It uses it to fix an absolute pressure value upon startup so that the relative pressures in the intake manifold are adjusted for elevation. I think.

I've driven through some very steep elevation changes, and it really has helped to stop, shut the car off, then restart the car. (I ALWAYS wait for the CEL light to go off before starting any car I drive.) If the ECU doesn't constantly look at ambient pressure, it can get out of it's frame of reference and won't be able to compensate for elevation changes.

I don't know about how a tuned engine will respond to something like that, though. That's a very interesting question.

 

On a mega with a Dual Port Map sensor. In effect you would not need to stop and re-start the car. This is my understanding.

 

I really don't see why it would matter since the absolute pressure determines your fueling requirements, not the relative pressure.

The only thing I can see it being useful for is determining when to disable closed loop operation (you need a lot more throttle in Denver than in NYC).

 

I don't know what you guys think a barometer can do that a MAP sensor can't do.

Elevation and temperature just increase or decrease air density. In a fixed volume like a manifold less density means less mass, which results in less pressure, since pressure is force per area, and force is mass times acceleration (of gravity in this case). Now, the acceleration of gravity changes with elevation, which is what a barometer actually detects. But if a barometer can detect this, a MAP sensor should be able to detect this as well, as a change in the acceleration of gravity will just show up as a change in air pressure, as measured by the MAP sensor.

I've always wondered if an increase in air temperature also had the potential to increase manifold pressure as well, since the volume of a manifold is fixed, and therefore by the ideal gas law an increase in temperature should create an increase in pressure. Maybe Dan can clarify...

The only thing I can see it being useful for is determining when to disable closed loop operation (you need a lot more throttle in Denver than in NYC).

I think TPS parameters should account for this.

The only reason I can think of for the barometric pressure being relevant with a speed-density system is that exhaust pressure will change.

This is the only thing I could think of as well. But even if exhaust pressure was higher, all that would mean is more throttle was required, raising manifold pressure, which the ECU could determine the fuel requirements for via the speed-density method.

Quite a mystery this barometer is.

 

I think it would matter a huge deal cant explain it right now.
If it didnt matter why would Honda got thru the Trouble of Providing the extra sensor in the first place ?

 

It didn't in flat countries.

 

At the very least, it's a good diagnostic tool for testing purposes.

 

it alters the timing and fueling, mainly the fueling. Since the air is less dense at high altitudes, the sensor decreases the demand for fuel to compensate for the lack of air at any given load and throttle %.

A car tuned at sea level will lack a lot of power and torque. the sensor does its best to keep the car from running pig rich. Timing is also tweaked to compensate for the lack of fuel, in most cases, retarding the timing by as much as 5-10* depending on the vehicle setup.

 

Isn't that what the MAP sensor does?

 

i heard that you can upgrade to obd2 but i cant imagine how that would help tho...

 

I'm still thinking the MAP sensor is relative pressure drop compared to absolute ambient pressure set at startup by the Barometric pressure sensor.

 

If that was the case, why does the MAP sensor read the same as the barometer when you key on?

MAP = Manifold Absolute Pressure, not relative pressure

 

ok ok, i'll let the cat outta the bag....its a corporate gimmick to get you to spend more money. Yes, the map sensor does the same thing. Except for switching over to a different timing table, which is what the baro does.

 

Where are you getting this information from?

The only time I've seen a barometer used like that is for boosted cars, like with AEMPro. I've never seen it used in Crome or Neptune.

 

obd2 cars all have them, boosted or not. some obd1 cars have them.

 

I know they have them, but how do they use them? You're saying there's a correction table, but how do you know it's there and used in that manner?

 

in the crome/uberdata and the like, you cant adjust for it like you can other ecu's.

For example- in crome (and the others) you have a low/high ignition table, for the ford eec-iv you have base/sea-level/altitude ignition tables. the altitude table has less timing than the other two. I know im comparing oranges to apples here but for the obd0/1 honda world, you cant change it. Now, i dont mess with hondata/megasquirt/aem ems so i cant say that with 100% confidence.

But as far as crome/uberdata and the like, you cant adjust for it unless you tune at sealevel and then save another tune for high altitude. For the Ford eec-iv, i can adjust the 3 mentioned above independently.

Im pretty sure that didnt answer your question, sorry. lol

Edit: Just trust me, it retards timing and modifies fueling. Disabling it will allow you to run say 32* timing ALL the time whether your at sea level or high altitude. But doing so may cause poor performance.

 

from Manifold Absolute Pressure MAP Sensors

When the ignition key is first turned on, the powertrain control module (PCM) looks at the MAP sensor reading before the engine starts to determine the atmospheric (barometric) pressure. So in effect, the MAP sensor can serve double duty as a BARO sensor. The PCM then uses this information to adjust the air/fuel mixture to compensate for changes in air pressure due to elevation and/or weather. Some vehicles use a separate "baro" sensor for this purpose, while others use a combination sensor that measures both called a BMAP sensor.

i think they forgot to mention that this applies mainly to open loop

 

Let me put it this way...

If you know the manifold pressure from the MAP sensor reading, why do you need to know the barometric pressure?

 

I've messed with Crome, Neptune, and Hondata, and unless I missed a table somewhere, there's no correction for barometric pressure. The AEM EMS does have a correction table for it (if desired). That's the extent of my knowledge of publicly available calibration software, so I can't say for anything else.

If you're running alpha-N, then you absolutely need to have a correction table for barometric pressure, which is probably why AEMPro has it.

For speed-density, the only physical reason I can think of to use a barometric pressure correction table is the change in exhaust pressure. Backpressure will be slightly higher if you're running 900 mbar at part load than when you're running 900 mbar at WOT, so your fueling and timing requirements will be slightly different. Is the difference large enough to need a correction table? I don't know. Even if it is different enough to require a dedicated table, few if any people will actually be able to properly use it.

On the intake side, I don't see how it matters. I thought it might be relevant for a boosted engine because your isentropic efficiency changes with pressure ratio, but now I'm second guessing myself. Even if efficiency changes, you'll see the difference in the MAP and IAT readings, so you can correct accordingly. All you need to know is MAP, IAT, and VE to calibrate the engine from that standpoint.

 

i can tell you with confidence, baro and map sensors REDUCE timing.

Baro sensor reads the atmosphere pressure and sends a signal to the ecu to reduce timing by whatever the parameters/integers are set to adjust.

MAP sensor reads what is GOING into the engine by manifold pressure(atmosphere pressure usually 29.7 or so) plus the positive pressure (boost psi from s/c or turbo), then REDUCES timing accordingly.


Edit: if im getting this wrong, then i need to do more learning.

 

You're still not seeing my point though... why does the barometer reduce timing? Just knowing that it has an effect is useless for a tuner because you don't know how to properly use the table. What is physically happening with barometric pressure that the MAP sensor can't account for?

 

Do you guys just want to know why there is a Baro sensor in hondas and what it does? The main reason they put the baro sensor in any honda and most cars is because at sea level the pressure is 14.7 psi, and it changes with the elevation (you probably know this) and then the ecu uses that to compare to the Map sensor. Because the map sensor would be 14.7 psi (or 29.2 in/hg) if it didn't know the outside air pressure, the ecu sees how the vacuum in the engine compares to the atmospheric pressure. That is the main function of the baro, this is what I was always taught. Then it changes fuel and ignition values based on the pressure changes in elevation. I didn't know if you guys knew this or not, just thought I would post this up anyway.

 

We understand the difference between atmospheric pressure and MAP (at least I hope so), but what I want to know is:

1) Exactly how does the ECU use the barometer to change fuel and timing (is it just a table of correction factor vs. barometric pressure? an offset vs. barometric pressure? Does it just use the value for reference and nothing more?)

2) The only reason I can think of for the barometric pressure being relevant with a speed-density system is that exhaust pressure will change. How much of an effect does this have? Does it affect anything else for which the MAP sensor can't compensate?

 

Oh, gotcha. I was told it's a correction factor based on the barometer value (I remember seeing a chart), that's what I read in some Honda training manuals and was told by some master techs. Don't know exactly how it uses the data or what it does with the map sensor values. Honda also made high-altitude ECU's (4500ft and above), with a different Barometer and correction factor. They had them for the CRX HF, and some JDM h-series ECU's (don't know for sure, just the ones I've seen). That's all I really know, the reason there isn't much info on it is because it's not meant to be tested or replaced (replace whole ecu or nothing)so honda didn't need to make it clear about how exactly it worked and processed the data. I'm curious to know. :bigok:

 

barometric sensor works with map sensor in real time. meaning it will always adjust, unlike the old system where the ecu takes a reference voltage from the pressure present at the time when the ecu powers up. that reference voltage is what the ecu assumes the pressure will be throughout the time the engine will be running.

you dont lower the timing when pressure drops.

 

It would seem there is a total lack of clarity on this subject here.

Ok first lets understand how other ECUs use Baro. The three types of ECUs.
Then we can be in a better position to understand how Crome / Neptune uses it.

The three systems in relationship to how baro is used can be split into
a. Not used at all (A very popular ECU sold locally has this problem if the baro changes the complete tune goes for a toss and the tuner needs to be called in to re-map.)

b. Used only once at start up.
This is when the ECU has only one sensor. Or its designed to take a reading at start up.
And store this value as a reference value. If you find stoping and starting your car while driving thru the hills you have this setup.

c. Constant Live Update of Baro.
In order to use this setup you must have two map sensors they are typically called the DP (Dual Port) Map sensor. Which keeps one sensor port measuring baro pressure at all times and the other measures map. Any changes in baro are constantly updating your fueling. The Honda OBD1 pre-dates the Dual Port sensor so maybe that is why they put the 2nd baro sensor on the ECU board.

Second part what does the ECU do with this Baro value.
It controls your fueling. In particular your Enrichment circuit.
The Formula it uses is
The definition of GammaEnrich is:
gEnrich = Warmup x Closed Loop x AirCorr x BaroCorr
100 100 100 100

For a clearer understanding of Gama Enrichment read this document
http://www.megamanual.com/v22manual/ve_tuner.pdf

It makes perfect scene when you consider that the VE of your Engine is going to change based on Baro.

The reason I asked this question is because we have two tracks one at sea level and one at high elevation and many teams have serious issues when switching tracks.

There is still no clarity on how Honda uses Baro. But I get the feeling in Crome its a static value. In Neptune I see it gets updated. But there is no way of telling if the updates effect your fueling. Guess Ill figure it out the hard way. When we hit the track.
Worst case we will need to separate maps for each track based on Baro. Best case the OBD1 ECU or code factors it in.

 

It makes perfect scene when you consider that the VE of your Engine is going to change based on Baro.

This is still not clear in my head, and the Megasquirt document only presented its calculation based on the difference in barometric pressure. It didn't explain why it's a first-order relationship.

Why does it make perfect sense to you?

 

I agree with 36 DDD's

who gives a fuck what the atmo. pressure is. It'll change the max vacuum reading in the manifold but the map will still be able to see what the pressure is and fuel accordingly. Plus there's feedback from the o2. You'd have to give the car more gas, as in like going up a hill, but you still use the 0/WOT column of the map.

Now if we where talking about carbs where there jetted and the fuel is fixed they have high altitude adjustments, on motorcycles too.

 

You'd have to give the car more gas, as in like going up a hill, but you still use the 0/WOT column of the map.

You wouldn't be using that column since you're not at 1 atmosphere, but I get what you're saying.