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Air Density

EFI Tuning Fundamentals

Discussion and questions related to the course EFI Tuning Fundamentals


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I think we can all agree that the main thing that we look at when making power is to consider air density.

However, I do have something that I disagree with (it's very likely that a lack of knowledge is to blame).

In one of the videos Andre said that Oxygen content reduces with altitude. The only part of that that makes sense (to me), is the fact that a lower air density would mean a lower physical number of Oxygen molecules in a given volume of air. However, the oxygen content still remains at 21%

In order to calculate air density, not only does atmospheric pressure and temp have to be taken into account, but perhaps more importantly humidity too.

If we look at actual measured atmospheric pressure, things become a bit more murky.

Let me use 2 cities in South Africa as an example.

Right now, Johannesburg (altitude of 1753m) has a temp of 12C, baro of 1027.1mb, and humidity of 44%.

Durban (altitude 9m), on the other hand, has a temp of 17C, baro of 1018.29mb, and humidity of 94%.

The reason I mentioned measured atmospheric pressure, is because when working with the standard model to calculate the change in pressure vs altitude, the pressure should be way less than what the measured figure actually is.

So, taking into account all those other figures, Johannesburg should have a higher air density (i.e more oxygen in a given volume) than Durban..which should equate into more power.

Perhaps Andre would be the better person to help here.

I think it's simply two different ways of discussing the same thing. You're right that the actual percentage oxygen doesn't necessarily change at altitude( however this wasn't strictly what I meant). The density is what effects the amount of oxygen as you correctly state. This was what I was meaning when i said the oxygen content was lower.

I apologise for any confusion.

Thanks for the reply.

Not at all Andre, I understood what you meant.

What I am trying to figure out is, whether or not altitude really does make a difference to how much power can be made.

If we calculate with the accepted standard model, then yes, it would seem that altitude makes quite a difference to the air density.

But, if we try and calculate what the predicted air pressure would be at a certain altitude, we get a number much lower than what the actual measured figure is. And in order to calculate air density, the pressure must be known.

This is where my confusion starts. Perhaps (and it is likely) my calculations are wrong. But at no point do I see the standard model take humidity into account; and since humid air is less dense than dry air, it would seem that to consider humidity when trying to understand everything involved with making power is essential.

What is your experience with the matter? Does altitude really make a difference?

My apologies if I'm asking what may be considered irrelevant questions, just trying to get my head around it all.

I can answer part of your question here - Yes altitude will make a difference to the power an engine can produce. As altitude increases, air pressure and hence air density decreases so for a given volume of air, the available oxygen is reduced. It's the oxygen we really care about and if we have less in the combustion chamber we can't make as much power.

I'll admit I haven't considered the effect of humidity and this is an interesting point. Humidity will have an effect on air density however I am not aware of any ECUs currently accounting for humidity in their calculations.

You have got me thinking here Theo ;)

Humidity is a measurement of moisture (water vapour) in the air. Water is H2O so the higher the humidity the more oxygen is available in the air. This is especially interesting, to me anyway, because air contains (I think) <20% oxygen so an engine will theoretically make more power at a location at sea level (not necessarily near waterfront but rather at 0 metres elevation) with higher level of humidity than a location at sea level with less humidity (assuming all other factors are exactly equal).

Well, no...not quite.

Humid air is less dense than dry air...therefore you can't have more oxygen in humid air. Regardless of the chemical composition of the water molecule, water will only split into H and O at over 2000*C

The reason I keep coming back to measured atmospheric pressure is because time and again the math doesn't line up with the science.

In my first post I used example of 2 cities, one at 9m above sea and the other at 1753m above sea. The math would say that the higher altitude should have a lower atmospheric pressure....but when measured, It doesn't..in fact, for those 2 cities the higher altitude one had a slightly higher pressure.

Then we take air temp into consideration...the higher altitude one has a lower air temp.

And finally, humidity....sea level has a higher humidity.

So taking all of those things into account, the higher altitude should make more power, if all that power is really concerned with is air density.

Yet, we get so many people who swear by the idea that you lose power at higher altitude. Please, don't get me wrong, I'm not saying that they are lying or incorrect in any way whatsoever. I'm just trying to understand, if you do lose power, why.

Previously when I started thinking about it, it kind of made sense that you would lose power with an N/A, but not much if at all with forced induction. If you do lose power with forced induction, I though "ok, maybe the thinner air means there is slightly more lag, or perhaps it puts the compressor out of its efficiency range as the turbo has to work harder which produces more heat, or perhaps the thinner air would influence the intercooler's ability to cool the charged air.......perhaps its all of those things"

But when I started thinking about how density is actually calculated....then things just didn't make sense to me anymore.

@Andre...so if I understand you correctly.....does the ECU automatically calculate what the air density "should" be? In which case, where does it get the atmospheric pressure reading from? If it is calculating the pressure rather than directly measuring it, it would somehow need to know what the altitude is.

But if it is that it measures the pressure directly, then in the case of Johannesburg and Durban, it should still read a higher pressure in Johannesburg (as the measured pressure from the weather station did show that it was higher than Durban)

This can be linked to the lack of atmospheric pressure. At low elevation, the pressure is higher because the molecules of air are compressed from the weight of the air above them. However, at higher elevations, the pressure is lower and the molecules are more dispersed. The percentage of oxygen in the air at sea level is the same at high altitudes. But because the air molecules are more spread out at higher altitudes, At the atmospheric pressure is lower the molecules are less compact, resulting in a lower percentage of actual oxygen. Example at sea level, because air is compressible, the weight of all that air above us compresses the air around us, making it denser. As you go up a higher, the air becomes less compressed and is therefore thinner.The important effect of this decrease in pressure is this: in a given volume of air, there are fewer molecules present. This is really just another way of saying that the pressure is lower (this is called Boyle's law). The percentage of those molecules that are oxygen is exactly the same: 21%. The problem is that there are fewer molecules of everything present, including oxygen. So although the percentage of oxygen in the atmosphere is the same, the thinner air means there is less oxygen to breathe too.

I think you can read the details http://en.wikipedia.org/wiki/Boyle%27s_law . Sorry for my bad english. i hope this help u bro.

"Well, no...not quite.

Humid air is less dense than dry air...therefore you can't have more oxygen in humid air. Regardless of the chemical composition of the water molecule, water will only split into H and O at over 2000*C"

I'm pretty sure you've got that wrong. Density is not measured by the amount of oxygen but rather the amount of molecules in a defined space. Water splits at boiling point which is well below 2000 degrees C. What you are referring to is the required heat energy to totally break the bonds between Hydrogen and Oxygen which would then stop them reforming into water. As an aside you don't need heat to do it you can do it with electrical currents (which are all around us in the natural environment anyway) and when a spark plug fires the contents of the cylinder are exposed to electrical currents.

Getting back to your original thought. You are concentrating on one example that isn't representative of global patterns. It isn't representative because there are many factors that determine air density-temperature-and (if you want to get really technical) the chemical composition of the air itself. This is, or can be, a highly technical discussion that delves deeply into the nature of climate and weather patterns and how we as humans utilise it (think Kenyan marathon runners because the exact same principle applies).

Thanks for the reply.

The problem I'm having with it all is the use of a standard model. I'm not entirely sure how or where the formula was devised...but if you compare it to actual measured pressure, it doesn't line up.

The formula to calculate pressure above sea level is

p = 101325 (1 - 2.25577 x (10^-5) * h)^5.25588

h is in m, and p is in Pa

I guess what I'm trying to ask is, how does this affect the whole tuning process? Do you just follow the standard model regardless of your location.....or for better (more accurate) results, should you not use the measured atmospheric conditions in which the car is located?

Or is it completely irrelevant and you just leave the ECU to make its own calculations?

Just to clarify, I understand how and why elevation should influence pressure, and such density. That is pretty darn basic science. I'm just asking why the standard model doesn't quite line up with measured conditions, and why the standard model doesn't take humidity into account...and if any of that is at all relevant to the tuning process

Any model is an approximation of many different things. The use of pre defined numbers to represent air pressure at sea level comes more from North American motorsport (drag racing especially) than it does from actual scientific studies of weather/climatic patterns around the world (meaning its just an average not an absolute). In order to compare different engines raced on different days with different weather conditions they needed a standard reference to base their outcomes on. Its the same concept as measuring 2 different cylinder heads airflow . The 1st at 10" H20 and the 2nd at 25" H2O and then calculating what the airflow for both would be at 28" H2O by using a conversion factor.

The ECU doesn't "see" barometric pressure as such, it measures either airflow or air pressure inside the engine compared to air pressure in the engine bay and uses IAT to calculate air density. We tune to the conditions the engine is actually subject to, not to some idealised standard reference that is used for comparative purposes, and let the ECU use its logic (maths) from its coding to adjust for conditions outside of where we have tuned it.

As an example, you're at sea level and you tune the engine, you don't drive it to the top of K2 or Everest and tune it to suit extremely thin air . Why not? because the ECUs logic will adjust fuel and timing to suit the differences in air pressure and density accordingly because the ECU manufacturer knows that thinner less dense air requires less fuel etc. through knowledge gained by testing real world testing.

I may have overlooked your original question about the relative baro difference between Johannesburg and Durban. I'm not familiar with the geography of South Africa but google tells me the two cities are approximately 600 km apart and Durban is on the coast while Johannesburg is inland. These factors will affect the atmospheric pressure of both locations which explains the pressure variation that you originally pointed out. Basically if you could measure the air pressure in Durban at sea level and then climb vertically to the same altitude as Johannesburg (vertically above Durban), the air pressure will be lower. Likewise if you could dig a hole in the ground in Johannesburg and burrow down to sea level, the air pressure would increase - It's all relative.

Remember that depending on your current atmospheric conditions the air pressure at sea level is also constantly moving target - Watch the weather report and you will see the barometric air pressure changes day to day and across the expanse of a country. That being said though on any given day at a fixed location, the air pressure will always be relatively lower as altitude increases.

So what effect does the baro pressure have on the engine performance and how do we deal with it? As air density decreases we have less oxygen available and hence engine power will be effected. This also means that to maintain a consistent AFR we need less fuel as barometric pressure drops.

If your ECU is using a MAP sensor as the load input, in a way the baro pressure is accounted for in the main fuel equation as manifold pressure is part of the ideal gas law. Let's say your N/A engine (turbo is a little more complex as we’ll see) is at WOT at sea level and 101.3 kPa baro. The MAP pressure will also be 101.3 kPa. Now at altitude we have a baro of 85 kPa and again at WOT the MAP sensor will read 85 kPa hence addressing the lower air pressure in the fuel calculation. BUT it's not quite that simple...

There are two considerations here - Manifold pressure and volumetric efficiency. There are two ways we can lower the manifold pressure in the engine: Close the throttle partially or lower the barometirc pressure. If we use MAP as the load input we are effectively saying that it doesn't matter what the throttle opening is, 85 kPa in the manifold is going to equal the same mass of air regardless of how we achieve it.

In reality there is a difference between 85 kPa in the manifold at let’s say 70% TPS and 101.3 kPa atmospheric pressure and 85 kPa at 100% TPS and 85 kPa baro because the TPS effects the VE of the engine and the MAP then defines the density of the air volume in the cylinder. It’s a subtle difference but a difference none the less.

Let’s put it another way that might be easier to picture. You have a tubocharged engine with a 1 bar spring in the wastegate. You tune the engine to achieve 0.80 lambda at WOT and 1.0 bar boost. Now you close the throttle to 70% but the turbo is so good at moving air, that you still have 1 bar boost in the manifold and this is what the ECU will base it’s fuelling off. In this situation you will likely see the lambda fall richer to maybe 0.77-0.78. This is because closing the throttle reduces the engine’s VE despite the manifold pressure remaining the same.

So how do you deal with it? Most ECUs don’t bother, allowing baro to be accounted for in the main fuel equation and usually this is just fine. If you aren’t experiencing large changes in baro pressure then properly accounting for baro is not going to be critical to you. What say you want to deal with it though? Read on.

Link offer an interesting option with their ECUs which is MGP or Manifold Gauge Pressure as the load input. This references the internal baro sensor to read the manifold pressure relative to the current barometric air pressure. This means that in a N/A engine at WOT, you will always be operating in the same load row regardless of the baro pressure. This means that VE isn’t effected and the baro is properly accounted for in the fuel equation.

That’s a neat solution for a N/A engine but what if you run a turbo? If the engine is turbocharged then it’s possible to maintain the same absolute pressure in the manifold regardless of altitude so on the face of it this seems to suggest that baro correction isn’t relevant - Surely 100% TPS and 200 kPa pressure in the manifold is the same, regardless of baro right? Well not really…

To maintain the same absolute pressure as the baro drops, the turbocharger ends up working at a higher pressure ratio (ratio between manifold pressure and barometric pressure). This has two effects - First the turbocharger will end up working in a different area of it’s efficiency, and secondly since the turbo is being worked harder, the turbine inlet pressure (back pressure) will rise. The ratio between manifold pressure and exhaust manifold pressure will therefore be effected by barometric pressure and this will affect the airflow through the engine.

Motec actually had a load mode on their Mx00 ECUs called MAP/EMAP which accounted for this although I haven’t ever been in a position to use it. Obviously on any ECU that has the option of defining a baro sensor, you could produce your own compensation table and account for baro pressure using that but this would require testing as it’s the ratio of MAP to EMAP that really counts and without measuring this directly you would be using a baro comp to sort of model this.

I’m not sure if the above novel will help clear up your questions or will simply aid to further confuse matters? Either way this is some relevant information on the topic that’s worth thinking about. Sorry if I’ve made matters worse :)

Thanks for the reply Andre. That does clear it up somehow.

As I said in one of my previous posts, I had considered the effects of a change in baro pressure in relation to a turbo's efficiency, and perhaps that of the intercooler too.

I'm rather new to this, so I can only really ask questions given the knowledge that I have so far (and the plethora of knowledge I've gotten from you guys), and what I hear from others.

One thing that stuck with me was that I heard form one of the tuners in South Africa that they see a 20% drop in power (from turbo'd motors) between Durban and Johannesburg.

Does that seem right to you? 20% is like quite a lot.

As I said, I'm new to this, but that sounds more like a misuse of the dyno correction factor that could have a hand in it

I'm gad that my novel helped rather than hindered :)

How much power you lose between Johannesburg and Durban will depend on the effective air density and if the air density drops by 20% then you expect a relative drop in engine power. It's all about the change in baro/altitude.

To date I haven't had the opportunity to test anything relating to large changes in baro. Having just moved to Queenstown though I am now in a position to do exactly that, hence the reason I've just fitted a baro sensor to our 86