How is Baro pressure measured?

[rally1]

Hi all, is barometric pressure measured from a sensor and if so where is the sensor? (Holden VE) Or is it calculated from something like MAP?

Thanks in advance for your reply’s.

[joecar]

Usually it's measured from the MAP sensor when key is turned on just prior to cranking/starting engine.

Also, in some cases, at WOT, if MAP sensor value exceeds the previously read BARO, then the BARO is updated with this new value.

Some vehicles may have a separate/dedicated BARO sensor (not sure which).

[rally1]

Thanks Joecar, thought it would be something along those lines.

[gmh308]

It also updates while driving the car normally. Have seen the key on baro number rise when coming down to sea level from altitude without any wot. No doubt due to lookups based on other sensor inputs. :)

[rally1]

We have a customer that’s asked us to look into building an LS based engine with twin turbos to take to Pikes Peak Hill climb, just wondering how if at all would baro update under the extreme elevation change? Or are we best to replace the E38 for a Motech ECU that has a external baro sensor.

[WeathermanShawn]

I think different OS's update BARO differently. I have done pleasure drives from 5000 MSL to 11,000 MSL and have only seen BARO update under 2 circumstances.

1. Key-on
2. WOT

I never saw BARO update as I climbed or under part-throttle. But, remember your MAP is constantly changing voltage with altitude, so the PCM is always properly calculating load. As I understand it, BARO is really only used as a reference to vacuum and injector flow. So, it may not really be an issue unless you are at WOT (which it instantaneously updates).

Different OS's and PCM's may give you more flexibility. But, again utilizing the LS1 PCM those are only the two times I found it would update.

That Pikes Peak climb would be a challenge, especially keeping your ECT/coolant working efficiently with altitude.

Good luck.

[rally1]

How does it get on when you are running boost? I would assume it can’t then update at WOT?

The customer would like around 1100hp at sea level, he has been there a few times with a 850hp Mitsubishi EVO that would not even start when they arrived there due to the height so they had to carry out some quick on the fly tuning which sounds like a recipe for disaster.

We don’t have many big mountains here in New Zealand so it’s going to be pretty hard to test any tuning. Really starting to think it won’t be worth the headach.

[redhardsupra]

hm, I think you just found one case for which GM's 'use MAP as BARO' idea doesn't work for.

What is BARO really used for though, other than MANVAC based tables, namely IFR? If it's just IFR, then just put a vacuum referenced fuel pressure regulator, and then your IFR table would become a constant value, making BARO or MANVAC irrelevant.

What other tables use BARO or MANVAC? I cant think of any others of the top of my head...

[joecar]

+1 on the MAP-referenced FPR.

I can't think of any other tables.

[gmh308]

How does it get on when you are running boost? I would assume it can’t then update at WOT?

The customer would like around 1100hp at sea level, he has been there a few times with a 850hp Mitsubishi EVO that would not even start when they arrived there due to the height so they had to carry out some quick on the fly tuning which sounds like a recipe for disaster.

We don’t have many big mountains here in New Zealand so it’s going to be pretty hard to test any tuning. Really starting to think it won’t be worth the headach.

A couple thoughts...

If you are running constant boost through a boost controller your intake air mass flow should be constant regardless of MAP. (?) At least until the compressor runs out of the ability to do this or gets so far off peak efficiency that the intake charge gets to hot for the intercooler to extract the heat as the altitude rises and the air thins.

plus maybe if you ran a MAF maybe you might not have the same challenges keeping the mixture right as the ECM would "know" the mass air flow vs looking it from the VVE which will no longer see the right MAP.

Just some thoughts. :)

[redhardsupra]

If you are running constant boost through a boost controller your intake air mass flow should be constant regardless of MAP. (?)

the formula for airmass clearly states that airmass depends on VE, displacement, MAP, and TEMP.



plus maybe if you ran a MAF maybe you might not have the same challenges keeping the mixture right as the ECM would "know" the mass air flow vs looking it from the VVE which will no longer see the right MAP.

MAF is not a solution for a 1000hp car, as you're gonna hit the 512g/sec limit. granted, it's a GM specific limit, and even that is going away nowadays with the zr-1 spec ECUs with all the upped limits for air and fuel.

[gmh308]

the formula for airmass clearly states that airmass depends on VE, displacement, MAP, and TEMP.


MAF is not a solution for a 1000hp car, as you're gonna hit the 512g/sec limit. granted, it's a GM specific limit, and even that is going away nowadays with the zr-1 spec ECUs with all the upped limits for air and fuel.

Oops I meant atmospheric not MAP. Under constant boost pressure and constant intake temp, air mass flow into engine should be constant for given throttle/rpm. Referenced to MAP at at start up this should run fine...though I am not thinking about this too hard. :)

On the 512g/s MAF, yes but most 08 OS's support at least 2 or 3 kg/s MAFs (LS7 being an exception). Most '09 OS's support same hi flow MAF + 16g/s injectors, so if it is a simple matter of using an 08 OS.....

[joecar]

Here is an example of BARO being updated from 88kPa to 104kPa when boost kicks in (see the log file):

3000-RPM-max-and-setting-codes-03-Z06-with-Magnuson-at-150kPa (http://forum.efilive.com/showthread.php?14119-3000-RPM-max-and-setting-codes-03-Z06-with-Magnuson-at-150kPa&p=126540#post126540)