Altitude effects
Registered User
Joined: Feb 2006
Posts: 150
Likes: 0
From: Alberta
BarryG seems to confirm my theories on performance. I think that comment was correct about boost because a waste gate would be affected by outside atmospheric pressure. After more thought I would think a boost gauge would have to be calibrated to the elevation you are at and then you would find you are getting less boost.
Registered User
Joined: Aug 2006
Posts: 3,113
Likes: 0
From: New Mexico
I get slightly less mpg's, right at a loss of 1mpg, at 7K ft elevation where I live vrs my mileage at 3-5K elevation where I frequently travel. Obviously the thinner air has an effect in fuel ratio.
The result IS my truck does smoke a little more. The answer lies in the smoke. The more we smoke, what is really happening with the fuel/performance?
CD
The result IS my truck does smoke a little more. The answer lies in the smoke. The more we smoke, what is really happening with the fuel/performance?
CD
Registered User
Joined: Aug 2002
Posts: 1,227
Likes: 50
From: Castle Rock, CO
SuperHP:
What you say makes sense and I can follow that logic. I guess Holiday Inn didnt quite do it for me wonder if I can get a refund
.
I do know for sure though my truck smokes way more at altitude than down lower and egts are higher and lag is increased. 15psi down here and I get on it results in little to no smoke but up there it will lay a hefty cloud. From a stop I can easily black out the sun up high and actually try to drive so that I wont. Now I do have question. The turbo charger doesnt spin any faster (produce more work) for a given amount of exhaust output regardless of altitude so how does the turbo do more work. Is it that it just takes longer to get to that pressure? However that doesnt make sense as the turbo is still only going to spin the same speed it cannot create more work on its own. Oh Oh Oh I may be getting it now - it takes more fuel thereby increasing exhaust output to spin the turbo charger faster.
So if that is the case then it is affected by altitude but it is overcome. Which would explain why power wise there does not seem to be an issue but lag, smoke & egts etc can be a problem up high. You dont suffer power wise as long as you have enough fuel to accomplish the extra work needed. The turbo size would become more important at altitude as too large of an exhaust housing or too small of a compressor would restrict how fast if at all the turbo could turn that extra work into pressure which could/would lead to overly high egts in which case you need to lay off the go pedal (ie no more extra work).
Thanks superhp I was just thinking out loud there and think I have it square in my head.
What you say makes sense and I can follow that logic. I guess Holiday Inn didnt quite do it for me wonder if I can get a refund
.I do know for sure though my truck smokes way more at altitude than down lower and egts are higher and lag is increased. 15psi down here and I get on it results in little to no smoke but up there it will lay a hefty cloud. From a stop I can easily black out the sun up high and actually try to drive so that I wont. Now I do have question. The turbo charger doesnt spin any faster (produce more work) for a given amount of exhaust output regardless of altitude so how does the turbo do more work. Is it that it just takes longer to get to that pressure? However that doesnt make sense as the turbo is still only going to spin the same speed it cannot create more work on its own. Oh Oh Oh I may be getting it now - it takes more fuel thereby increasing exhaust output to spin the turbo charger faster.
So if that is the case then it is affected by altitude but it is overcome. Which would explain why power wise there does not seem to be an issue but lag, smoke & egts etc can be a problem up high. You dont suffer power wise as long as you have enough fuel to accomplish the extra work needed. The turbo size would become more important at altitude as too large of an exhaust housing or too small of a compressor would restrict how fast if at all the turbo could turn that extra work into pressure which could/would lead to overly high egts in which case you need to lay off the go pedal (ie no more extra work).
Thanks superhp I was just thinking out loud there and think I have it square in my head.
Registered User
Joined: Feb 2006
Posts: 150
Likes: 0
From: Alberta
I think you've got it there but I don't think the truck fuels more at altitude. I could be wrong though but given the power losses up high I don't think the truck would compensate with more fuel, if anything a bone stock truck might fuel less to keep air to fuel ratios the same (and emissions) at all altitudes like gas engines. But I'm just speculating here.
The turbo could actually spin faster with a given amount of exhaust in less dense air because the compressor has less drag in the thinner air, but in my eyes it's causing power/efficiency losses by taking longer to make that boost and keeping the wastegate shut longer (which causes exhaust restriction).
I think the main thing with the smoke is that you're getting the same amount of fuel when you slam the throttle but it's taking longer to make boost. Same fuel with less boost = black out the sun smoke
Burning less of the fuel makes less power and is less efficient so that would cause the MPG loss as well.
It's the same reason my diesel doesn't smoke much at all in the winter vs the summer.
If I'm wrong anywhere here, correct me guys. I'm running on low sleep
The turbo could actually spin faster with a given amount of exhaust in less dense air because the compressor has less drag in the thinner air, but in my eyes it's causing power/efficiency losses by taking longer to make that boost and keeping the wastegate shut longer (which causes exhaust restriction).
I think the main thing with the smoke is that you're getting the same amount of fuel when you slam the throttle but it's taking longer to make boost. Same fuel with less boost = black out the sun smoke
Burning less of the fuel makes less power and is less efficient so that would cause the MPG loss as well.
It's the same reason my diesel doesn't smoke much at all in the winter vs the summer.
If I'm wrong anywhere here, correct me guys. I'm running on low sleep
Thread Starter
Registered User
Joined: Aug 2004
Posts: 295
Likes: 0
SuperHP:
What you say makes sense and I can follow that logic. I guess Holiday Inn didnt quite do it for me wonder if I can get a refund.
I do know for sure though my truck smokes way more at altitude than down lower and egts are higher and lag is increased. 15psi down here and I get on it results in little to no smoke but up there it will lay a hefty cloud. From a stop I can easily black out the sun up high and actually try to drive so that I wont. Now I do have question. The turbo charger doesnt spin any faster (produce more work) for a given amount of exhaust output regardless of altitude so how does the turbo do more work. Is it that it just takes longer to get to that pressure? However that doesnt make sense as the turbo is still only going to spin the same speed it cannot create more work on its own. Oh Oh Oh I may be getting it now - it takes more fuel thereby increasing exhaust output to spin the turbo charger faster.
So if that is the case then it is affected by altitude but it is overcome. Which would explain why power wise there does not seem to be an issue but lag, smoke & egts etc can be a problem up high. You dont suffer power wise as long as you have enough fuel to accomplish the extra work needed. The turbo size would become more important at altitude as too large of an exhaust housing or too small of a compressor would restrict how fast if at all the turbo could turn that extra work into pressure which could/would lead to overly high egts in which case you need to lay off the go pedal (ie no more extra work).
Thanks superhp I was just thinking out loud there and think I have it square in my head.
What you say makes sense and I can follow that logic. I guess Holiday Inn didnt quite do it for me wonder if I can get a refund
.I do know for sure though my truck smokes way more at altitude than down lower and egts are higher and lag is increased. 15psi down here and I get on it results in little to no smoke but up there it will lay a hefty cloud. From a stop I can easily black out the sun up high and actually try to drive so that I wont. Now I do have question. The turbo charger doesnt spin any faster (produce more work) for a given amount of exhaust output regardless of altitude so how does the turbo do more work. Is it that it just takes longer to get to that pressure? However that doesnt make sense as the turbo is still only going to spin the same speed it cannot create more work on its own. Oh Oh Oh I may be getting it now - it takes more fuel thereby increasing exhaust output to spin the turbo charger faster.
So if that is the case then it is affected by altitude but it is overcome. Which would explain why power wise there does not seem to be an issue but lag, smoke & egts etc can be a problem up high. You dont suffer power wise as long as you have enough fuel to accomplish the extra work needed. The turbo size would become more important at altitude as too large of an exhaust housing or too small of a compressor would restrict how fast if at all the turbo could turn that extra work into pressure which could/would lead to overly high egts in which case you need to lay off the go pedal (ie no more extra work).
Thanks superhp I was just thinking out loud there and think I have it square in my head.
For the most part, I agree with you, and we are seeing the same side-effects, but I DO see a difference in timed runs, and if there was no difference in power output, the e.ts should be lower up here due to a decrease in the air resistance. Even if 20#s boost is the same universally, that is being added to a lower initial charge (lower EFFECTIVE compression ratio). A simple compression test varifies this. If our c.r. is 17, that is 17 times (say for example 11 p.si at 10,000 ft, yielding 187 p.s.i compressionn natural and 207 p.s.i. with the 20 stated from the turbo). at sealevel, I believe the atmospheric pressure is 14.7#s, so you start out with 269.9 and tack the 20 onto that............BIG Difference in my book. Another way of looking at it is to go to the extremes.........a 100:1 c.r. would do you zero good in deep space, while a 1.1:c.r. would be way more than you want if under water.
Registered User
Joined: Dec 2003
Posts: 896
Likes: 0
From: Minnesota
I live at 6300' here and routinely get up to 10k and above. Running empty I do not notice much difference except more smoke and higher egts and more lag when going slow . Towing a trailer up that high I still dont have a problem power wise but the egt issue is worse and the smoke is even more pronounced due to the increased turbo lag. When my truck has been at sea level I was quite suprised at how much less lag there was, lower smoke and my egts were much easier to control when towing heavy.
Now I am no engineer but did sleep at holiday inn the other night . So take this for what its worth(which by the way is very little). 20 psi at altitude is not quite the same as 20 psi at sea level. The pressure at sea level that reads zero on your gauge is higher to begin with than it is at altitude. At sea level the atmospheric pressure is like 14.2 or something close to that. Here it is 12.5 or something close due to the fact that the air is less dense(ie less o2)the higher the altitude. Your boost gauge is comparing the ambient pressure or whatever that technical term is not the pressure at sea level(unless you are at sea level). So at sea level your engine is really seeing 14.2 psi at idle or zero boost condition. At altitude here for example it is 12.5 at idle or zero boost condition. While that is only about a 1.7 psi difference that equates to approx 12%. So there is approx 12% less 02 at altitude regardless of boost pressure compared to the same boost pressure at sea level. A good example of this when you are here or anywhere in the mountains at altitude go in the local store and check out a bag of potato chips or can of pringles. If they were packaged at sea level the bag will be really puffed out ready to explode or the lid may be getting ready to blow off the pringles and when you open it you will hear the air pop or rush out of the bag or can. They didnt blow the air in there it is just the pressure differential from where it was packed to where it is now. Even scuba diving here you have to make allowances for the increased altitude for no reason other than the lower air pressure as the dive tables are made for diving at sea level.
That being said I have never had a problem power wise but it is noticeable in some ways ie lag and its brothers smoke and egts. I have had difficulty starting at 12k after sitting overnight unplugged in below zero temps but never at home at even colder temps.
Now I am no engineer but did sleep at holiday inn the other night
. So take this for what its worth(which by the way is very little). 20 psi at altitude is not quite the same as 20 psi at sea level. The pressure at sea level that reads zero on your gauge is higher to begin with than it is at altitude. At sea level the atmospheric pressure is like 14.2 or something close to that. Here it is 12.5 or something close due to the fact that the air is less dense(ie less o2)the higher the altitude. Your boost gauge is comparing the ambient pressure or whatever that technical term is not the pressure at sea level(unless you are at sea level). So at sea level your engine is really seeing 14.2 psi at idle or zero boost condition. At altitude here for example it is 12.5 at idle or zero boost condition. While that is only about a 1.7 psi difference that equates to approx 12%. So there is approx 12% less 02 at altitude regardless of boost pressure compared to the same boost pressure at sea level. A good example of this when you are here or anywhere in the mountains at altitude go in the local store and check out a bag of potato chips or can of pringles. If they were packaged at sea level the bag will be really puffed out ready to explode or the lid may be getting ready to blow off the pringles and when you open it you will hear the air pop or rush out of the bag or can. They didnt blow the air in there it is just the pressure differential from where it was packed to where it is now. Even scuba diving here you have to make allowances for the increased altitude for no reason other than the lower air pressure as the dive tables are made for diving at sea level. That being said I have never had a problem power wise but it is noticeable in some ways ie lag and its brothers smoke and egts. I have had difficulty starting at 12k after sitting overnight unplugged in below zero temps but never at home at even colder temps.
also, tyring to make 20psi at 8000ft vs. making 20psi at sea level (given all other conditions the same) results in overspeeding the turbo at the 8000ft mark. well, maybe not overspeeding in all conditions but it does have to spin faster.
Registered User
Joined: Aug 2002
Posts: 1,227
Likes: 50
From: Castle Rock, CO
What I was trying to say was that the turbo alone isnt just going to start spinning faster and create more boost without more fuel to the combustion chamber to increase the exhaust flow. When I say added fuel I mean you push more on the go pedal not that the truck does it by itself though it is possible I guess with the ecm. I know on my 96 gas dodge there was a bug in the ecm software and the check engine light would come on anytime you went over 12k elevation The dealer mechanic told me the software had only been programmed to 12k so hence the mil light coming on.
The ETs being higher at altitude does make sense as it takes the turbo longer to do the extra work and build boost.
The absolute pressure is what I was thinking of. So then 20psi at altitude is actually a bit less than at sea level because the gauge is reading the ambient pressure as zero. The 20psi may be 20psi but they are starting at different points. That would mean that at altitude you are really seeing approx 6% less 02 at 20psi compared to sea level (14.7+20 = 34.7psi vs 12.5+20=32.5psi 2.2psi/34.7psi= 6.3%)
I guess I am not sure about this anymore all I know is that at altitude my truck runs pretty darn good except for the increased lag time, smoke and higher egts.
The ETs being higher at altitude does make sense as it takes the turbo longer to do the extra work and build boost.
The absolute pressure is what I was thinking of. So then 20psi at altitude is actually a bit less than at sea level because the gauge is reading the ambient pressure as zero. The 20psi may be 20psi but they are starting at different points. That would mean that at altitude you are really seeing approx 6% less 02 at 20psi compared to sea level (14.7+20 = 34.7psi vs 12.5+20=32.5psi 2.2psi/34.7psi= 6.3%)
I guess I am not sure about this anymore all I know is that at altitude my truck runs pretty darn good except for the increased lag time, smoke and higher egts.
Registered User
Joined: Feb 2006
Posts: 150
Likes: 0
From: Alberta
The absolute pressure is what I was thinking of. So then 20psi at altitude is actually a bit less than at sea level because the gauge is reading the ambient pressure as zero. The 20psi may be 20psi but they are starting at different points. That would mean that at altitude you are really seeing approx 6% less 02 at 20psi compared to sea level (14.7+20 = 34.7psi vs 12.5+20=32.5psi 2.2psi/34.7psi= 6.3%).
So say you're wastegate is set to only allow 35 psia at max , you're cab gauge might say 20 psig in Seattle, while when you go up to Colorado it says 22psig. You really can't use your boost gauge as an indicator to how much oxygen is truely reaching your engine unless you factor the atmospheric pressure.
This is partly the reason for the efficiency loss up high, your turbo is actually having to make more boost just to make the same amount of power. If you need 150hp to cruise at 60mph, then at high altitude your boost gauge will be reading higher and the turbo working harder than down at sea level.
Registered User
Joined: Dec 2003
Posts: 896
Likes: 0
From: Minnesota
Wastegates work off absolute pressure not gauge though. Regardless of atmospheric pressure they will function the same and open at the same absolute manifold pressure.
So say you're wastegate is set to only allow 35 psia at max , you're cab gauge might say 20 psig in Seattle, while when you go up to Colorado it says 22psig. You really can't use your boost gauge as an indicator to how much oxygen is truely reaching your engine unless you factor the atmospheric pressure.
This is partly the reason for the efficiency loss up high, your turbo is actually having to make more boost just to make the same amount of power. If you need 150hp to cruise at 60mph, then at high altitude your boost gauge will be reading higher and the turbo working harder than down at sea level.
So say you're wastegate is set to only allow 35 psia at max , you're cab gauge might say 20 psig in Seattle, while when you go up to Colorado it says 22psig. You really can't use your boost gauge as an indicator to how much oxygen is truely reaching your engine unless you factor the atmospheric pressure.
This is partly the reason for the efficiency loss up high, your turbo is actually having to make more boost just to make the same amount of power. If you need 150hp to cruise at 60mph, then at high altitude your boost gauge will be reading higher and the turbo working harder than down at sea level.
yes, the boost is only an approximation for the amount of 02 entering the engine. a mass airflow sensor would be better.
Registered User
Joined: Aug 2006
Posts: 115
Likes: 0
From: Trussville, Alabama
You guys may want to search "bernulli's law" & "reciprocating engine operation at altitude" to determine if the zero reference used during waste gate operation is affected. Also boost guages & transmitters may also be affected due to ambient reference pressure being lower at altitude. Everyone agrees that "more smoke" is present at altitude. This indicates incomplete combustion, ie. too much fuel or not enough air for complete burn. To sum it up, these engines were designed considering sea level as datum. The operational envelope is greatly expanded with the turbo, but is not unlimited in operation at high altitudes. Water injection can help as it greatly increases the density of the intake air and provides a slight cooling effect. Large turbocharged reciprocating aircraft engines routinely use both water injection & and excess fuel for cooling at takeoff power settings. They also have a second stage in the supercharger that is usually engaged at 10,000 ft density altitude (altitude corrected for temperature) due to running out of throttle and not being able to maintain a specified manifold pressure with variable pitch props due to decreasing air density & turbo efficency. Sorry post is so long but this is a subject that routinely took a day to teach & 2 days to get across to pilots & engineers. Without knowing the high secrets of the Cummins ECU, there will always be room for discussion when it comes to operation of this engine in non-standard environments.
Thread Starter
Registered User
Joined: Aug 2004
Posts: 295
Likes: 0
Might as well muddy things up a bit more.......At 8000 ft. 29#s is the max boost. Back east in MI at about 600 ft. I see 31+ psi.
You guys have turned this into a great post.......thanks for sharing your knowledge!
You guys have turned this into a great post.......thanks for sharing your knowledge!
Registered User
Joined: Feb 2006
Posts: 150
Likes: 0
From: Alberta
nope, the wastegate is using gauge pressure. at a relaxed state, the wastegate is just sitting there with atmospheric pressure on both sides. lets say cold side creating the boost is side C and the hot side about to get its hot gases expelled is side H. as the turbo begins to make boost, it is only making it on side C of the wastegate (otherwise the wastegate wouldnt move). the atmospheric pressure on both, C and H, sides of the wastegate cancel one another out and all you are left with is gauge pressure. think of it this way: where does the in-cab gauge get its reading? where does the wastegate 'read' the boost? each read it in the piping between the turbo and intake.
yes, the boost is only an approximation for the amount of 02 entering the engine. a mass airflow sensor would be better.
yes, the boost is only an approximation for the amount of 02 entering the engine. a mass airflow sensor would be better.
The 2004.5+ trucks have electronically controlled wastegates, not sure about before that. The engine has a MAP sensor (manifold absolute pressure) and operates the wastegate based on that reading. The main advantage of this is for more consistant operation at different altitudes and temperatures.
HaulinBut, are you talking spikes or that's where it holds? My truck before the boost fooler would spike higher in colder air and lower altitudes, but it would hold at lower points than in the hotter air/higher altitudes. I haven't tested this since putting in the boost fooler.
Registered User
Joined: Nov 2002
Posts: 630
Likes: 1
From: Az
At altitude the wastegate will close a bit allowing the turbo to do abit more work to maintain the pressure with less air available. Turbocharged, piston driven aircraft produce the same power at altitude as they do on the ground all because of the wastgate maintaining the Manifold Air Pressure. The turbocharger also allows same a/c to fly much higher than thier aspirated counterparts. So I agree with the fact the trucks performance should be the same..
Thread Starter
Registered User
Joined: Aug 2004
Posts: 295
Likes: 0
That's for manually controlled wastegates. I got into the CTD only a few years ago and hadn't realized they still used manual wastegates so recently... OEM turbo gas jobbers have been using the EBC/MAP system for a much longer time.
The 2004.5+ trucks have electronically controlled wastegates, not sure about before that. The engine has a MAP sensor (manifold absolute pressure) and operates the wastegate based on that reading. The main advantage of this is for more consistant operation at different altitudes and temperatures.
HaulinBut, are you talking spikes or that's where it holds? My truck before the boost fooler would spike higher in colder air and lower altitudes, but it would hold at lower points than in the hotter air/higher altitudes. I haven't tested this since putting in the boost fooler.
The 2004.5+ trucks have electronically controlled wastegates, not sure about before that. The engine has a MAP sensor (manifold absolute pressure) and operates the wastegate based on that reading. The main advantage of this is for more consistant operation at different altitudes and temperatures.
HaulinBut, are you talking spikes or that's where it holds? My truck before the boost fooler would spike higher in colder air and lower altitudes, but it would hold at lower points than in the hotter air/higher altitudes. I haven't tested this since putting in the boost fooler.
PC.....Think of it this way.....Until the turbo reaches max. boost, I think we all agree the truck is gonna be weaker up in the thin air, right? (I know this for a fact because I can't hardly squeek the tires up here, but will put down two long strips when down in the thick air). Now, disregarding the fact that I get more boost down "there" than I do up "here", let's figure I get 30#s at either place, doesn't it just make sense that a stronger running engine is still going to run stronger if it gets the same boost? This is exactly what the stopwatch, seat of the pants, and tire deposits support. And then if we want to get a bit scetchy, technically the truck is pushing less air up high and even weighing just a bit less too, both supporting the truck being faster up "here".
Registered User
Joined: Jul 2005
Posts: 150
Likes: 0
From: Alberta Canada
Altitude Chart
http://www.sablesys.com/baro-altitude.html
Sorry, I'm canadian and kpa is the way I play. 101.325 at sea level to 69.74 at 10 000 ft (3050m)
pv=mrt
101.325 x 1 = m x .287 x 289
=1.2216kg/m3
69.74 x 1 = m x .287 x 289
=0.8408kg/m3
.8408/1.2216=0.6883
or 68%
it's a bit like the canadian dollar a while ago. I drive too far south, the money starts lookin funny and they only gave me 68cents for my dollar. A naturally aspirated engine will only have 68% of it's rated horsepower. The turbo does help significantly because it's a dynamic compressor instead of a posative displacement type, but since none of us have a set of flow curves for the hx35 (or what ever yer running) showing the "pumping" efficiency at various suction pressures it's hard to predict. Holset clames a 3:1 pressure ratio or 191.49kpa (27.77 psi fer all the yankee's here). Sea level at 101.325 at 3:1 gives 303.975kpa (44.086 psi). Stock boost calibration is 20-22 psi, then they defuel automaticly. Thank god for electronics, this is how they can work so well at all altitudes, in stock form. Those of use who have everything turn up are going to notice power losses sooner at that height as well as our EGT's will go nuts. I recall working with a 966 Caterpiller that had a plate that said the fuel system was calibrated for anything less than 7 or 8000 ft or something, and that if it was to be used above that it had to be adjusted by a service tech. Twin turbo guy's who are set up for towing and not for sled pulling shouldn't suffer as much as the singles. my $.02
Sorry, I'm canadian and kpa is the way I play. 101.325 at sea level to 69.74 at 10 000 ft (3050m)
pv=mrt
101.325 x 1 = m x .287 x 289
=1.2216kg/m3
69.74 x 1 = m x .287 x 289
=0.8408kg/m3
.8408/1.2216=0.6883
or 68%
it's a bit like the canadian dollar a while ago. I drive too far south, the money starts lookin funny and they only gave me 68cents for my dollar. A naturally aspirated engine will only have 68% of it's rated horsepower. The turbo does help significantly because it's a dynamic compressor instead of a posative displacement type, but since none of us have a set of flow curves for the hx35 (or what ever yer running) showing the "pumping" efficiency at various suction pressures it's hard to predict. Holset clames a 3:1 pressure ratio or 191.49kpa (27.77 psi fer all the yankee's here). Sea level at 101.325 at 3:1 gives 303.975kpa (44.086 psi). Stock boost calibration is 20-22 psi, then they defuel automaticly. Thank god for electronics, this is how they can work so well at all altitudes, in stock form. Those of use who have everything turn up are going to notice power losses sooner at that height as well as our EGT's will go nuts. I recall working with a 966 Caterpiller that had a plate that said the fuel system was calibrated for anything less than 7 or 8000 ft or something, and that if it was to be used above that it had to be adjusted by a service tech. Twin turbo guy's who are set up for towing and not for sled pulling shouldn't suffer as much as the singles. my $.02
Thread
Thread Starter
Forum
Replies
Last Post
KATOOM
Performance and Accessories 2nd gen only
6
Aug 31, 2003 11:41 PM
Parvenu
Performance and Accessories 2nd gen only
7
Aug 30, 2003 10:19 AM
whiteknight
12 Valve Engine and Drivetrain
7
Jan 8, 2003 10:51 PM