Elevation Correction

Tuning concepts, methods, tips etc.

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suberimakuri
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Re: Elevation Correction

Post by suberimakuri »

Sedd, perhaps consider running percent baro and handling the map variance effect in your baro table along with everything else the baro correction is handling (back pressure).
Trial and error.
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Re: Elevation Correction

Post by Yves »

sedd wrote:I recently logged a trip from 450 feet elevation to 6400 feet elevation. My baro sensor went from around 99 to around 78. I have not been able to figure out all of the data yet. I am trying out the new Megalog HD viewer, getting used to it still. Mostly I want to make a post so I can find this set easier.......... I don't think I can add much to this yet.

I used a set of baro corrections that was similar to ones suggested in the posts. My afr error was pretty small with my baro correction that I used. I think my baro correction is pretty close.

I had set the limit for correction at 118.8% and 85 baro. I used 100% at 101.3 baro. My curve was almost linear.

A quick look shows that for idle at elevation and baro around 78, the Idle valve steps opened to 30, which is 15 more than when at around 94 baro. It appears there was not enough air flow past the throttle body so the steps had to open to flow more air? So I assume a slightly higher map due to the larger steps?

I didn't see a linear change in the map at idle that follows the change in elevation baro. I show 46 map at idle and 78 baro with 56 map at idle and 94 baro. I get a 10 map change with a 16 baro change. Possibly the Idle valve steps account for this, not certain of that.

I need to look at the temperature change yet, it was a lot cooler ambient.
It would be logical that the higher you go, the more idle steps you need in order to inhale the same grams of air so the mixture burn is the same. The normal situation would be that the engine would inhale less grams in air and therefor correct the fuel weight accordingly. So less power is produced.
However, if you open the throttle air (or increase airflow) resistance to flow decreases as well so you could end up with an increased VE at the same time, necessitating more fuel. I supposed that might be the reason some need more fuel the higher they go. Just thinking here.

Good thinking on the idle valve position.
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Re: Elevation Correction

Post by sedd »

See the post: Expanded Baro Correction Limits for more details on this subject.

The values below are for a naturally aspirated engine. I have large exhaust pipes so my calculations show 0.5 psig backpressure at around 3200 rpm, my approx. cruising rpm. Back pressure affects the equations used for the values noted below. The curves for various backpressure give very close values so there is not significant errors by choosing one over the other.

Based on my experiments and evaluations of the data from the climb to 6300 feet, I am going to use the following values. I chose 60 as the minimum baro because that is the likely lowest value in the US. My baro correction is set for 100% at sea level which is 101.325 baro. I wanted to get this table set so I can tune at any baro I want and it will still be correct or at least close enough to prevent large errors. I doubt this accounts for every factor that baro can affect, but possibly these are the major contributors? EGO should correct for these. These are shown to correct for baro with minimal error in afr and/or call for ego correction over the entire trip, approx. 98 to 80 baro.

60 166.6
70 143.5
80 126
90 112.3
93 108.8
96 105.4
99 102.3
101.325 100
104 97.5

The values I found from my logs match pretty closely to those predicted by the equations. I will try to simplify many pages of others work below.

I found that the ratio of exhaust pressure / intake pressure is roughly constant, and that was what the data shows. This rough ratio being constant is noted in the paper I found the equations. My actual values of map matched predicted values for changes in map for the same intake flow. I assume the same idle rpm needs same air flows. That should mean the same cylinder filling and same backflows, and that means the same VE filling at any elevation.

Basic equation is: (exhaust press/baro pressure)power 0.6) / (intake pressure/baro pressure) This solves for the changes in the filling for the cylinder and that affects VE. Spent gases left over in the cylinder affect the VE and the amount of spent gases changes with the baro.

There are two effects from back pressure = backflow, one from exhaust gases entering the cylinder from the exhaust pipe thru the exhaust valve. the other is from the cylinder sending combusted gas into the intake manifold. VE is more dependent on the intake backflow thus the top term is to the power of 0.6. 0.6 was found to be the most accurate value for natural aspirated engines per those doing testing. If one (1) is used for the power, the equation simplifies to the ratio of exhaust pressure / intake pressure.
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Re: Elevation Correction

Post by Yves »

Could you explain that spreadsheet ?
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Re: Elevation Correction

Post by sedd »

thanks for the interest. this spreadsheet needs review by others to figure out if it is accurate. The math gets a bit tricky and that is the part I am not certain of. the spreadsheet uses proportional ratios noted in the article I based it on. reading the article is interesting but a bit over my level of understanding. The spreadsheet tries to show how the backflow changes with baro. So although the equations appear to reflect my engine I still have some doubts.

one of the tabs calculates the backpressure, that is a factor in the equations on the other tab. at idle you have less backpressure and of course much more at higher rpm. I added the graphs to show the effect of backpressure and you can see this does change the values but not dramatically. I chose to use an in between value.

the spreadsheet calculates the equation that reflects what happens to the backflow. it reflects the change to VE for various barometric conditions. Mueller Pr2 is the calculated correction for the VE.

you have to enter the exponent to use, that changes depending on how much you feel the intake pressure backflow is more of a factor than the exhaust backflow. the team that provided the equation stated that 0.6 was a good value to use for natural aspirated engines.

I found in real testing that the intake map (pmap on the spreadsheet) reduced in value with baro. My theory is that the baro calc needs to reflect the same rpm and volumetric efficiency so I decided to use the same VE factor for the calculations. I figured the engine would run at the same rpm if it had the same volumetric efficiency. I used the solver program found under Data/drop down, to change the pmap value and solve for the same VE factor (Mueller Pr2 ) that was found at 101.3 .
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TheSilverBuick
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Re: Elevation Correction

Post by TheSilverBuick »

Looking at your MSQ, I wonder if mine is reversed of yours because I run %Baro on my fueling and you are running Speed Density. That's the only real difference I see in our settings. I run %Baro on both my cars because my normal tuning elevation pressure was 80kPa, so the only way to tune WOT at home for sea level is to use %Baro.

Have you looked at your actual pulsewidths at a given AFR at different elevations/baro pressures?
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sedd
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Re: Elevation Correction

Post by sedd »

Silver, I don't understand the question about pulse widths?

I will think about the % baro versus speed density and see if the math makes sense to get to the same spot. I figure speed density only uses map on the intake side to get an adjustment for the flow change. I will have to look up the factor for % baro, is it baro / map or map / baro?

I was trying to use a more standardized approach so that you could use a common set of factors and tweak them for an engine. Similar to the temperature compensation.

I did have to adjust the program to allow for more range in my settings, is that where you had issues in your use of speed density?
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KenK
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Re: Elevation Correction

Post by KenK »

After many months, I have now had a chance to run my car at a location where I could do some elevation tuning testing.

During the testing, I ran the car from about 1500 feet ASL to over 6500 feet ASL. Although the temperature did change, the change was not significant (about 10 degrees F.) and I have already setup a temp correction curve (MAT) from about 40 degrees F to over 118 degrees F.

I my case I did the temperature curve was developed at a fixed elevation and under a fixed load and at a fixed RPM. The curve as seems to work pretty well.

Now, when I did the elevation testing I did OK until I got above about 4500 feet ASL. Above that, I ran out of room with the correction curve limit set by default at 120. How do I increase the limit?? I see that one of the commenters above had to go to some 140%. From the results I got at 6500 feet ASL and above, I am sure I am going to have to do the same to get to 8600 feet ASL.

In my case, at low load and lower RPMs, the engine would run OK. But as soon as I brought the engine RPM above about 2500 RPM and increased the engine load above about half throttle, the engine went real lean to the point of miss-firing. In excess of 17/1 AFR.

Is there an easy way to change the preset Baro Corr Limit?? I could change the fuel injector flow rate by some percentage and then lower the Baro Correction table values by the same percentage but I would rather not do that.

KenK
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Re: Elevation Correction

Post by Yves »

KenK wrote:After many months, I have now had a chance to run my car at a location where I could do some elevation tuning testing.

During the testing, I ran the car from about 1500 feet ASL to over 6500 feet ASL. Although the temperature did change, the change was not significant (about 10 degrees F.) and I have already setup a temp correction curve (MAT) from about 40 degrees F to over 118 degrees F.

I my case I did the temperature curve was developed at a fixed elevation and under a fixed load and at a fixed RPM. The curve as seems to work pretty well.

Now, when I did the elevation testing I did OK until I got above about 4500 feet ASL. Above that, I ran out of room with the correction curve limit set by default at 120. How do I increase the limit?? I see that one of the commenters above had to go to some 140%. From the results I got at 6500 feet ASL and above, I am sure I am going to have to do the same to get to 8600 feet ASL.

In my case, at low load and lower RPMs, the engine would run OK. But as soon as I brought the engine RPM above about 2500 RPM and increased the engine load above about half throttle, the engine went real lean to the point of miss-firing. In excess of 17/1 AFR.

Is there an easy way to change the preset Baro Corr Limit?? I could change the fuel injector flow rate by some percentage and then lower the Baro Correction table values by the same percentage but I would rather not do that.

KenK
Ken,

I made the comment about that maximum value of 120%, but I calculated the baro curve on air density change.
The comment you made about it going lean when under load seems to imply that it would inhale more air, which might be because of reduced pumping losses on the exhaust stroke and backpressure. What curve are you running now.
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Re: Elevation Correction

Post by KenK »

08/01/17

Yves:
I suspect that you are correct in your statement about pumping efficiency gains due to lower back pressure on the exhaust side of the motor are correct. I would have never expected the magnitude of the effect but I have to go with what I am seeing.

First Round of Actual Condition Testing:
The testing was done on Memorial Day weekend (May 24, 25 & 26 2017) near Mojave CA. The base elevation where the testing was done was about 1500 feet above sea level (ASL). The maximum elevation available at this location is about 8600 feet ASL. Ambient temperature during the testing ran between 85 and 100 degrees F.

The curve I was using basically starts at about 96% for ambient pressures at sea level (98kpa) and runs pretty flat until it reaches about 93kpa. From that point it goes pretty much straight up to 120.

I was able to get the engine to run close to correct until I reached about 4000 feet anything above that the motor kept getting leaner and leaner. By 6500 feet I could not run the motor much above half throttle without going lean to the point where the motor would fall on its face and start missing badly (in excess of 17:1 Air Fuel Ratio (AFR)).

At that point I gave up and came down off the mountain.

The preliminary table I have after the run looks like the following:
65.1--- 120.0
72.2--- 120.0
80.7--- 120.0
84.0--- 118.5
86.6--- 115.9
92.6---- 99.5
97.9---- 99.3
105.4--- 97.0
110.0--- 96.2

With respect to the table above, it is NOT any ware near correct. The table represents the point where I gave up trying to tune the engine at the given elevation.

Engine Details:
The motor I am working with is an air cooled VW motor that has been stroked and bored to 2331cc.
The exhaust system is an open Tri-Mil system with a 2” diameter stinger about 18” long.
The cam I am using (Engle 120) is running about 293 degrees of duration.
The compression ratio used is about 11.8 to 1.

The motor starts to come on at about 2200 RPM and the torque starts to fall off at about 5000RPM. Due to dilution issues, the AFR reading below about 1800 or 1900 RPM is pretty unreliable unless the engine is loaded pretty heavily or I run an extension pipe on it. As such, I do most testing in the 2500 to 4000 RPM range.

I did some experimenting with the setup today to get an idea on how much effect the “Barometric Correction” (BARO) table actually had.

Engine Tune Changes:
To get some room to play with, I lowered the fuel injector size from 483 cc/min to 350 cc/min. That works out to be 136 cc/min or about 28%.

To get the AFR back to close to where it was, I lowered the sea level “BARO Correction” factors by about 27%. I
setup a straight line between about 63 and 120.

The test table looks as follows:
60.1--- 120.0
68.3--- 112.8
74.0--- 106.0
81.1---- 97.9
89.2---- 86.0
95.2---- 63.8
99.8---- 63.4
104.1--- 63.1
109.7--- 60.0

Test Conditions:
The test absolute pressure was about 98 4kpa.
The test manifold air temperature (MAT) start at about 110 degrees F and cooled down to about 104 degrees F.
At 2700 RPM the manifold air pressure (MAP) was running about 50 kpa.
To simulate elevation changes at least to the electronic control unit (ECU), I attached a hand vacuum pump to the input of the BARO sensor. This allowed me to change the BARO pressure the ECU saw.

Test Objective:

This test was designed to see how the ECU responded to changes in the BARO pressure and to see what effect it had on the engine. Bear in mind, this has NO effect on the BARO pressure on the exhaust side of the motor. It only shows how the ECU responds to the pressure change and how much of an effect the change in fuel injected has on the AFR.


Results:
I have included an MSL file from the engine for detailed review.

RPM-------BARO-------MAT-------MAP----Fuel Flow-----AFR-----BARO
----------- kpa-------- Deg F.-----kpa-------ms--------------------Corr
2600------- 97.7-------104.2-----48.8-----2.056---------13.9-----63.6
2590------- 97.9-------103.7 -----48.9-----2.085---------13.6-----63.6
2485------- 50.0-------103.2 -----54.3-----3.171---------11.4----120.0
2463------- 50.0-------102.6 -----54.0-----3.191---------11.3----120.0
3261------- 97.9-------102.5-----49.8-----1.782----------13.2-----63.6
3012--------50.0-------102.2-----55.2-----3.079----------11.1----120.0
3650------- 98.1-------102.0-----49.1-----1.69-----------12.7-----63.5
3235------- 56.2-------101.8 ----58.2-----3.237----------10.8-----120.0

Summary:
As you can see, the software does what it is suppose to do. When the BARO correction goes up, the ECU adds fuel and the engine goes rich.

I am surprised by the response of the engine to the change. The span of the BARO change during this test was 63.6 to 120 or a change of 56.4%. The change in fuel injected went up by 1.106 ms @2500RPM or about 53%. The change in ARF went down by 2.2 points or about 16.2%.

Now, this test does NOT accurately test the effect of the BARO correction on the engine as the change in BARO pressure was not applied to the exhaust side of the engine. That in my case cannot be done except on the field under actual conditions.

I still think I am going to need more room to move the BARO correction than I can currently get. I need to be able to increase the correction limit higher than 120%.

Next Step:
The next step will occur on Labor Day weekend (Sept 2, 3 & 4). At that time, I will take the car back to the desert and try the test run again.
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Re: Elevation Correction

Post by Dennis_Zx7r »

KenK wrote:I suspect that you are correct in your statement about pumping efficiency gains due to lower back pressure on the exhaust side of the motor are correct. I would have never expected the magnitude of the effect but I have to go with what I am seeing.
I don't think this has that much effect to explain your data. Would be interested to see a log though.

1) You are using SpeedDensity. This by itself also means that the MS cuts fuel on altitude.
2) You are using MultiplyMap. This means that as you go higher up and your MAP goes down, the MS is cutting fuel.

Both combined mean that you are cutting fuel to the square of the loss of ambient pressure. Your baro correction therefore imo for the most part compensates for the fuel cuts resulting from the two effects mentioned above. You can check your PW in the logs, and I guess it's slightly less on high altitude even though your baroCorr is set this high.
As for 1), you may want to check out Baro%. It seems to handle changes in altitude better and converting your tables using a spreadsheet should be easily doable. The MS would still pull fuel proportionally using MultiplyMap, and I think this may result in much less need for baro correction. As a reference, I need about +6% baro corr for a drop of 10kpa ambient pressure.
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Re: Elevation Correction

Post by suberimakuri »

Yeah try percent baro...
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Re: Elevation Correction

Post by KenK »

Sorry, I thought I had included a log. I see I actually attached an MSL. My Bad.

I'll include the log file in this post.

You are correct in that I am using the SD algorithm and I do have and am using a second sensor for BARO.

With respect to the issue of elevation having an effect on the MAP, I would have thought that elevation would have very little effect on the MAP until the engine was in a wide open throttle condition as the throttle plate position and intake plumbing play a very large part on the MAP. As such, I would think that the ambient pressure would have little effect on the actual MAP (I could be and have been wrong before) until you were at or near a wide open throttle condition and the MAP was approaching the BARO pressure.

In reviewing the test logs I ran the other day, I could not help but notice that the BARO correction I have setup in the test gave me about a 56% correction, and the fuel injection rate was increased by about the same amount (plus or minus a small amount) but the AFR only changed by about 16%. What's up with that, I would have expected a much bigger change in AFR than that.

With respect to the %Baro technique, I will take a serious look into the technique. Someone else on a different post stated that he converted a system from SD to %Baro using the same VE tables and finished the conversion in one outing. I hate the thought of having to completely rebuild the VE tables.

With respect to the VE table used for %Baro, What range would you guys suggest for the VE table. Since my application is a naturally aspirated engine, my current VE table goes from 40 kpa to 100 kpa. With my extremely limited understanding of the %Baro technique, I would think that a VE table with the same range should work fine.

What do you guys think.

KenK
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Re: Elevation Correction

Post by Dennis_Zx7r »

KenK wrote:Sorry, I thought I had included a log. I see I actually attached an MSL. My Bad.
I'll include the log file in this post.
This log seems to be of your test checking the baroCorr with the car in a standstill. I meant your driving up the mountains.

You can easily plot Baro% versus Map or the other way around in MLV. Create a custom variable with [MAP]/[Barometer]*100 as the formula. Baro% shouldn't be too much off on a NA engine. using this formula, you can easily convert your VE-Table to a baro% table. If your baroCorr was 100% at 100kpa, the tables would even be identical.

For example, if we have 100kpa baro pressure and a MAP of 100kpa, we have 100Baro%. Still 100kpa baro pressure and a MAP of 70 -> 70Baro%.
However, when baro drops to a lower value, say 70kpa, a MAP of 70kpa would still equal 100Baro%. So you're still in the top row of the VE table under full load instead of somewhere in the middle for full load. This eliminates point 1) from my previous post and should substantially reduce the need to add a lot of fuel again. Of course this means that you would have to recalculate or otherwise redo your baroCorr curve.
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Re: Elevation Correction

Post by KenK »

Dennis:
Sorry, I don't have a log for the mountain climb testing. I have a purpose built dash that provides me MAT, MAP, RPM and AFR along with a host of other things so I didn't really need to log the data to get done what I wanted to do. Sorry, I kind of wish I had setup a log now. I was using my tablet to tune the ECU so I could have just as easily started a log. The down side of running a log is that my dash talks to the ECU through the same blue tooth interface as does the tablet so I can one or the other but not both. I would kind of like to find a way around this but I don't think it is going to be easy as the MS2 only has one (1) serial port. This is a project for another day. I have included a picture of the dash.

I converted the system to %Baro yesterday.

I reset the Baro correction curve to a flat 95%. This will give me a little more room than the standard 20% limit set by the software.

I reset the injector flow rate back to where it was when the VE table was developed.

I started out with the same VE table as was used for the SD technique.

The first test was to do a "No Load" test from 2000 RPM to 4000 RPM. The engine ran a little bit rich but I am using 95 (Baro Corr Table) as the baseline and the table was setup with the baseline at 100. I fixed that.

The second (two runs actually) was to do some road work. I made a few passes down the street yesterday afternoon and found that the engine went real rich at %Baro numbers greater than about 80%Baro (10.5 to 11.5 AFR) through out the RPM range. I did not get any run data where the engine load was between 50 and 80% so I can't tell what is happening there. With respect to the Idle RPM (1000-1400RPM , 0 TPS), the AFR data is corrupt due to a dilution problem. The engine actually is running in the high 12s and low 13s. I have included a couple of logs for the tests done yesterday afternoon.

I will try today to fix the high load problem and will try to get some mid load data.

I have to be careful running the car on the street as it is NOT street legal and is very loud. Johnny Law would not be happy with me if he caught me. Besides, it annoys the heck out of my neighbors.

I'll generate a follow up post when I get the testing done this afternoon.

Ken
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Re: Elevation Correction

Post by whittlebeast »

We need to find someone with a well tuned (at some known altitude) that runs a Ultimate or Evo, that can drive up and down a mountain. All we need is a data log from running up and back down the the hill and we will learn more that 4 pages of assumptions and guesses will ever learn. We need hard data.

Andy
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Re: Elevation Correction

Post by racingmini_mtl »

whittlebeast wrote:We need to find someone with a well tuned (at some known altitude) that runs a Ultimate or Evo, that can drive up and down a mountain. All we need is a data log from running up and back down the the hill and we will learn more that 4 pages of assumptions and guesses will ever learn. We need hard data.

Andy
I agree that hard data is needed but what you will get is only valid for that specific engine. Remember that the elevation correction corrects for parameters that are not measured such as exhaust backpressure and other intake, exhaust and generic engine characteristics that are affected by ambient atmospheric pressure.

It would be very interesting to see the hard data from different engines. And having variations such as naturally aspirated, turbo and supercharged should provide very different correction needs.

But this is more speculation which would be confirmed or refuted by actual data.

Jean
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Re: Elevation Correction

Post by whittlebeast »

All points by Jean above are true.

Andy
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Re: Elevation Correction

Post by R100RT »

Jean and Andy are "Right".
See movie clip
https://www.youtube.com/watch?v=3NaQxUEfxt0
However, reading post with great interest (no dis intended)
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Re: Elevation Correction

Post by ol boy »

I've found that all 5 fueling types(SD, %baro, alpha-n, MAF, alpha-n w/map) end up with different looking baro curves. Just the way the math sorts out to PW.

I had a customers fly an unmanned UAS with a microsquirt based system to over 21K feet using alpha-n w/map. No baro correction at all. The engine makes nearly zero MAP change across the rpm range but moves only vertical with altitude change. So as the plane flies higher the MAP falls lower and naturally pulls fuel while doing so.

If we ever do a small 4 stroke I'll need to add a baro sensor and sort through all this again.
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