[ddd4114]

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.

See, that's different because that's completely wrong. With a speed-density system, your timing advance and fueling will always change with elevation because your MAP sensor reading changes with barometric pressure (at WOT).

 

[mightyjoecivic]

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.

 

[ddd4114]

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?

 

[ZCHONDACRX91]

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.

 

[ddd4114]

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?

 

[ZCHONDACRX91]

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:

 

[koji_kabuto]

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.

 

[rdc]

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.

 

[ddd4114]

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?

 

[guest]

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.

 

[koji_kabuto]

i think one factor thats being neglected is that map sensor have a sealed vacuum reference behind the diaphragm. some are bled straight to the air but most are sealed, so if th pressure doesnt change the other side of the intake manifold then another sensor must be used as a control, this is then the barometric sensor. wot open loop disables o2 feedback as per 1lambda isnt the fastest for acceleration. standalone systems have full time o2 feedback support thats probably why they implemented the barro correction with the o2 feedback at the same time

 

[guest]

i think one factor thats being neglected is that map sensor have a sealed vacuum reference behind the diaphragm. some are bled straight to the air but most are sealed, so if th pressure doesnt change the other side of the intake manifold then another sensor must be used as a control, this is then the barometric sensor. wot open loop disables o2 feedback as per 1lambda isnt the fastest for acceleration. standalone systems have full time o2 feedback support thats probably why they implemented the barro correction with the o2 feedback at the same time

right, at wot o2 isn't used. I was making the point that if your running an o2 it picks up slack in a tune.

 

[hotrodj]

SO just make a tune for each track??? Sounds simple enuff. Or use a wb o2 and adj fueling accordingly if its changing at the different altitudes. Just my .02 cents lol

 

[ddd4114]

i think one factor thats being neglected is that map sensor have a sealed vacuum reference behind the diaphragm. some are bled straight to the air but most are sealed, so if th pressure doesnt change the other side of the intake manifold then another sensor must be used as a control, this is then the barometric sensor.

What do you mean? If it's not sealed, wouldn't it not be an absolute pressure sensor?

 

[ddd4114]

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.

 

[koji_kabuto]

What do you mean? If it's not sealed, wouldn't it not be an absolute pressure sensor?

you have to understand that if you use the surrounding air as a basis for map reading, ecu will inject fuel that it is programmed to do.... regardless of elevation, as far as i know they are tuned for sea level.

high altitude is to fewer molecules of air in the same volume, this volume is the factor that dictates the map sensor, map sensor cant tell how dense air is. and we all know that stoich in sea level is 14.7 parts air mass and 1 part fuel mass ie. 14.7 kg of air and 1 kg of fuel. a 10cubic meter 10k ft air is not as heavy as it is in sea level but they have the same volume. use a non baro correction ecu in these condition and it will run rich.its just like mr. guest pointed out, carburated engines needs jet adjustment when in high altitude. same applies to efi.

 

[ddd4114]

That's the whole point of a MAP sensor. It reads absolute pressure, not relative pressure, and absolute pressure is what determines fueling. If it doesn't have a static reference pressure, it can't do its job.

 

[xile6]

To much to read but I'm going to throw my 2 cents in here. The senor is used to find out how much above sea level you are. The higher you go up the less oxygen in the air. So the ecu has to correct the fuel and igntion time . There was one in my p06 and p28. I don't know about my p2p because there not point in opening it to me.

 

[ddd4114]

I think you're all confusing the purpose of a MAP sensor with a barometer.

 

[Senoryandel]

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.