The truth about elevation effects on turbo engines?

Jehu · Jul 8, 2003

I know what the title says, but

thiazole said:
My current hypothesis is that people at high elevation get doubly screwed by its effects and therefore run considerably slower than people at sea level contrary to the popular belief that turbo engines in fact suffer much less from elevation effects than N/A engines (I suggest the effects are about same between the two).

This suggests that the MEAT of the post is arguing this point...to me, the actual content is more important than the title...call me old fashioned...

As I said in my discussion, neglecting temperature effects, on the same car with NOTHING ELSE being different, ONLY THE ELEVATION, why is the pressure not a valid indication of RELATIVE HP differences? Holding the temperature and volume constant, guess what, pressure will be directly proportional to your mass. That's what simplification is all about, and is almost always what you have to do when applying simple theory to complex real-world applications. Sure it'll be nice to do a thorough calculation for mass flow, but you'll be making assumptions about the temperatures, volumetric flow rates etc anyway...this is just an easy way of figuring out relative effects.

Taboo · Jul 8, 2003

Jehu said:
Holding the temperature and volume constant, guess what, pressure will be directly proportional to your mass.

Nope. You forgot to consider the reduced air density at high elevation - which is the point of the entire discussion.

Mass = Density x Volume

If you want to compensate for the reduction of density by pressure (boost) increase, you HAVE TO consider the increased air temperature as well since density is based on both temperature and pressure. Try to run a 14B with the stock intercooler at 16 psi and 20 psi and see if the temperature and volume of the intake charge stays constant - as you stated.
Ignoring important facts is simply not "an easy way of figuring out relative effects when applying simple theory to complex real-world applications", it's a way to obtain worthless data.

Pressure is never a valid indication of relative HP since increasing the pressure doesn't necessarily increase the mass of the air going to the engine. That's also the reason why a turbocharged car might produce the exact same HP numbers at lower boost at sea level as at high boost at Bandi. It does not matter how many psi of boost one runs, but how much HP he produces.

LaserRS90 · Jul 8, 2003

I try reading a little bit about this topic from book called 'fuel systems and emission controls' and it had portion about supercharging and turbocharging, so I wanted to share that info if I may. I know this is or could be something you already knew since it is basic material.

"The volumetric efficiency of any normally aspirated engine is related to the density of the air drawn into it. Since atmospheric pressure and air density are greatest at (or below) sea level, both pressure and density normally decrease at altitude above sea level increases. For example, atmospheric pressure at sea level is about 14.7 psi (101 kPa); at higher elevations atmospheric pressure may be only 8 or 9 psi (55 or 62 kPa). Therefore, the volumetric efficiency of any egine will be greater at sea level than at altitude above sea level.
Pumping air into the intake system under pressure forces it through the bend and restrctions at a greater speed than it would travel under normal atmospheric pressure, allowing more air to enter the intake port before it closes. By increasing the engine's air intake in this manner, more fuel can be mixed with the air while still maintaining the same air-fuel ratio. Th denser the air-fuel charge entering the engine during its intake stroke, the greater the pontential energy released during combustion. In addition to the ncreased power resulting from combustion.
A turbocharger pressurzes air to greater than atmospheric pressure. The pressurization aove atmosphericpressur, or boost, can be measured in the same way as atmospheric pressure. Atmospheric pressure drops as altitude increases, but boost pressure remains the same. If a turbocharger develops 12 psi (83 kPa) boost at sea level, it will develop the same amount at a 5,000-foot altitude."

Jehu · Jul 8, 2003

Taboo said:
Nope. You forgot to consider the reduced air density at high elevation - which is the point of the entire discussion.

Mass = Density x Volume

If you want to compensate for the reduction of density by pressure (boost) increase, you HAVE TO consider the increased air temperature as well since density is based on both temperature and pressure. Try to run a 14B with the stock intercooler at 16 psi and 20 psi and see if the temperature and volume of the intake charge stays constant - as you stated.

Sigh...I'm going to try one last time...just TRY for a second to take this in context...

Why is reduced air density not taken into consideration? Let's say both runs are taken at standard room temperature of 25 deg. C. In the ASL example, atm is 12psi. That's all you need to account for the lower density.

You example of running 16 vs 20psi on a 14b is neither here or there. It is NOT analogous to the scenario of running 15psi on a 14b at sea level and running 15psi on a 14b at 5800 ft. At the higher elevation, the only impact on the turbo is only that the pressure ratio is now slightly more, and it might lose a few percent effeciency. If you look at the calculations, you will see the the reduced density is taken into account in the 12(atm) term. And relative to the turbo, the effect is stated in the 2nd last line of my original post.

Taboo said:
Ignoring important facts is simply not "an easy way of figuring out relative effects when applying simple theory to complex real-world applications", it's a way to obtain worthless data.

I'd argue that not taking the time to understand and to thoroughly read what is written is a way to make data worthless. There are valid simplifications that can be made when you look at a complex problem, the key is knowing where to make them, and that comes with having a healthy understanding of all the nuances involved.

thiazole · Jul 8, 2003

Taboo and Jehu,

Let me help out. Using the ideal gas equation (not my favorate equation as stated previously, but it will make the point) we have PV=nRT. We want to assume that at the same pressure ratio (ie, same boost gauge reading) that T is constant, and of course R is constant, and since my intercooler and piping aren't expanding (much) V is constant too. So, the variables are P and n. Since they are on opposite sides of the equation and there aren't any exponents, they will have a positive correlation of 1. It is good that these are the only 2 terms that vary at a constant pressure ratio, because they are also the most important to making power. The importance of n is obvious. n is the expression of how many molecules are air we are talking about. The more n we can get in our engine, the more fuel we can burn and the more power we make. P is more complicated, though. Increasing P gives us TWO advantages. The first is by increasing n (remember, they have a perfect positive correlation in our example, so if P gets bigger, so does n). The second is by forcing air into the engine that normally wouldn't want to enter under reduced pressures. Remember that you can have all the n in the world, but without P to force it into the engine, it doesn't help much. The pressure allows us to substantially exceed 100% VE. But, Jehu, you are right in that P perfectly (ideally, of course) correlates with n assuming constant T, which should be the case assuming the pressure ratio and outside temp is constant. So therefore, using P to compare air mass from sea level to Bandimere is valid assuming we only compare at the same pressure ratios.

LaserRS90,

I think the author of that book missed it on that statement. 12 relative psi at sea level is not the same as 12 relative psi at high elevation (other than the force it exerts on the walls of your intake). If he is talking about absolute pressure, then pressurizing lower pressure air to the same absolute pressure as when pressurizing higher pressure air will create a higher temperature and less air density. Ie, compressing 70deg 1psi air to 16psi will make it hotter than hell (and the heat will contribute to the pressure without contributing to the air density). Pressurizing 70deg 15psi air to 16psi will barely change the temperature - same final pressure - not the same fuel burning, knock preventing air mass.

Taboo · Jul 8, 2003

Jehu said:
Why is reduced air density not taken into consideration? Let's say both runs are taken at standard room temperature of 25 deg. C. In the ASL example, atm is 12psi. That's all you need to account for the lower density.

Jesus Holy f*cking Christ...

x1000. How hard is it to understand that while air has a density of 0.0765 lb per cubic foot at sea level, it has a density of only 0.0565 lb per cubic foot at 10,000 feet altitude? Consequently, even if the pressure ratio and inlet temperature stay the same, the turbo efficiency is affected by the change in the air density ratio - which directly affects the air volume and consequently also the mass of air. You're NOT compressing the air of the same density at different elevation levels. This "discussion" is like trying to nail Jell-O to a tree... If change in air density at different elevation levels doesn't matter, then also the temperature of the intake charge doesn't mater either (since it directly affects the air density as well) - in which case we obviously don't need intercoolers and the only thing that matters is the volume of air we push to the engine since mass = volume now, right?

Van · Jul 9, 2003

thiazole said:
I would still think that someone at Bandimere could safely boost about 5-10% more "gauge boost" than someone at sea level. So, if someone with a 16 g can only boost 20 gauge psi at sea level, then he should be able to get away with 21-22psi with the same parameters at Bandimere. Anyone disagree?

Van, if you move to Arizona, make sure you let us know how much faster your car runs there. I am VERY curious. I've heard you say you thought you could only pick up a half a second, but as long as you race on a cool day, I bet you could pick up 7-8tenths.

I disagree with the 5 to 10% more gauge boost IF the turbo is now out of the same efficiency island on the comp map. The turbo would have to stay in the same efficiency island to keep it the same. So that is not necessarily safely with some turbos.

I'm in Az now but the car has changed alot since last season. The 20 psi vs 21-22 psi is not quite right. We/I typically use 12 as the atmospheric pressure number (instead of 14.7) when figuring the pressure ratio and where you are on a comp map up there. That number seems to hold water IMHO. It's roughly 20% less to start with.

14.7 + 29psi / 14.7 = pressure ratio of 2.97
12 + 29 psi / 12 = pressure ratio of 3.42

Those are two very different places on most compressor maps.

It's interesting because at 29-30 psi at Bandi I was equalling the MPH of similar weight dsms at 25 psi at much lower elevations. ET is driving, so MPH is better used here. There's that roughly 20% again, eh.

It took roughly 20% more boost psi at the gauge to equalize the mph and I was still a little short.

I felt it would be about a half second because with the same driving skills it would be that much easier and the MPH would be better equalized without an absolute perfect run.

Someone else said it would be richer in this thread. The MAS figures that for you. Any tweaking you do after that initial signal has that accounted for.

Jehu · Jul 9, 2003

Okay, let's see if you believe the equations...
Let's start with everybody's favourite equation, the ideal gas law.
PV=nRT. What is density?
basically n/V
P=(n/V)RT
Holding T constant, P is directly proportional to n/V.
So, P is directly proportional to density. Basically, by looking at P, 12psi atm pressure IS saying that the air is less dense. If you bother to calculate it out, you will see that the % reduction in atmospheric pressure at a particular altitude is exactly the % reduction in density. Do YOU get that yet?

What should be pretty obvious by now is that I'm saying the turbo does NOT CARE what the original density is. It just happily, for example, halves the volume, i.e. doubles the density. So, if you compress less dense air, you get less dense compressed air as a result. Hence, compressing air at sea level to 14.7psi gauge pressure gives you a final absolute pressure of 29.4psi. Compressing air at 5800 ft to 14.7psi gauge pressure gives you a final absolute pressure of 26.7psi. So, you ARE getting less mass flow at higher altitudes, no if ands or buts about it. And the point is that relative to a N/A car though, it is still less of a relative drop in air pressure/air mass in a turbo car.

Can you not understand that pressure, mass, volume and temperature is all tied together? Under the right situations, you CAN relate pressure to mass directly, or pressure to volume, etc etc.

Van,
You are correct, the boost gauge is reading gauge pressure and if anything else, you'd want to reduce your boost pressure a little to be safe to keep the turbo in the same efficiency island. If you are already on the ragged edge, keeping the same boost pressure will result in a higher pressure ratio, push the turbo efficiency down and that COULD just push you over the edge. It is NOT safer for you to run more boost at higher elevations.

thiazole · Jul 9, 2003

Ok, everybody seems hell bent on assuming that 14.7 psi gauge pressure is really 14.7 psi greater than atmospheric regardless of what atmospheric pressure is in spite of my objections. Well, I have a trick question for anyone who still believes that. The cylinders on my car are supposed to have 178psi during a compression test. Assuming my cylinders test at 178psi at sea level, what would a pressure gauge show at Bandimere?

While we are at it, another "trick" question that I don't know the answer too... how do we get 178psi with 8.5:1 compression?? 8.5X14.7psi=125psi.

Taboo · Jul 9, 2003

Taboo said:
Mass = Density x Volume

If you want to compensate for the reduction of density by pressure (boost) increase, you HAVE TO consider the increased air temperature as well since density is based on both temperature and pressure.

Jehu said:
Can you not understand that pressure, mass, volume and temperature is all tied together? Under the right situations, you CAN relate pressure to mass directly, or pressure to volume, etc etc.

Duh...

Jehu said:
What should be pretty obvious by now is that I'm saying the turbo does NOT CARE what the original density is.

Duh...

Taboo said:
In the long run, it's simply about supplying the engine with the same mass of air (while elevation doesn't have any significant effect on the mass of fuel) of the same temperature at any altitude level in order to maintain the same A/F ratio and produce the same HP numbers without approaching the detonation threshold of the engine.

Taboo said:
Consequently, even if the pressure ratio and inlet temperature stay the same, the turbo efficiency is affected by the change in the air density ratio - which directly affects the air volume and consequently also the mass of air.

Jehu said:
So, you ARE getting less mass flow at higher altitudes, no if ands or buts about it.

Finally... zzzz Jehu, do you actually have any idea why and what you're arguing about or do you argue just for the sake of arguing? :dunno: :slam:

Jehu · Jul 9, 2003

Well, let's see

Taboo said:
Hmmm, in that case you must be interpreting the topic of this thread - "The truth about elevation effects on turbo engines?" a little bit differently than I do... :dunno:

That's the exact fundamental point you missed. Boost does not equal HP or air mass. That's why looking just at boost pressures is literally worthless and miles from being sufficient enough to draw some conclusions or make some justifications.

THIS is originally what started the argument. Do YOU know what YOU are arguing about? Why don't you read through the thread a take a look at the developement of the posts.

You go from arguing that looking at boost pressure being worthless, to now suddenly only about the points you stated? I don't think so. My point, and it has ALWAYS been this, is that using the pressures of the air (boost and atmospheric) lets you get a good idea of what's happening in the SAME engine, at different altitudes. Maybe you are unable to interpret the equations and understand all the implications behind them, but all that YOU'VE said has all been in the equations and in my original statement. All I've been trying to convince you of is the it IS valid looking at pressures. Have you at least understood that???

And Thiazole...
Gauge pressure is exactly just that. The pressure above ambient. That's how a bourbon gauge works. It is a curled tube that uncurls as the internal and external pressure differential rises. Hence, it will measure pressure relative to the atmospheric pressure.

Taboo · Jul 9, 2003

Jehu said:
Why don't you read through the thread a take a look at the developement of the posts.

Yes, I have - and not even once you mentioned air mass in any of your posts (most likely because you had no idea that such a thing even existed) - until I mentioned it as the very fundamental point that was completely missed while you kept babbling about pressure.

Jehu said:
You go from arguing that looking at boost pressure being worthless, to now suddenly only about the points you stated? I don't think so. My point, and it has ALWAYS been this, is that using the pressures of the air (boost and atmospheric) lets you get a good idea of what's happening in the SAME engine, at different altitudes.

OK, try to concetrate and read the sentences bellow very slowly:

Taboo said:
Boost does not equal HP or air mass. That's why looking just at boost pressures is literally worthless and miles from being sufficient enough to draw some conclusions or make some justifications.

Now, look at the chart bellow (in case you missed it the first time) and in case you still don't understand it, I may tell you how to calculate volume flow for given turbocharger operating at peak conditions based on the volume flow of the engine, pressure and density ratios and how to figure out efficiency of any turbocharger based on the obtained data.

Jehu · Jul 9, 2003

And how many times do I have to say that in this case, it is reasonable to say the pressure IS directly proportional to density IS directly proportional to air mass???? Do you NOT know that without any mass, there won't be any air, hence there won't be any pressure???? By holding T steady (same efficiency point, the increase in pressure ratio leading to low efficiency IS mentioned), the fact that those terms are directly proportional IS already right there in the equation. Like I said, if you understood the equation, you would have grasped that already. Do I have to spell EVERY assumption out like I'm talking to a high school student? I had assumed this was advanced talk and people would already have a grasp of basic physics.

And your curve illustrates my point perfectly. Pressure ratios are directly proportional to density ratios...(you do understand what directly proportional is right? i.e. y=bx, b can be ANY constant). So now let me ask you again...why is it wrong to use pressures to talk about density/air mass?

Talk is cheap. Why don't you quantify what you are saying with some equations? Back to the first implied question...does a change in elevation affect a turbo car more/less than a N/A car, and how. Equations please. Everybody can spew words...let's see some actual calculations. Try to make a good engineering estimate, which I thought was the whole point of this exercise???

thiazole · Jul 9, 2003

Ok, everybody seems hell bent on assuming that 14.7 psi gauge pressure is really 14.7 psi greater than atmospheric regardless of what atmospheric pressure is in spite of my objections. Well, I have a trick question for anyone who still believes that. The cylinders on my car are supposed to have 178psi during a compression test. Assuming my cylinders test at 178psi at sea level, what would a pressure gauge show at Bandimere?

I'll take it that no one knows the answer to this one. That's ok, I'm not sure I know either. I asked it because since the piston takes air volume A and compresses it to volume B, then while the pressure ratio stays the same, the absolute pressure would change between different elevations. If I am right, then the gauge would read 178 psi no matter where you tested the compression. I still win though, even if I am wrong, because that means that the 150psi that I measured on my 125000 mile engine at 5000ft is corrected to almost perfect stock compression.

Jehu · Jul 10, 2003

Actually, it will not be 178 psi at elevation. Even if the pressure ratio remains the same, the pressure ratio is end pressure/start pressure. So, at Bandimere, where the starting pressure is 12psi instead of 14.7psi at sea level, the final pressure will be correspondingly lower.

Van · Jul 10, 2003

Right even the compression check numbers are lower than normal up there. I should do a check down here to compare. But anyways, I used to live up in the mountains at about 8500 ft elev. I showed a difference in the compression numbers just between my home's elevation and down in Denver at 5280 ft elev.

I think I was at about 120 psi average in all the cylinders up at 8500 ft and up to about 130 psi in them down in Denver. It wasn't quite 10 psi difference though. I just figure it was about 20% lower than what it would have read down at sea level.

When I get around to it I'll check.

Taboo · Jul 10, 2003

Jehu said:
Talk is cheap. Why don't you quantify what you are saying with some equations? Back to the first implied question...does a change in elevation affect a turbo car more/less than a N/A car, and how. Equations please. Everybody can spew words...let's see some actual calculations. Try to make a good engineering estimate, which I thought was the whole point of this exercise???

OK..

Here are equations of turbo at its maximum efficency at sea level and 10,000 feet (which is exaggerated just for the demonstration). In both cases, the boost is set at 20 psi and the ambient temperature is the same (70'F) with 80% engine efficiency and 70% intercooler efficiency..

Sea level
Inlet temperature = 70
Boost=20 psi
Engine volume flow = 200 cfm (7.5K RPM, 80% efficiency)
Ambient pressure = 14.7
Absolute pressure = 34.7
Pressure ratio = 2.36
Y=0.27
T ideal = 0.27*(70+460) = 143F
T actual = 143/0.7 = 204F
Intercooler effectivness = 70%
Intake manifold temperature = (204 + 70)*0.7 = 191
Density ratio = 530/651 * 69.9/29.9 = 0.814 * 2.33 = 1.89
200*1.89 = 378 @ 2.36
Mass = 0.0765 * 346 = 28.91

10,000 FT
Ambient pressure = 10.15
Absolute pressure = 30.15
Pressure ratio = 2.97
Y=0.36
T ideal = 0.36*(70+460) = 190F
T actual = 190/0.6 = 316F
Intake manifold temperature = (316 + 70)*0.7 = 270
Density ratio = 530/730 * 69.6/29 = 0.726 * 2.33 = 1.69
200*1.69 = 338 @ 2.97
Mass = 0.0565 * 338 = 19.09

Since HP is all that really matters and what's affected by air mass reduction due to several factors, one can't simply apply Ideal Gas Law and call it done. As a rule of thumb, turbocharger speed will increase approximately 2% per 1000 feet of altitude - which further contributes to intake charge temperature increase (that was already increased by pressure ratio increase and decreased compressor efficiency) which results in even bigger air mass decrease dirrectly affected by density ratio drop. If one starts with maxed out turbo at sea level, he can't simply base the equations (with increasing altitude) just on pressure without considering the effect of increased outlet temperature on density and consequently the mass of air. One could possibly do this only if the turbo doesn't run out of its maximum efficiency range (as I stated in my very first post in this thread) at the increased altitude - in which case the turbo wouldn't more than likely ran at its peak efficiency at the sea level.

In the long run, change in elevation affects a turbo car less than a N/A car, but how much less is going to be dictated namely by the efficiency of the turbocharger and intercoller while turbos that are able to operate at their maximum efficiency at broad shaft speeds will effect the reducation in air mass less even at increased pressure ratios.

thiazole · Jul 10, 2003

Taboo,

Nice explanation of what you were previously saying. Another of the factors that we have ignored is the fact that an intercooler's cooling ability will drop with the decreased air density at elevation(less air molecules to absorb heat). With 20% less air molecules(31% in your example), this would definately be a significant factor (especially when we are tring to cram more than 80% of the sea level mass of air through the engine). My conclusion is this. One might be able to squeak out a small improvement with a turbo over a NA engine, but not much. I would say from the info that I have so far that the larger the ratio between turbo CFM and the calculated NA CFM of the engine, the more you can make up with the turbo. I still think you need at LEAST 30psi to see much difference and probably much more. I think that the turbos that most of us use are just too small to make that significant of a difference.

Van · Jul 10, 2003

thiazole said:
I still think you need at LEAST 30psi to see much difference and probably much more. I think that the turbos that most of us use are just too small to make that significant of a difference.

Not really. A turbo dsm is the shiznit at Bandimere. Not much else actually keeps up. Vette's are regular DSM food up there, with not many mods either.
Whereas the N/A cars really suffer, it only takes the Honduh boys a couple of races to see how outgunned they really are by a DSM up there.

Taboo · Jul 10, 2003

I'm convinced that any generalization expressed in percentage of difference between N/A and turbocharged engines at given altitude is just as "accurate" as estimate of driveline loss on RWD and AWD cars... You're absolutely right about the cooling ability of the intercooler, there's just so many factors that affect the final conclusion that we could play with all kinds of equations for ever - and finally come up with something fairly accurate that would apply only to some particular turbo, inetrcooler and engine setup running in very specific conditions - and the very next combination of all the elements and factors would throw everything completely off. There's just way too many variables... :dunno:

The truth about elevation effects on turbo engines?

Jehu

Taboo

LaserRS90

Jehu

thiazole

Taboo

Van

Jehu

thiazole

Taboo

Jehu

Taboo

Jehu

thiazole

Jehu

Van

Taboo

thiazole

Van

Taboo

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