Barometric correction

[Erik88GT]

What exactly does the barometric correction do in MS in regards to final pulsewidth? Does anyone know the calculation?

I have an AEM EMS on my other car and some of the guys on their forums claim that barometric correction is not necessary on MAP sensed engines. People actually seem to be split 50-50 on this, but the so called "EMS engineer" says it's not necessary.

Here's a link to the discussion:
http://forum.aempower.com/bbs/viewtopic.php?t=11157

Also, what about the correction for air temperature? Is that just gas law?

Thanks for any input.

Erik

[efahl]

Erik88GT wrote:

> What exactly does the barometric correction do in MS in regards to final
> pulsewidth? Does anyone know the calculation?

Erik,

It's

barocor = 100.0 * (1.0 - 0.0047 * (pressure - 100.0))

where pressure is expressed in kPa. This was roughly derived from (if
I remember right) an old SD fueled 'Vette motor, would be different for
different motors, but it's probably close enough for most uses.

> I have an AEM EMS on my other car and some of the guys on their forums
> claim that barometric correction is not necessary on MAP sensed engines.

They are wrong, unless they think retuning because of altitude changes
is appropriate. Here's the Cliff's Notes version (Lance probably has
this stashed, I should find it so I don't have to retype it):

1) MAP only gives an indicator of conditions on the intake side of the
engine.

2) Changing barometer/altitude affects what's happening on the exhaust
side. Imagine putting your tail pipe way up in the sky, into a perfect
vacuum, where it would extract the end gasses very effectively. It
should now be obvious that dropping the barometer improves VE on the
exhaust side of the motor.

3) Unless you have something to correct for this improvement in VE,
you'll go lean as the altitude goes up, therefore baro correction
richens the mix as the altitude goes up and the barometer drops.

HTH,
Eric

--
Eric Fahlgren http://www.not2fast.com/



(posted by email)

[colinnwn]

E> What exactly does the barometric correction do in MS

Oh I do savor when this comes up. I think your questions will be
answered by the link. Feel free to point the AEM people there too.

http://thegebharts.com/megabaro.html

--
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Colin. mailto:colin@thegebharts.com

My wicked email client is http://www.ritlabs.com/the_bat/index.html
An operating system for strong constitutions http://debian.org/
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(posted by email)

[Erik88GT]

Thanks guys, that's what I was looking for!

So, barocor = 100.0 * (1.0 - 0.0047 * (pressure - 100.0))

then for a pressure of 85kpa:

barocor = -1493 ?

[efahl]

Colin Gebhart wrote:

> http://thegebharts.com/megabaro.html

Ooh, nice, Colin. I'll have to bookmark that.

--
Eric Fahlgren http://www.not2fast.com/



(posted by email)

[efahl]

Colin,

If you want a pdf of the patent referenced at the bottom of your MegaBaro page, grab this:

<http://www.not2fast.com/patent/p5522365.pdf>

I've got a cgi script that builds pdfs given a patent number, so I can snatch them on a moments notice.

Eric

--
Eric Fahlgren http://www.not2fast.com/



(posted by email)

[Erik88GT]

barocor = -1493 ?

So what relevence is that number?

Oh, and the AEM "engineer" isn't budging...
http://forum.aempower.com/bbs/viewtopic ... 6776#66776

And he says: "I don't know if I would take what mega squirt has to say as gospel. "

[Erik88GT]

barocor = 100.0 * (1.0 - 0.0047 * (pressure - 100.0))

= 100 * (1.0 - 0.0047 * (85 - 100.0))

= 100*(1.0 - 0.0047*(-15))

=100*(1.0 + 0.0705)

= 107.05%

Well, it's good to have that info on this board for people to search for, anyway. I never did like the old yahoo board.

Okay, that equation makes more sense. Thanks.

I don't know about trolling, I'm just trying to motivate some people to get to the bottom of this, that's all.

I own both sytems, and they're both great, IMO.

[colinnwn]

Hi Erik88GT,
My comments are embedded
Wednesday, January 12, 2005, 11:44:22 AM, you wrote:

E> Oh, and the AEM "engineer" isn't budging...
E> http://forum.aempower.com/bbs/viewtopic ... 6#66776'89

I think you have done a great job of decimating your debaters.

When JS was talking about oxygen content not changing, you could have
laid the term "partial pressure" on him and see how he responded.

Several seem to be arguing on grounds that miss the point of absolute
versus differential pressure. Like when JS mentions flow is all about
the delta. I would argue that is a poor comparison because flow
calculations are based on constant restrictions. An engine is not a
constant restriction. It is an iterative process with the pressure
signals from each side cut off from each other during most of the
cycle (ie the closing and opening of the valves.) The process lends
itself to using absolute values rather than differential values.

I found it amusing when gw342 was talking about 100% efficiency and
WOT. Engines do not necessarily convert energy to work most
efficiently at WOT. I think he was getting the terms "efficiency" and
"maximum power" confused.

BLKMGK thinks the AEM is compensating for altitude when it reduces
fueling based on its measured intake pressure at altitude. He is half
correct because it is compensating for the altitude change on the
intake side. He goes on that exhaust backpressure differential would
be negligible unless the exhaust is free flowing. In fact it is
reverse.

When the cylinder is evacuating freely, it is at some absolute
pressure and is venting into an atmosphere that represents its
backpressure. When you add a tailpipe/muffler, it acts as a throttle
that increases the "atmospheric" pressure the cylinder sees the same
way the intake throttle decreases "atmospheric" pressure. The more
restricted the exhaust tract, the higher backpressure and greater
effect of increased VE with altitude increase.

If JS's pro cars had free flowing exhausts, that would explain why
they ran satisfactorily without baro correction. He and BLKMGK both
mention baro correction being unnecessary in closed loop, and of
course this is true. But the point of baro correction is to get the
fueling as close as possible when you are not in closed loop. There is
no way to tell what you are missing in power until you try both and
tune the baro correction for the engine/exhaust it is being used on.

--
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Colin. mailto:colin@thegebharts.com

My wicked email client is http://www.ritlabs.com/the_bat/index.html
An operating system for strong constitutions http://debian.org/
You are richer today if you have laughed, given or forgiven.
-unattributed



(posted by email)

[Erik88GT]

Thanks a lot Colin, very insightful.

Too bad that turned into an argument, I was really just wanting to figure out how to setup my table (it's a 2d graph of fuel% vs baro pressure) with the proper numbers. I doubt it will ever happen or be correct. The AEM doesn't take into account injector open/close time so doing percent +or- would likely be a futile attempt, anyway. They have both the baro and air temp correction zeroed out without even providing guidelines of what they should be, it's crazy. And they went to the trouble to put an internal baro sensor in the thing that is constantly updating, but not one of their "engineers" recommends using it! The AEM has more bells and whistles than you could ever ask for, but I think the people that made it know more about software programming than actual engine dynamics. Don't get me wrong, my AEM car runs excellent, and it plugs into the stock Mustang wiring harness. But it just bugs me to not have the baro and air temp correction.

I can't wait to get my MS car back running again (and add MSII to it) so I can tune a system that makes sense again...

[Erik88GT]

I can't win that argument, it's like trying to argue that the world is round 500 years ago. We all know it's true, but where's the evidence?

I need more sources for baro correction on map sensor engines. Anybody got any? Books, anything?



Thanks.

[colinnwn]

Hi Erik88GT,
My comments are embedded
Sunday, January 16, 2005, 11:58:14 PM, you wrote:

E> I can't win that argument,

Reading over the new posts, I immediately choked on this "If the atmospheric pressure changed today to 200kpa, it would be no different than the guys that have been running 14.7psi of boost, and engines power outputs would double!"

Um, nope. If atmospheric went to 200kpa on an engine, it would make less power than a 100kpa turbocharged engine, after compensating for temp increase of compression. This is because the turboed engine has 100kpa lower pressure on the exhaust helping the cylinder evacuate fully. You would have to put a 100kpa vacuum on the exhaust of the 200kpa atmospheric engine to make the power output comparable.
And "Ask anyone who knows about planes, and they will tell you that they don't have to trim any fuel for altitudes. Can you imagine the guy in an old turbo pro ascending from take off trying to keep up his fuel trim as he climbed to 14000'....yeah.....right! Since his manifold pressure was constant, he didn't have to touch any "trims"

From someone who has actually flown piston airplanes, I can tell you every carburetor plane has a fuel trim lever and you have to use it, turbocharged ones included. Now turbo-normalized engines may not have to use it until they are no longer able to maintain 100kpa. But that is mostly because a human is unable to trim fuel accurately enough to take advantage of the higher ve of the engine as it ascends.

"Blip your throttle at idle, and how much manifold pressure do you get? 5 or 10% throttle can easily net 100kpa, so can 100% throttle, but the fuel required is different,"

Exactly, blipping the throttle can get you to atmospheric briefly as the engine lags to accelerate to the new power setting. But hold it there till the engine quits accelerating and the vacuum will recover. This does nothing to prove lower pressure in the intake is any different whether it is because of an intake restriction or lower atmospheric pressure.

E> I need more sources for baro correction on map sensor engines.
E> Anybody got any??

It is going to be hard for you to find this information because manufacturers don't volunteer what is going on in their ECUs. You will have to look for people who have attempted to dissassemble the raw hex code like here (read around where the word baro appears)

http://www.diy-efi.org/diy_efi/archive/archive_num_60

Or in patent material like at the bottom of column 8 here

http://www.not2fast.com/patent/p5522365.pdf

Or you could try to infer information by looking at diagnostic charts for OBD-II trouble codes (which have several baro error codes)

http://www.ford-trucks.com/article/idx/ ... Codes.html

Or by looking at what other aftermarket efi makers are doing.

http://www.mrgasket.com/ftp/pdf/EMIC.pdf

But as you pointed out in your last post on that forum, I don't know if this is worth your effort. This "engineer" sure seems to think he has a solid science background to argue his real world perceptions. But in the end, it sounds more like pseudoscience.

--
Later, http://thegebharts.com
Colin. mailto:colin@thegebharts.com

My wicked email client is http://www.ritlabs.com/the_bat/index.html
An operating system for strong constitutions http://debian.org/
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nothing good in war. Except its ending. -Abraham Lincoln



(posted by email)

[dsleepy47]

If you want some light reading You can go here: http://eecanalyzer.webhop.net/ and click on the GUFB strategy document. It is Ford's strategy document for their EEC IV controllers. It covers both MAF and SD systems. If you look at page 357 (of 591) you will see their percent load calculation (their version of a VE table), if you follow the calculation lowering BP will raise percent load putting you in a richer part of the fuel tables. This BP compensation is used on SD and MAF cars even though MAF compensates for BP on the inlet side.

Deems

[Erik88GT]

Can we say with certainty that no one has complained about the MS baro correction being wrong in actual usage? I mean, I haven't read of anyone having a head scratching moment with regards to the baro correction, it always seems to work correctly for everybody.

And the same correction factor should apply to every piston driven, gasoline engine with a map sensor, right?

[efahl]

Can we say with certainty that no one has complained about the MS baro correction being wrong in actual usage?

Erik,

No, indeed we have had people that had problems with the current implementation. I recall one guy who tried to use it on a race car with an open exhaust and high CR, where the hard-coded correction factors were overcorrecting badly. I think that the ability to tune the algorithm (as in MS-II) is crucial to making it work in general, as the chamber size and exhaust plumbing should impact the required correction quite a bit.

Eric

[Erik88GT]

Yeah, I can see after browsing some of the PDFs posted that there is a lot more to it than most people think. Although I'll bet the MS baro correction works good for most people that run normal street car setups with mufflers, etc.

That's cool that you'll be able to tune it in MS-II.

Btw, Colin shut a lot of people up and changed some minds over on the AEM forums. Good show Colin!

[colinnwn]

Hi Erik88GT,
My comments are embedded
Saturday, January 22, 2005, 1:22:48 AM, you wrote:

E> Yeah, I can see after browsing some of Colin's PDFs

I wish I could take credit for finding those patents; but in fact they were directed to me through an intermediary. And I don't mean my guardian angel told me about them ;)

--
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Colin. mailto:colin@thegebharts.com

My wicked email client is http://www.ritlabs.com/the_bat/index.html
An operating system for strong constitutions http://debian.org/
Justice is never advanced in the taking of a human life. Morality is
never upheld by legalized murder. -Coretta Scott King



(posted by email)

[deepnsteep]

Sounds like this topic might be kind of a can of worms after reading this post and the link to the other.......but I have some questions regarding baro pressure correction and MS on a 2-stroke. This may need a new topic, but for the time being...........

Let me make sure I have this straight first. I guess since we're using MAP and RPM to estimate what the VE is, this is why we need baro correction. Because the ECU achieves alt. compensation by going to a different site on the map, the VE value the ECU picks is going to be off from what it actually should be. Hopefully my logic is right.....at the same RPM and outlet pressure, an engine operating at lower intake baro pressure will have lower MAP value???

That being said, I'm going to be putting MS on a 2-stroke snowmobile. Since 2-strokes achieve a pretty good shot of VE from the returning exhaust pressure pulse, would it still be useful to use baro correction???

Thanks!

Levi

[efahl]

I guess since we're using MAP and RPM to estimate what the VE is, this is why we need baro correction. Because the ECU achieves alt. compensation by going to a different site on the map, the VE value the ECU picks is going to be off from what it actually should be.

Levi,

You've got half of it. Yes, MAP and RPM combined with the VE table give us an estimated flow, but they only take into account the parameters of the intake side of the equation, assuming that the exhaust side has constant behavior. Well, that ain't true, the pressure on the end of your exhaust pipe can quite significantly change the VE (try artificially raising the apparent barometer by stuffing a potato up the tailpipe). When the barometer drops, the pressure differential from the cylinder to the end of the tailpipe increases, with a concurrent increase in VE (you get better evacuation, which then helps the intake stroke get more charge in).

Hopefully my logic is right.....at the same RPM and outlet pressure, an engine operating at lower intake baro pressure will have lower MAP value???

It will have lower maximum MAP (assuming naturally aspirated), but that's really of no consequence, since if you have the throttle at some position which gives 90 kPa, it doesn't matter what is the ambient pressure is, just that the manifold-side of the intake valve sees 90 kPa...

Eric

[mbohn]

barocor = 100.0 * (1.0 - 0.0047 * (pressure - 100.0))

where pressure is expressed in kPa. This was roughly derived from (if
I remember right) an old SD fueled 'Vette motor, would be different for
different motors, but it's probably close enough for most uses.

Eric:

This equation is hardwired into the barofactor4250.inc file, correct? So basically the raw map value is used to lookup into that file to produce the barocor variable?

Thanks, Mark