 2200 RPM than idle.

That's not actually the case - for a zero load condition (eg in neutral, so the engine is only generating enough power to turn itself over) vacuum is lowest at idle, and on typical engines starts to drop off quickly below about 700-800 rpm.

That's not surprising because when you look at how little you have to open the throttle to reach 2200 rpm (hardly anything) you now have nearly 3x the rpm (and therefore a vacuum pump that is working 3x harder) for only a slight throttle increase, hence more vacuum.

On my Xantia V6 the vacuum at idle is about 17" while if I hold the throttle at 2000 rpm it's about 20", so the figures you quote are consistent with this.

If you're not convinced, all I can really say is try connecting a good old fashioned vacuum gauge (I have one) on an engine and try it to see how it behaves.

Also we all know that when the throttle is closed and the car decelerates (high rpm with closed throttle), high amount of vacuum is created.

In this condition MAP reads a higher number like 400 - 500.

So assuming ambient pressure of 1000 mbar:

Vacuum of decelerating = (1000 - 500) / 33.86 = 14.76 inches of mercury

14.76 < 18

It is again contradictory because vacuum @ idle (800 - 850 RPM) should be much lower than vacuum at decelerating with closed throttle (3000 - 4000 RPM).

What is the idea behind these contradictory results?!

Did you measure this 400-500 mbar reading while the car was being driven coasting down a hill ?

That is an unusual result, however I can think of a possible explanation.

Don't forget the idle control valve or ICV, this valve provides an alternative flow of air around the throttle plate - it is primarily there so that the ECU can precisely regulate the idle speed to the desired speed (which changes depending on coolant temperature etc) but a little known fact is that on many (most ? not sure) petrol engines when you open the throttle wide to let the engine accelerate the idle control valve also opens wide at the same time so that you have the combined air flow of an open throttle plate and ICV.

I haven't really seen the reason for this explained anywhere, but I believe this is done primarily to provide a modern equivalent of the old dashpot you used to find on a carburettor, so that when you take your foot off the throttle suddenly (for example when changing gear) you don't get a sudden lurch due to the engine torque suddenly dropping to nothing.

At the moment you release the throttle (from a reasonably wide throttle) the ICV is still wide open so provides some air so that effectively the throttle is still open somewhat, the ICV then smoothly closes back to a small opening over about half a second or so, preventing a sudden lurch. On some engines if you snap the throttle open briefly from idle the rpm will linger at about 1500 rpm before dropping again - this is the effect of the ICV opening wide when you snapped the throttle but remaining open for a brief moment after you released the throttle.

Modern EFI petrol engines on the overrun for example coasting down a hill with engine rpm held above about 1600rpm by the pull of gravity will actually shut off the injectors completely when you take your foot of the throttle, however there would still be a lot of engine braking (thus wasted momentum) due to the high vacuum. It is foreseeable in these conditions that the ECU decides to leave the ICV wide open while the injectors are cut off in overrun to reduce engine braking - this could account for a lower than expected vacuum reading.

This is speculation on my part though as I have not observed a low vacuum on my car on the overrun. If you have a Lexia you could measure the ICV opening percentage in those driving conditions to see if it is wide open.

[LFY]

Thanks for your answers specially Mandrake! You seem to be a very sophisticated man! Thanks for your nice explanations!

Did you measure this 400-500 mbar reading while the car was being driven coasting down a hill ?

I saw it in a car but i don't remember exactly. I will test it and let you know.

But there is something i know for sure. When you turn A/C on, MAP pressure increases to about 480 - 500 and ICV also opens wider (about 70 steps).

This is speculation on my part though as I have not observed a low vacuum on my car on the overrun. If you have a Lexia you could measure the ICV opening percentage in those driving conditions to see if it is wide open.

Yes, the Stepper Motor will be wider open at higher revs.

I have both Lexia and PP2000.

This is my car @ idle (number of steps = 21)






This is my car @ 2400 RPM (number of steps = 45)






But the thing i don't understand is that:

We can think of ICV as an extra way of entering air into the engine. So, we can almost say:

Throttle totally Closed, ICV Wide Open = Throttle a little Open, ICV a little open

A little open throttle (say 1300 RPM) as said in the first post produced 294 mbar of MAP pressure.
BUT! Closed throttle and almost Wide Open ICV results in around 480 - 500 mbar of MAP pressure (the case with A/C on - @ idle).

In both cases we have a narrow opening for air to enter the engine but MAP readings are very different! Why?


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How can i interpret MAP data according to your formula?

For example, my car MAP reads about 310 mbar @ idle while my friend's car (same car) reads about 350 mbar @ idle.
Both cars were tested in same place, same conditions. Ambient pressure was about 852 mbar. (Elevation = 1500 meters above sea level)

According to your formula it means that my car produces about 16 inches of mercury but my friend's car produces about 14.8 inches of mercury.

16 > 14.8

My car produces more vacuum.

Does it mean that my engine is better in terms of piston rings condition, valve (cam belt) timing, valves sealing, vacuum leak, etc?

How do you interpret MAP data?

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By the way, i love your car! ES9J4! What an engine! I read in a German magazine road test that the MT version could reach 100 km/h in just 7.4 seconds!
That's fast indeed even by today's standards!