Boost and altitude?
#21
Originally Posted by Q45tech
The question is "Does the atmosphere in Denver contain the exact number of oxygen molecules CORRECTED for barometric decrease as the atmosphere at sea level.
Denver is about 0.85 of sealevel so normal Denver pressure is 12.5 psi and you would need 2.2 PSI more boost to equal the same power at sealevel.
http://www.phoenixelectrode.com/lvl2/oxygenguide.html
A pound is not the same everywhere on earth as gravity varies around the earth by up to 1%.
Denver is about 0.85 of sealevel so normal Denver pressure is 12.5 psi and you would need 2.2 PSI more boost to equal the same power at sealevel.
http://www.phoenixelectrode.com/lvl2/oxygenguide.html
A pound is not the same everywhere on earth as gravity varies around the earth by up to 1%.
#22
8 psi is 8 psi no matter what altitude your at. I use to own an Evo an sea level guys would tell me all the time that running 25lbs of boost is too much for the stock 4G63T. But as mention the air is thinner so you can run more boost. The altitude does affect boosted cars just not as much as a N/A car.
#23
Originally Posted by roneski
Ok, I see what you're getting at. BUT!
At sea level let's lets say that you have 14.7 psi of atmospheric pressure add our 8 pounds of boost to that and you have a net 22.7 psi.
Now we go to Denver where atmospheric pressure should be slightly less than an atmosphere. Let's just pretend that the atmospheric pressure is 14.2 there (because I have no idea about what it actually is) that gives you a net pressure of 22.2 psi.
This is where I am getting confused. Wouldn't the boost be indexed to the atmospheric pressure of where you are actually measuring it?
At sea level let's lets say that you have 14.7 psi of atmospheric pressure add our 8 pounds of boost to that and you have a net 22.7 psi.
Now we go to Denver where atmospheric pressure should be slightly less than an atmosphere. Let's just pretend that the atmospheric pressure is 14.2 there (because I have no idea about what it actually is) that gives you a net pressure of 22.2 psi.
This is where I am getting confused. Wouldn't the boost be indexed to the atmospheric pressure of where you are actually measuring it?
The pressurized intake is a closed environment and has no correclation to what is happening to the atmosphere outside. The wastegate is telling the turbocharger to make 8psi. The wastegates job is to send enough exhaust to the turbocharger to make that boost.
JET
#24
Originally Posted by Q45tech
The question is "Does the atmosphere in Denver contain the exact number of oxygen molecules CORRECTED for barometric decrease as the atmosphere at sea level.
Denver is about 0.85 of sealevel so normal Denver pressure is 12.5 psi and you would need 2.2 PSI more boost to equal the same power at sealevel.
http://www.phoenixelectrode.com/lvl2/oxygenguide.html
A pound is not the same everywhere on earth as gravity varies around the earth by up to 1%.
Denver is about 0.85 of sealevel so normal Denver pressure is 12.5 psi and you would need 2.2 PSI more boost to equal the same power at sealevel.
http://www.phoenixelectrode.com/lvl2/oxygenguide.html
A pound is not the same everywhere on earth as gravity varies around the earth by up to 1%.
Right about the gravity thing. But that's simply too immeasurable and insignificant to be a factor.
JET
#25
Originally Posted by Robert_K
8 psi is 8 psi no matter what altitude your at. I use to own an Evo an sea level guys would tell me all the time that running 25lbs of boost is too much for the stock 4G63T. But as mention the air is thinner so you can run more boost. The altitude does affect boosted cars just not as much as a N/A car.
JET
#26
Originally Posted by 350zDCalb
I think Gurgen is gonna have to get on here and bust out some crazy Gurgen Physics...be warned, the mad scientist is alerted
#27
I'm an airline pilot and endured 4 years of aviation physics not to mention about two thousand hours in a turbocharged Piper Arrow. I think I know how altitude affects turbocharged motors. Jet engines are axial flow turbochargers. The same principles apply to a turbojet as they do to a turbocharger. Last time i checked I had over 5000 hours in turbojet aircraft. The above facts I stated stand true.
JET
JET
#31
Originally Posted by JETPILOT
I'm an airline pilot and endured 4 years of aviation physics not to mention about two thousand hours in a turbocharged Piper Arrow. I think I know how altitude affects turbocharged motors. Jet engines are axial flow turbochargers. The same principles apply to a turbojet as they do to a turbocharger. Last time i checked I had over 5000 hours in turbojet aircraft. The above facts I stated stand true.
JET
JET
#33
Originally Posted by JETPILOT
The reason being that high altitude locations such as Denver don't get 93 octane. The highest they get is 91 I believe.
JET
JET
#34
OK...here it goes... lets hope i read everything thoroughly enough.
Increased altitude affects the working of an internal combustion engine two-fold (mainly): volumetric efficiency and tubro compressor efficiency. The latter affects NA cars to a larger extent than FI cars, and the latter is MOSTLY relevant to the turbo cars and somewhat to SC cars.
1) Volumetric efficiency.
At higher altitudes, there is less air density available. Hence, two things happen, less oxygen (ratio of which is about 21%) and less backpressure (the latter one is almost never mentioned). Less oxygen effectively decreases the oxygen-based (not total) volumetric efficiency of the engine, directly decreasing combustion output, while the exhuast backpressure in a way works in the opposite direction, allowing for greater efficiency of the oxygen that IS present. Of course, the former beats the latter with respect to to the overall effect on engine output. If you were to magically be able to run an NA car in space, or, say, in a specialized chamber with the intake pipe in an atmosphere filled with air, with the tailpipe outputting into a 0 absolute pressure environment (full vacuum), this would significantly volumetric efficiency, since not only the pistons on their intake stroke would be doing the 'sucking', but also the atmosphere. This is especially true with OUR cars, since the internal EGR (i.e. EGR due to cam overlap) is also affected, allowing
for a greater VE, especially on high overlap cams. Onto turbo cars and VE. The same that i said for NA (which also applies to SC cars) applies to turbo cars, but to a different extent. This is because the backpressure seen at the engine and the effective internal EGR is a much bigger factor, given the high exh. manifold pressures of turbo cars imposed by the turbine blades' load. But, there are other things, discussed next that are a far bigger factor.
2) Turbo Compressor Efficiency
Say you tune a car in L.A. for 0.5 bar boost. That's 1.5 bar absolute, and the absolute and not relative pressure is what matters, since 1) it's the number that is directly proportional to the mass, or, more accurately, moles (# of molecules) of gas (read: oxygen) entering the intake manifold (hence the term manifold absolute pressure "MAP"), and 2) it's the number that's therefore used for tuning. Now, 1.5 bar MAP is the same everywhere...it's the matter in which that pressure is achieved that's relevant. So, you tune your car perfectly in L.A. (by Sam at GTM ), and drive to Albuquerque to party with Todd. When you get there, and you have a turbo car (of course, that's the only way to race against Todd on the way to the vegan restaurant of choice), and not change your boost controller setting, your car will NO longer develop 1.5 bar absolute. If you have a regular analog boost gauge, it will still say 0.5bar boost. However, that boost setting in L.A. meant that you are getting 0.5bar "boost" over the ambient pressure of 1 bar (assuming that's what hte pressure in L.A. was), or 1.5 bar absolute. But your car is not developing 1.5 bar of MAP, but it's developing about 0.5bar+0.816=~1.22 bar. Why? Because the compressor generates pressure according to PR (pressure ratio). It starts with an ambient, and compresses it a certain percentage. When you start with 0.82 bar ambient and compress it X amount (the amount regulated by the tubine's wastegate and consequently by the turbine/compressor spin rate), you get pressure Y, that's PR. This is why the use of the term "boost" is not always accurate. Starting with a lower ambient pressure, is going to get you less pressure in the intake manifold, i.e. lower MAP. Also, to achieve 1.5 bar MAP again you will need to change the boost setting on your wastegates to a higher value, because you need to compress the existing smaller pressure by a greater amount to get back up to 1.5bar. Now, your car was of course tuned for everythign up to 1.5 bar MAP, including 1.22 bar MAP, right? So to get to 1.5 bar MAP you will need to just up the boost setting? Well, no, not exactly. The MAP of 1.22/1.5 bar is THE the same as 1.22/1.5 bar MAP in L.A. and NM only with respect ot pressure and NOT with respect to oxygen content in the manifold, because oxygen content is determined by temp AND pressure. Look at how you develop 1.22 bar MAP at sea level and at 5500'. To get 1.22 bar MAP in L.A. the compressor will have to compress 1 bar 'into' 1.22 bar, that means a pressure ratio of 1.22bar/1bar, equalling PR of 1.22. 1.22 bar at Todd's is 1.22bar/0.82bar equaling 1.49 PR. Now, look on a compressor map, which you will notice are ALL contructed in a PR vs flow, not boost vs flow manner. 1.49 PR will for our turbos mean a lower compressor efficiency. The lower the compressor efficiency, the more of the pressure is 'made up' of by temperature (higher temp meanshigher pressure everythign elese being equal) and less by number of gas molecules. This is why on a turbo car 1.22 bar MAP in L.A. is NOT the same as 1.22 bar MAP in NM or CO, an precisely why baro pressure correction is necessariy. The same is true if oyu try to up the boost back up to 1.5 in a 5500' ft elevation environment. 1.5bar/0.86 bar equals a PR of 1.83 PR, which is higher than 1.5 PR (1.5bar/1bar) that would be required for 0.5 bar boost at sea level. You can see why your tune would be off, or more precisely richer at higher elevations, not because there is less pressure (because you are still tuned for the lower MAP when you tune at hte higher map) but because the pressure is made up of less amout of air and higher temperature. If for example the compressor efficiency is 3-5% less at 1.83PR compared to 1.5PR, and assuming you ARE in fact perfectly tuned with baro press compensation (to fix the AFR), you will still make less power at elevation.
if oyu thought the compressor efficiency was complex, it doesn't stop there. Volumetric efficiency comes into play as well... and i am not giing into that here.... too long/complicated. Just try to integrate the two discussions.
One thing to note, none of this is any longer an issue if you use MAF for tunign and not map, since MAF measures MASS of molecules directly (which is not dependent on temperature or anything else to be extrapolated into number of molecules - which is what's relevant). MAF however has its limitations, mostly range/voltage filtering issues (the voltage output is less stable compared to MAP...hence less resolution), and is just not possible with very high boost cars. But, if you are tuned on MAF, you are tuned period, since, using the above example, it would measure the mass of air going into the engine irrespective of compressor efficiency. The compressor is less efficient at altitude, it will pump less air and more heat...but we don;t care b/c the heat factor is out of the equation.
This gives you an idea how complex car tunign is, and why you shouldn't attempt it on your own.
I am spent. HTH
Increased altitude affects the working of an internal combustion engine two-fold (mainly): volumetric efficiency and tubro compressor efficiency. The latter affects NA cars to a larger extent than FI cars, and the latter is MOSTLY relevant to the turbo cars and somewhat to SC cars.
1) Volumetric efficiency.
At higher altitudes, there is less air density available. Hence, two things happen, less oxygen (ratio of which is about 21%) and less backpressure (the latter one is almost never mentioned). Less oxygen effectively decreases the oxygen-based (not total) volumetric efficiency of the engine, directly decreasing combustion output, while the exhuast backpressure in a way works in the opposite direction, allowing for greater efficiency of the oxygen that IS present. Of course, the former beats the latter with respect to to the overall effect on engine output. If you were to magically be able to run an NA car in space, or, say, in a specialized chamber with the intake pipe in an atmosphere filled with air, with the tailpipe outputting into a 0 absolute pressure environment (full vacuum), this would significantly volumetric efficiency, since not only the pistons on their intake stroke would be doing the 'sucking', but also the atmosphere. This is especially true with OUR cars, since the internal EGR (i.e. EGR due to cam overlap) is also affected, allowing
for a greater VE, especially on high overlap cams. Onto turbo cars and VE. The same that i said for NA (which also applies to SC cars) applies to turbo cars, but to a different extent. This is because the backpressure seen at the engine and the effective internal EGR is a much bigger factor, given the high exh. manifold pressures of turbo cars imposed by the turbine blades' load. But, there are other things, discussed next that are a far bigger factor.
2) Turbo Compressor Efficiency
Say you tune a car in L.A. for 0.5 bar boost. That's 1.5 bar absolute, and the absolute and not relative pressure is what matters, since 1) it's the number that is directly proportional to the mass, or, more accurately, moles (# of molecules) of gas (read: oxygen) entering the intake manifold (hence the term manifold absolute pressure "MAP"), and 2) it's the number that's therefore used for tuning. Now, 1.5 bar MAP is the same everywhere...it's the matter in which that pressure is achieved that's relevant. So, you tune your car perfectly in L.A. (by Sam at GTM ), and drive to Albuquerque to party with Todd. When you get there, and you have a turbo car (of course, that's the only way to race against Todd on the way to the vegan restaurant of choice), and not change your boost controller setting, your car will NO longer develop 1.5 bar absolute. If you have a regular analog boost gauge, it will still say 0.5bar boost. However, that boost setting in L.A. meant that you are getting 0.5bar "boost" over the ambient pressure of 1 bar (assuming that's what hte pressure in L.A. was), or 1.5 bar absolute. But your car is not developing 1.5 bar of MAP, but it's developing about 0.5bar+0.816=~1.22 bar. Why? Because the compressor generates pressure according to PR (pressure ratio). It starts with an ambient, and compresses it a certain percentage. When you start with 0.82 bar ambient and compress it X amount (the amount regulated by the tubine's wastegate and consequently by the turbine/compressor spin rate), you get pressure Y, that's PR. This is why the use of the term "boost" is not always accurate. Starting with a lower ambient pressure, is going to get you less pressure in the intake manifold, i.e. lower MAP. Also, to achieve 1.5 bar MAP again you will need to change the boost setting on your wastegates to a higher value, because you need to compress the existing smaller pressure by a greater amount to get back up to 1.5bar. Now, your car was of course tuned for everythign up to 1.5 bar MAP, including 1.22 bar MAP, right? So to get to 1.5 bar MAP you will need to just up the boost setting? Well, no, not exactly. The MAP of 1.22/1.5 bar is THE the same as 1.22/1.5 bar MAP in L.A. and NM only with respect ot pressure and NOT with respect to oxygen content in the manifold, because oxygen content is determined by temp AND pressure. Look at how you develop 1.22 bar MAP at sea level and at 5500'. To get 1.22 bar MAP in L.A. the compressor will have to compress 1 bar 'into' 1.22 bar, that means a pressure ratio of 1.22bar/1bar, equalling PR of 1.22. 1.22 bar at Todd's is 1.22bar/0.82bar equaling 1.49 PR. Now, look on a compressor map, which you will notice are ALL contructed in a PR vs flow, not boost vs flow manner. 1.49 PR will for our turbos mean a lower compressor efficiency. The lower the compressor efficiency, the more of the pressure is 'made up' of by temperature (higher temp meanshigher pressure everythign elese being equal) and less by number of gas molecules. This is why on a turbo car 1.22 bar MAP in L.A. is NOT the same as 1.22 bar MAP in NM or CO, an precisely why baro pressure correction is necessariy. The same is true if oyu try to up the boost back up to 1.5 in a 5500' ft elevation environment. 1.5bar/0.86 bar equals a PR of 1.83 PR, which is higher than 1.5 PR (1.5bar/1bar) that would be required for 0.5 bar boost at sea level. You can see why your tune would be off, or more precisely richer at higher elevations, not because there is less pressure (because you are still tuned for the lower MAP when you tune at hte higher map) but because the pressure is made up of less amout of air and higher temperature. If for example the compressor efficiency is 3-5% less at 1.83PR compared to 1.5PR, and assuming you ARE in fact perfectly tuned with baro press compensation (to fix the AFR), you will still make less power at elevation.
if oyu thought the compressor efficiency was complex, it doesn't stop there. Volumetric efficiency comes into play as well... and i am not giing into that here.... too long/complicated. Just try to integrate the two discussions.
One thing to note, none of this is any longer an issue if you use MAF for tunign and not map, since MAF measures MASS of molecules directly (which is not dependent on temperature or anything else to be extrapolated into number of molecules - which is what's relevant). MAF however has its limitations, mostly range/voltage filtering issues (the voltage output is less stable compared to MAP...hence less resolution), and is just not possible with very high boost cars. But, if you are tuned on MAF, you are tuned period, since, using the above example, it would measure the mass of air going into the engine irrespective of compressor efficiency. The compressor is less efficient at altitude, it will pump less air and more heat...but we don;t care b/c the heat factor is out of the equation.
This gives you an idea how complex car tunign is, and why you shouldn't attempt it on your own.
I am spent. HTH
#35
Oxygen content is determined by temp AND pressure
JET
#36
Originally Posted by JETPILOT
The oxygen content of air is constant regardless of altitude. Always approx 78% oxygen 21% nitrogen, and 1% trace gasses. The density is what changes. The comressor has to work harder to comprss the less dense air at higher altitudes to lets say 8psi. But again. 8psi is 8psi is 8psi.
JET
JET
Gurgen, my head is spinning..see guys; beware what questions you ask, you may get a Gurgen response that takes two encyclopedias and a physics professor to understand
Good response Gurgen, as always, you are talking over my head
-TODD
#37
Originally Posted by JETPILOT
The oxygen content of air is constant regardless of altitude. Always approx 78% oxygen 21% nitrogen, and 1% trace gasses. The density is what changes. The comressor has to work harder to comprss the less dense air at higher altitudes to lets say 8psi. But again. 8psi is 8psi is 8psi.
JET
JET
Second, you are taking what I said out of context. If you read that area of my write up, and always, and once interchangebly, refer to oxygen content not as it pertains to total air content (21%) but as part of the content inside the manifold.
This is what I said:
The MAP of 1.22/1.5 bar is THE the same as 1.22/1.5 bar MAP in L.A. and NM only with respect ot pressure and NOT with respect to oxygen content in the manifold, because oxygen content is determined by temp AND pressure.
The comressor has to work harder to compress the less dense air at higher altitudes to lets say 8psi.
#39
Somebody is gettin schooled! I love it when two power brains go at it.
Lets cut the scientific mumbo jumbo here for a moment. Bottom line this what is really being asked.
What would be the HP difference between my stock 350Z with a ST TN kit at 5000 Plus elevation vs the same exact car at sea-level? Aproximately.
No I have another question that is somewhat related.
At elevation would I be throwing off my tune by using 93 octane? Would I gain any safety net by running 93 octane?
Lets cut the scientific mumbo jumbo here for a moment. Bottom line this what is really being asked.
What would be the HP difference between my stock 350Z with a ST TN kit at 5000 Plus elevation vs the same exact car at sea-level? Aproximately.
No I have another question that is somewhat related.
At elevation would I be throwing off my tune by using 93 octane? Would I gain any safety net by running 93 octane?
Last edited by 2fast4thelaw; 10-13-2006 at 11:35 AM.