Elevation Correction

Tuning concepts, methods, tips etc.

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jsmcortina
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Re: Elevation Correction

Post by jsmcortina »

I'll leave it to someone else to explain then.

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Speedy_G
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Re: Elevation Correction

Post by Speedy_G »

In my opinion there are three effects to think of, when it comes to BaroCorrection

1) Intake Air Density: Higher Baro -> Higher Density - > More Air Mass -> Leaner AFR : So you need a positive correction factor (enrichen) to inject more fuel to get the same AFR like before
2) Exhaust Back Pressure: Higher Baro -> Normally Less air mass -> richer AFR: You you need a negative correction factor to inject less fuel to get the same AFR like before
3) Fuel differential Pressure: Some systems are not referred to MAP. So in this case: Higher Baro -> Less differential pressure -> less fuel mass -> Leaner AFR : You need positive correction factor ....

Now the question is which of those three factors has the most influence.
Maybe this depends on your exhaust system how big the influence of the lower back pressure (due barometric pressure) is.
Because i've seen some people who have positive correction for less ambient pressure and i've seen people who use negative correction for less ambient pressure.

The other thing is the influence of your Fuel Load.
Pure SD: You will need the lowest baro correction
Hybrid: Something between.
Pure AN: You definitely need the baro correction

Now the question concerning this is: Is there really a 1:1 influence of the atmospheric pressure to the MAP ?
I would say for WOT - yes there is !
But do also have 20kPa less MAP when idling on mountain in comparision to sea level ?? I don't think so ! So there is additional some kind of load influence.
With this maybe it would be easier to tune the Baro-Correction with a fuel load without MAP influence!

But now there is another problem. Image you're driving a motorcycle with RAM-Air effect.
Now it gets very difficult with the Megasquirt because you dont have a RAM-Air correction function!
On stock bikes they use MAP + Airbox-Pressure + Ambient Pressure Sensor.
So three Pressure sensor.
The MAP is used for some similiar to megasquirt ITB mode.
The Airbox pressure sensor is only for the RAM-Air effect. So higher pressure -> positive Correction factor
The Ambient Pressure sensor has the opposite behaviour of the Airbox-Pressure-Sensor. So at higher pressure -> negative correction factor.

So with megasquirt on a bike with RAM-Air effect youre somehow screwed.
There is a reason that the OEM spend more money on a additional sensor!
I dont think that the MAP would compensate the RAM-Air Effect perfectly. Because in this case they just would have save the money for the third pressure sensor.

@James: Maybe it's possible to add another Correction Curve which depends on a selectable Generic Sensor Input.
So the motorcycle guys could select their airbox-pressure sensor there? Then it would be the same like on OEM ecu's
Then the (old) correction takes care of the effect due to lower exhaust back pressure. And the sensor inside the airbox takes care of RAM-Air effect.
Furthermore the (pressure) input for the ram-air correction needs to be relative to the baro-sensor. (And not absolute)
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Re: Elevation Correction

Post by whittlebeast »

My gut feeling is MAP is MAP no matter how the air got in the intake manifold. My bet is the real effect of baro will end up on the exhaust side. If you have a stim, it should be fairly simple to verify what the code does when the baro changes. Simply put in a flat line "curve". Twist on the knob controlling baro and see if the PW changes. Now change the line to say 110 and see if the PW goes up 10% Keep in mind the injector dead time is about 1ms so the first .001 sec does not count in the math.

For example at any given MAP and RPM if the PW comes up 11ms, the injector will only spray 10ms. A 10% increase in fueling will show up as 12ms at the injector PW or 11ms of fueling.

As a side note, Intake manifold heat transfer correction and Intake air temperature correction are the same sort of thing. If you tune in the dead of summer but you want the SAME tune to run perfect in the dead of winter, you need to sweet this sort of thing like the OEMs do. I have been around Megasquirt for about 15 years. Years ago, I was helping a guy that was running a pikes peak hill climb motor. This stuff started showing up in the tune.

Andy
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Re: Elevation Correction

Post by Sam280Z »

My understanding is that the pressure (MAP) and temperature (MAT) determine how much air is available for the engine to ingest. The volumetric efficiency (VE), a function of the MAP and engine speed (RPM), determines how much of that available air is actually ingested.

Barometric correction allows for adjustment of the VE values to account for changes in VE due to changes in backpressure.
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Yves
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Re: Elevation Correction

Post by Yves »

Lots of different opinions

Imo our map is a measure of the level of vacuum vs absolute vacuum. So assuming the baro is 90 kpa and your vacuum at idle is 30 kpa, you will end up with a reading of 61 kpa compared to sea level baro (101,325).
It's however only the change of alt that changed density.
As for the exhaust , it only sees a reduction of 10 kpa. not much
KenK
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Re: Elevation Correction

Post by KenK »

09/12/16


Gentleman:
Very interesting conversations to date.

In my case I am running a naturally aspirated engine using the SD calculation for fuel.

I agree with Jean’s assessment of the situation. When using the SD calculation, the VE table is developed using the MAP and engine RPM. As such, regardless of the elevation, an engine running with a, 75Kpa MAP is the same at sea level as it is at 10,000 feet above sea level.

In my case, most of my VE table was developed at or around 2200 feet above sea level (ASL) so my Baro zero point is at about 95 Kpa.

Now there maybe some effect on the intake side due to ambient pressure caused by differences in the swirl patterns as well the disturbance caused by the effects of the throttle plates themselves. In my case, on the intake side, I have a straight intake tube with an eight (8) diameter upstream length. This was done to insure that the flow pattern is homogeneous as possible before the throttle plates. Remember the velocity stakes on the old Hilborn injection systems??

As for crank case pressures, maybe. In my case, I am running an air cooled VW engine which has a relatively small crank case volume with respect to the volume of the engine. One thing that must be understood here is this is a four (4) cylinder motor using the boxer design. As such the actual displacement of the engine for the most part is irrelevant as when piston # 1 is going up, piston # 4 is 180 degrees out of phase and is going down. Piston #2 is 90 degrees out of phase with piston #1 and piston #2 is 180 degrees out of phase with Piston #4. The big and unpredictable variable here is the amount of blow by that is going on. I certainly hope the effect of this is minimal as this will change over time as the engine wears.

I may be wrong but I have to believe that the big variable here is the effects the ambient pressure has on the exhaust system. One of these would be the effect the elevation has on the speed of sound.

At this time I do not have enough data to better demonstrate what is actually happening and won’t get any more until the Thanksgiving weekend. For those if you that are not in the US, that would be the third weekend in November. I don’t know how that is going to go as at that time if the year, it can get pretty cold at 9000 feet ASL.

Food for thought

Thanks for all of your input

KenK
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Re: Elevation Correction

Post by whittlebeast »

If you happen to own a copy of MegaLogViewer HD, live near a mountain, and live in a part of the country that actually has seasons (like the midwest) I will be happy to show you how to tune all of this.

Step one, tune the car to be near perfection in the spring based on the the GEGO changes very little as you drive around at the same elevation. Stop playing with your VE tables and your AFR target tables for the next 12 months.

Step two, wait for summer to hit one of this 97 degree days and slide your IAT correction to get as close to zero GEGO correction as possible at this new IAT.

Step three, wait for winter to drive around on one of those 10 degree days and repeat step two on this horrible cold day.

Step four, wait for spring to hit where the temps at home are reasonable and drive up to the top of the mountain and play with you Baro correction to get as little GEGO as you can when it is close at 10 degrees at the top of the mountain.

Step five, wait for dead of summer and drive up the mountain and hope everything stays close.

The OEMs do this in a barometric chamber with some serious heaters, air conditioners and have the ability to change the pressure in the chamber. I have a friend that used to run the one owned by the EPA for verifying engine calibration for EPA certifications. If I recall, it was worth like 9 million. He could simulate a drive from Death Valley in the dead of summer to the top of Pikes Peak in the winter in an hour or so. Way cool stuff.

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Re: Elevation Correction

Post by turbo conversion »

What whittlebeast said works, that is how I tuned my two MS projects. :D

This is why I always say if you have seasons it will take a year to get a consistent tune for all conditions.

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Re: Elevation Correction

Post by Yves »

:D Great, yet another year.

I'm getting the impression this is pretty much a hit and miss deal
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Re: Elevation Correction

Post by whittlebeast »

Look at the positive side, you have mountains to climb. The closest one to my home is a 20 hour drive.
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Re: Elevation Correction

Post by Yves »

Just something else that someone said about baro correction on another forum :
If map is included in the load calculation, when going to a higher altitude the load shifts naturally downward. This means usually into the regions where you run leaner....This could explain some of the behavior seen. (see also my comment on vacuum above)
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Re: Elevation Correction

Post by Yves »

whittlebeast wrote:Look at the positive side, you have mountains to climb. The closest one to my home is a 20 hour drive.
:D Well, Belgium is rather flat. We have mountains, but not the type that will reduce my baro enough. I would have to drive to Germany and not just into Germany, but way South.
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Re: Elevation Correction

Post by TheSilverBuick »

Dennis_Zx7r wrote: Maybe TheSilverBuick uses AlphaN?
I run %Baro on the VE table and straight speed density on the spark table.

I'm always watching the baro and baro correction when driving the car. In the right middle, 79.5kPa and 88.5 correction.
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Re: Elevation Correction

Post by suberimakuri »

Yves, yes, if map in load then you may hit differently curved rows and run lean.
Percent baro safer for this.

Notice this in a big way on itbs with speed density.
Alpha n needs more fuel with elevation.
As does SD on every turbo motor I've tuned.
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Re: Elevation Correction

Post by Yves »

So, how to set it?

Btw : how is baro accounted for in the code ?
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Re: Elevation Correction

Post by suberimakuri »

Sorry I don't understand what or whom you're asking?

We just need to give the motor what it wants.
Baro is just a multiplier in the fuel equation.

If SD is okay for your setup then use that and make a baro curve as described above.
Else if the variation in map puts you into rows that aren't a good use percent baro and make a baro curve.
?
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Re: Elevation Correction

Post by Yves »

That question was for you suberimakuri.

If I read James' comments it seems as if baro is not included in the load calculation and the graph is used to account for it...
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Re: Elevation Correction

Post by suberimakuri »

Okay. Sorry.
Baro correction is applied like all the other corrections (CLT/MAT/ETC).
The curve is now exposed so we can all adjust the baro correction to suit our engine configurations.

So at your 0 altitude (whether this is sea level or up on high plains), tune your VE and accel like normal. Your baro correction should be 100 at this point.
Then as you go up or down in elevation, adjust the baro curve.
If you were tuning at sea level with 100kpa baro.... maybe at 95kpa baro you have 102. At 90kpa you have 105.
Put baro correction on a graph in Tunerstudio or on a line in MegaLogViewer and you will see the effect.
Just give the engine what it needs, not what any laws say it should be.

And I highly recommend getting a second sensor for baro. It's so cheap to get the MapDaddy dual map sensor board or to just put another in the proto area. We've been doing this for over 10 years now and we're only dealing with 100->90kpa baro change here.

Make sense?
Last edited by suberimakuri on Wed Sep 14, 2016 12:09 pm, edited 1 time in total.
jsmcortina
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Re: Elevation Correction

Post by jsmcortina »

Yves, Have you read the fuel calculations section of the manual?

http://www.msextra.com/doc/pdf/html/Meg ... 4-151.html

James
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Re: Elevation Correction

Post by Yves »

jsmcortina wrote:Yves, Have you read the fuel calculations section of the manual?

http://www.msextra.com/doc/pdf/html/Meg ... 4-151.html

James
Thanks.

I still have a question remaining, when reading the definition for barocorr in the formula it seems that this is based on the initial map or the dedicated sensor. So the correction for altitude is done internally ? Is this right.
If so the barocorr curve is a corr on top of this if needed to include exhaust effects and such...
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