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Discussion Starter · #1 ·
I know there are some good mappers on this forum. Got a question about mapping a car. If you are doing speed density mapping (MAP based mapping) is it required to also do an altitude compensation (baromatric pressure compensation), or is this not required? And if yes, why and if no also why :)
Then if yes, what kind of compensation is needed?

Also another one: What kind of compensation is required for intake air temperature?
 

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I wouldnt think so... .As barometric pressure differences will be accounted for by the map itself!

I think temperature compensation is still needed most mapping software should have a default temp compensation table. As its pretty much set in stone that x degrees warmer = y% less fuel
 

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Less fuel, because it will be running rich due to less air.
Unless boost is controlled in such a way that it gives you a set absolute pressure rather than a set pressure ratio ;)

Although then of course the turbo working harder as you reduce atmospheric pressure which means the charge is heated more. Which could offset the increase in boost if the turbo is close to its limit so still require less fuel. Thats what the temperature compensation should deal with though!
 
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Unless boost is controlled in such a way that it gives you a set absolute pressure rather than a set pressure ratio
Boost controllers just work on pressure thou , thin air ,turbo works harder to make the same boost.

A air flow meter system (MAF) self compensates agaisnt baro pressure changes. Not like a MAP system

I do know the Pikes peaks car always have hassle when running turbo,s due to the altitude change , thats why they run massive turbo,s at lowish boost pressures, so they are using the volume of the air not the boost pressure to make power.

Mapping is always a juggling act , hot air is thinner so you need less fuel but hot air os more supscetiable (SP) to det than a cooler charge , so I tend to add fuel when air temp gets very high and remove ignition timing. With Barometric compensation this can help if you constantly logging the AFR (ie track work/drag runs) but for a road car I would not bother to trim the map agaisnt baro pressure.

good thread thou.


Mark
 

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Gotta agree about the juggling act. Im sure when I map my car I'll learn LOADS. I think I understand the theory but that doesnt mean it'll be easy.

I still cant understand how barometric compensation would be needed though. As air density is dependant on temperature vs pressure. MAP + charge temp compensation should cover it all in theory!
 

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calculating mass air

In the equation you need to take Volumetric Efficiency (VE) in account. VE changes as altitude changes, as a result of the effects of residual gas (leftover exhaust gas in cylinder). Residual gas in fact replaces air and has therefore a tremendous effect on VE.

The higher the altitude the leaner AFR will be, because VE increases as a result of less exhaust backpressure.

So my answer would be: YES, on a S-D based engine management system your fuel table will require barometric pressure compensation.

- Henk
 

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Id assume for when mapping with TPS as load.

Its a pity there arent any mountains around here to test whether an additional baro compensation is needed when using MAP as load!
 

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In the equation you need to take Volumetric Efficiency (VE) in account. VE changes as altitude changes, as a result of the effects of residual gas (leftover exhaust gas in cylinder). Residual gas in fact replaces air and has therefore a tremendous effect on VE.

The higher the altitude the leaner AFR will be, because VE increases as a result of less exhaust backpressure.

So my answer would be: YES, on a S-D based engine management system your fuel table will require barometric pressure compensation.

- Henk
AAAAh so thats what ive not been thinking of! If im understanding right, the baro compensation would be to compensate for the difference in how things work on teh exhaust side? Lower atmospheric pressure = better exhaust scavenging?
 

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Yes, also, to make it even worse, taking in account valve overlap, VVT, turbo wastegate etc.
It's the Holy Grail in calibrating OEM ECUs.

- Henk
 

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Discussion Starter · #14 ·
ah this discussion is exactly going where I wanted it to be. Henk big :thumbsup: to you, you got the point right where the discussion should be continued.

I think M Skinner has the baseline right: You DO NOT need altitude compensation on MAP based engines, because the MAP sensor itself already 'reads' the barometric value. However, and that is what is interesting to discuss more, there are other things in the whole system that do change with barometric pressures (as Henk is posting).

What do you guys think of the following statement: If you tune a car at sealevel (100 kPA outside pressure) and have 70 kPA in the intake manifold (e.g. you are running on some vacuum) it gets the same amount of mass air in as the car running on a mountain where it has 70 kPA outside pressure and also running 70 kPA of intake manifold pressure.

Asim: The ECU is not discussed here. I'm talking about all ECU's in general.

So what is the exact answer to the baro question? Is it "no the actual barometric pressure is not of importance to the fueling of the engine, but take into account that with baromatric pressure the engine itself changes it's efficiency so in that situation you have to adjust things"?
Also what would the adjustments be that has to be made?


I have never set the barometric value in my maps. My cars always run on sea level and I use MAP based maps only. Still I'm interested in what others think.


About the intake temperature v.s. fuel requirements. I heard 3% per 10 degrees celsius. And take 20 degrees celsius at sealevel as your baseline (0% compensation). What are other peoples thoughts? Looking at basemaps from the ECU's that I use often, I see 'only' 1 or 2% enrichment for every 10 degrees lower than 20 celsius and above 20 celsius they just do 0% (e.g. nothing).
 

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What do you guys think of the following statement: If you tune a car at sealevel (100 kPA outside pressure) and have 70 kPA in the intake manifold (e.g. you are running on some vacuum) it gets the same amount of mass air in as the car running on a mountain where it has 70 kPA outside pressure and also running 70 kPA of intake manifold pressurre.
Only the same density if charge temp is the same in both cases too.

And i'd guess there ARE differences, as 70kPA at sealevel means you arent at WOT. but in the second instance you would be at WOT but only 70kPA. throttles have to alter the way the air flows, even if MAP is the same at different throttle openings.
 

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Short answer is yes you need baro comp for fuel calc with speed density.
Best solution is to map fuel using MGP Manifold Gauge Pressure which is the most common way with Link ecus.
Resaults in dead stable AFRs regardless of altitude and or changing baro at same altitude.
Good example of this is the gold rush hill climb(similar to pikes peak) Testing proved using this method resaulted in the same afrs top to bottom.
Note Ign needs to be mapped off MAP.
Jason
 

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Discussion Starter · #17 ·
mapping on manifold pressure IS speed density mapping my friend. Also don't mention any ECU's as this is an ECU unspecific discussion. If we're gonna discuss what ECU is best, we can talk forever.
 

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mapping on manifold pressure IS speed density mapping my friend. Also don't mention any ECU's as this is an ECU unspecific discussion. If we're gonna discuss what ECU is best, we can talk forever.
Clearly you need to re-read my first sentence my friend.
Jason.
 
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