Elevation Correction

Tuning concepts, methods, tips etc.

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KenK
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Elevation Correction

Post by KenK »

9/07/16

Gents:
I am running an air cooled VW engine in an Off-Road car. The engine is pretty big as far as VWs go being 2331 ccs. Compression ratio is about 11.7/1. I am also running an open exhaust. I am also using the SD algorithm in the MSII system.

The system is setup as a sequential injection and am getting pretty close on the tune. Last week-end I was out at the desert and where we can get some pretty significant elevation changes as well as significant temperature changes during the day.

During the week-end, I noticed a couple of things that seemed counter intuitive to me. The first involved the MAT temperature correction. I noticed that when the MAT temp went from about 75 degrees to about 118, the engine went lean. The tune at that time was running the default curve built around the ideal gas law which I would think would or should be close. I fixed my problem or reduced the effect by making the curve flat between 80 and 120 degrees F.

The second issue I was working with was the elevation correction curve. The base elevation I was tuning at was around 2200 feet above sea level. During the runs in the day, I was going as high as 6000 feet above sea level. The elevation curve started out as the default (flat at 100). The engine went lean during the run. Real lean at the 6000 feet level. I made a couple of changes and a couple of runs but will need a couple more runs to get it right. I am increasing the table number (above 100) to correct for the problem. I will need to get the curve built to allow me to go the around 10,000 feet.

Intuition tells me that the motor should go rich not lean. Less air, to much fuel without correction. The documentation indicates that the elevation correction and for that matter temperature corrections are just a simple multipliers. Is it possible that the effect of the lower pressure on the open exhaust has more influence on the air flow through the motor than the reduced pressure on the intake side of the motor?? What are your thoughts??

Ken
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Re: Elevation Correction

Post by racingmini_mtl »

The SD algorithm uses MAP so the decreased atmospheric pressure is compensated for on the intake side but not on the exhaust side hence the leaner reading. That what the baro correction is for.

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KenK
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Re: Elevation Correction

Post by KenK »

09/08/16

That is kind of what I thought. But I found it hard to believe that the the exhaust effect would be so dramatic. I suspect that the same effect would also be true for the ambient temp compensation as well.

Would the same effect be true and dramatic with a turbo charged engine??

When I took on this project, I knew this would be a learning experience. I have not been disappointed.

Thanks again for your input.
KenK
kaeman
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Re: Elevation Correction

Post by kaeman »

I know what you mean, the first time I took my vehicle to reno my engine ran way lean.. 17 to 1 afr. I live at 200 feet and reno is above 5000... its completely backwards from everything I had ever experienced. You will have to tune the baro correction and adjust it at different elevations then it will make the needed adjustments for you..
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Yves
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Re: Elevation Correction

Post by Yves »

I thought that, when using a second map sensor as baro, the system automatically corrects for altitude changes. Correct me if I'm wrong.
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Re: Elevation Correction

Post by jsmcortina »

Yves wrote:I thought that, when using a second map sensor as baro, the system automatically corrects for altitude changes. Correct me if I'm wrong.
It uses the numbers you give it in the baro curve, yes.

James
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Yves
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Re: Elevation Correction

Post by Yves »

jsmcortina wrote:
Yves wrote:I thought that, when using a second map sensor as baro, the system automatically corrects for altitude changes. Correct me if I'm wrong.
It uses the numbers you give it in the baro curve, yes.

James
Not sure I understood what you want to say. I think that what you are saying is that when the baro correction curve is set to 100% it follows the internal correction of the code. The barocurve is a correction on top of that...

I'm not seeing what purpose it could have since the density of air is dependant on altitude and air temp mainly, so I assume that a theoretical calculation would be pretty accurate.

I think KenK uses initial map for baro. So if he makes large changes in altitude when driving and never stops, the car will not correct. A second map sensor used as baro sensor would correct this automatically.
TheSilverBuick
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Re: Elevation Correction

Post by TheSilverBuick »

At the end of the day it's a tunable parameter either way. Both my cars use curves very similar to this one. This is from my Firebird. After the rate and total vacuum values were changed to 0/0, the curve seemed to have straightened out (they had a x^3 shape before). I live at 78-80kPa atmospheric, and typically only go down hill from here, but have seen 64kPa in the Skylark. Also to note on mine, sea level is at 120%, which is quite a change. I've been running this way for years.

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Re: Elevation Correction

Post by jsmcortina »

Yves wrote:Not sure I understood what you want to say. I think that what you are saying is that when the baro correction curve is set to 100% it follows the internal correction of the code. The barocurve is a correction on top of that...
No, that's incorrect.

With current firmware there is no hidden built-in correction - the curve is the correction. So if you set it to 100% you are getting no adjustment for barometric changes.

James
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Re: Elevation Correction

Post by Yves »

:o

That way you will chase your tail imo as air density is both dependant on altitude AND air temp...

Edit : I found a way to set them. There was a table (see below) that gave air density corrections :

http://www.greenheck.com/library/articles/12

And then I found a site that would let you calculate the air pressure in Pa at a certain altitude.
http://www.mide.com/pages/air-pressure- ... calculator

So with these things I created a barocorrection curve.

Something strange I wouldn't have expected was that the curve seems to want to increase fuel while being at lower air pressures and lower it while at higher air pressures.
I found however some comments on old threads here on the forum that seem to indicate this is correct.
http://www.msextra.com/forums/viewtopic ... 0&start=20
As Kaeman stated in that thread, calculating with the above I also get 102.5% barocorrection at an air pressure of 98 kPa.
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Re: Elevation Correction

Post by jsmcortina »

Remember that the intake side is already allowed for by the MAP sensor. The default air-density correction curve matches the ideal physics.

The baro curve is taking into account other effects due to barometric pressure changes. Be they exhaust back pressure, crankcase pressure or flow effects through the throttle.

James
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Yves
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Re: Elevation Correction

Post by Yves »

You've lost me there. I seem to read here that in older firmwares the curve was internal to the ecu and could not be changed. Then I read that in the newer version the correction follows the barometric correction curve under general settings...

Assuming you have both a map sensor and a seperate baro sensor. What does the ecu use to correct for instance for elevation changes and or changes in baro due to good/bad weather.

BTW : I use ITB load, which is a mix between map and tps.
Dennis_Zx7r
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Re: Elevation Correction

Post by Dennis_Zx7r »

Yves wrote:Assuming you have both a map sensor and a seperate baro sensor. What does the ecu use to correct for instance for elevation changes and or changes in baro due to good/bad weather.
The way i understand the formula in the manual, baro-corr is just multiplied on top of the calculated fuel. So if it's 100%, it's multiplied with 1, or any other number in the table at the corresponding kpa-value. No other effects.

I also use ITB and am currently working on this. If you've got any sort of MAP in the calculation, the right way seems to richen it up with decreasing ambient pressure. I've only got about 3 hours of logdata collected between 90 and 100kpa, but there definately is a visible trend. Without baro-corr the leaning is even visible at 100% TPS with reduced pressure (Multiply Map). For my case, i would need about 102% to 104% barocorr at 90kpa (100% at 98kpa) - still gathering data.
I think setting baro corr to values below 100% with decreasing ambient pressure may be the way for AlphaN-Pure (without MultiplyMap) though, just from a mathematical view. But I guess the majority uses SpeedDensity, AlphaNHybrid or ITB (With multiplyMap).
Maybe TheSilverBuick uses AlphaN?

What I think many people miss is that the mixture ultimately DOES lean when theres a Map-factor in it, even when setting a baro-corr higher than 100% (up to a point).
Example: If we've got a VE-value of 100 with multiply map (AlphaN-Hybrid or only the Map-value using SpeedDensity), at 90kpa ambient pressure the effective VE-value is 90 instead of 100 (all other corections neglected for simplicity). If we assume our VE-table was tuned at 100kpa (100kpa aus 100% baro-corr) and set baro correction to 105% at 90kpa (richening it up) we'll end up with an effective VE-value of 100*0,90*1,05 = 94,5! Compared to our base value of 100, we effectively reduced the leaning the Map-sensor on the intake side was responsible for, but still leaned the mixture compared to 100kpa.
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Yves
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Re: Elevation Correction

Post by Yves »

Well based on the info with the above mentioned sites, I got the following calculated barometric correction curve :
baro- Corr
78.2 120.0
84.4 116.0
90.8 112.0
94.2 108.0
97.7 104.0
94.2 108.0
99.5 102.0
101.3 100.0
104.5 96.5

If I take your example above at ambient 90 kPa, I would have 100*0,90*1,12 = 100,8. So 0,8% rich (value at 90,8...) So this would work out correctly if what you have written is correct.

I'm also running, multiply map.

In my case the baro changes from good to bad weather would mean a difference of + or + 2%. In AFR's that would mean going from 13.7 to 14.3

PS: the value at 78.2 is not correct, but 120 was the maximum allowed value.
Dennis_Zx7r
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Re: Elevation Correction

Post by Dennis_Zx7r »

With your calculated table, you would only compensate for the leaning effect of MAP - I don't think that's the way to go, because then you could just shut off multiplyMap and wouldn't need baro-corr, which doesn't seem to be the case.
In my example and with the data i gathered for my engine, I would set baro-corr to about 103% at 90kpa, which would effectively be 90,8% of my VE-value. I think this might be highly dependent on the engine, especially for the exhaust system, and I'm only talking about a NA-engine here I should add.
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Re: Elevation Correction

Post by Yves »

I think the effect on the exhaust system is small. What's the exhaust pressure ? 10 bar or so or the equivalent to 1000 kPA. 10 kPA lower backpressure is only 1% max.

The real thing in this equation is air density changes, which is what is used to calculate the amount of fuel to be injected. Volume is volume. So when an engine consumes 400 cfm at 100 kPa ambiant pressure it's going to be 400 cfm at 90 kPa ambient pressure. The difference is however in the density. 400 cfm at 101.325 weighs 1,29 kg/m³ and at 90 kPa about 1.122 kg/m³ so the density is reduced by 13%, all other factors being equal.
In essence this would mean you would have to reduce your fuel 13% at that point to get the same AFR. That was what I thought the whole system was doing before I read this thread.

Assuming the map measures 90 kPa, it would lean fueling with 10% and you would still be running 3% rich. Assuming that you have 1% less backpressure, you would still be 2% rich at that point.
In the thread I posted above this seems not to be the case. Kaeman apparently had to richen things up to get the same AFR...

Maybe I'm not smart enough to understand it.
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Re: Elevation Correction

Post by jsmcortina »

Yves wrote:Well based on the info with the above mentioned sites, I got the following calculated barometric correction curve :
baro- Corr
78.2 120.0
84.4 116.0
90.8 112.0
94.2 108.0
97.7 104.0
94.2 108.0
99.5 102.0
101.3 100.0
104.5 96.5
That sounds like air density correction not baro correction.

James
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Re: Elevation Correction

Post by Dennis_Zx7r »

@Yves:
Kaeman runs a turbo according to the signature, I'm only talking about my naturally aspirated engine. I neither have experience nor data regarding turbos.
My exhaust system (Akrapovic Racing 4-1, short, large diameters) and intake pressures are much more dependent on ambient pressure. I doubt a drop to 90% ambient pressure in my case would be as little as 1% exhaust pressure reduction.
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Re: Elevation Correction

Post by Yves »

Dennis_Zx7r wrote:@Yves:
Kaeman runs a turbo according to the signature, I'm only talking about my naturally aspirated engine. I neither have experience nor data regarding turbos.
My exhaust system (Akrapovic Racing 4-1, short, large diameters) and intake pressures are much more dependent on ambient pressure. I doubt a drop to 90% ambient pressure in my case would be as little as 1% exhaust pressure reduction.
No, a turbo 350 is the name for his transmission. He runs ITB's.
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Re: Elevation Correction

Post by Yves »

jsmcortina wrote:
Yves wrote:Well based on the info with the above mentioned sites, I got the following calculated barometric correction curve :
baro- Corr
78.2 120.0
84.4 116.0
90.8 112.0
94.2 108.0
97.7 104.0
94.2 108.0
99.5 102.0
101.3 100.0
104.5 96.5
That sounds like air density correction not baro correction.

James
James,

With all due respect, you're an intelligent guy obviously but if your explanation stays this cryptic we will probably never get this right.

Probably it's a density correction curve, but then again, read what I said in my previous post about it.
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