Curious - How Much Difference Will Altitude Make In Performance?
07-27-2009, 03:43 AM
#47
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when they tell you barometer is at 29.92 in denver, its adjusted to sea level.
Yes, absolulte atmospheric pressure IS density (with a temperature factor a well).
You abolutely feel the 12% hp gain at 4k feet, maybe not at 2k feet as Ive done that test before as well. but at 6000 feet, your car feels like it is pulling a trailer.
mk
Yes, absolulte atmospheric pressure IS density (with a temperature factor a well).
You abolutely feel the 12% hp gain at 4k feet, maybe not at 2k feet as Ive done that test before as well. but at 6000 feet, your car feels like it is pulling a trailer.
mk
George -
Now you've got me thinking...
Are you saying that atmospheric air pressure is just as important (or nearly as important) as air density?
To my feeble automotive mind, I can envision that in the combustion chamber of an engine, increased air density would still be in play. IOW, at a lower altitude, more dense air would still be more dense air as it flowed into the combustion chamber.
But once inside the combustion chamber of an engine - where it was no longer open to the outside atmosphere, would that air still be subject to higher atmospheric pressure? It would seem to me that the air would now be within a closed system where outside atmospheric pressure would no longer be in play.
Or even if it was in play, it would seem to me that inside an engine, the outside air pressure would be meaningless when compared to the air pressure generated by the advancing piston.
Now you've got me thinking...
Are you saying that atmospheric air pressure is just as important (or nearly as important) as air density?
To my feeble automotive mind, I can envision that in the combustion chamber of an engine, increased air density would still be in play. IOW, at a lower altitude, more dense air would still be more dense air as it flowed into the combustion chamber.
But once inside the combustion chamber of an engine - where it was no longer open to the outside atmosphere, would that air still be subject to higher atmospheric pressure? It would seem to me that the air would now be within a closed system where outside atmospheric pressure would no longer be in play.
Or even if it was in play, it would seem to me that inside an engine, the outside air pressure would be meaningless when compared to the air pressure generated by the advancing piston.
07-27-2009, 03:56 AM
#48
Rennlist Member
Power required to go faster goes up with the cube of speed, while aero drag goes up with the square. What you mean to say below is 3% per 1000 feet roughly is lost to lower air density as you go up in altituded. (tempurature and humidity are also factors).
Planes get better fuel performance because the air is thinner provideing less drag. Mixtures should be kept the same regardless. carburators do a fair job, modern fuel injection is much more accurate. MAFs do the job well, and the other sensors are there to fine tune the accuracy for emissions. Even AFMs are accurate to 10,000ft. (quoted in the bosche book)
all you need to do to figure out where georges car would do best is work out the numbers. If he did 200mph at 6000ft then we can determine approximately how fast he could go at 10,000ft or sea level.
mk
going down in power would
Planes get better fuel performance because the air is thinner provideing less drag. Mixtures should be kept the same regardless. carburators do a fair job, modern fuel injection is much more accurate. MAFs do the job well, and the other sensors are there to fine tune the accuracy for emissions. Even AFMs are accurate to 10,000ft. (quoted in the bosche book)
all you need to do to figure out where georges car would do best is work out the numbers. If he did 200mph at 6000ft then we can determine approximately how fast he could go at 10,000ft or sea level.
mk
going down in power would
bigs - Barometric pressure is air density. The fact that the weather changes it from location to location is another one of those variables that have an effect on performance. Not a whole lot when you are only talking about a 3900 foot elevation difference, but when it is cumulative to many other things they begin to add up.
I am not disputing driving your car over the Continental Divide versus at sea level will result in better performance when near the ocean, it defintely will. All of my comments are relative to "bigs'" question relative to where he lives now, Provo Utah and where he will be, Arlington Texas. The altitude difference and the resultant 3hp decrease per 1000 feet of elevation is not enough to override other variables such as temp/air density/humidity in a way that make the car perform notable more powerful.
Bill - I certainly am no expert in the workings of our cars, especially of the LH at different altitudes, but if it funtions like most other mid 90s FI systems it is doing a lot of metering and adjustment. Just consider for a moment how aircraft compensate for air density by going more rich/lean depending upon altitude to get optimum performance. Why planes get best performance (fuel burn and speed) at higher altitudes as opposed to sea level. This is why I mentioned how George's car, even yours running at 150mph, at higher altitude must consider the aerodynamic efficiencies due to thinner air as opposed to just higher engine hp output. Your cars move at speeds commensurate and indeed faster than most modern production NA personal aircraft (Cirrus/Cessna) where the difference between flying just above the ocean as opposed to 5,000 - 6,000 feet asl makes a significant different in final speed and performance. The faster you go at higher altitude the greater the aerodynamic benefit of thinner air. I don't know but there must be some crossover between hp decline per 1000 feet of elevation versus aerodynamic effect as elevation increases. Perhaps, just perhaps, a car like George's, has a higher final top end speed at 5,000' than at sea level due to the aero advantage of thinner air. Tough to know unless you can assert consitency of all environmental variables.
I am not disputing driving your car over the Continental Divide versus at sea level will result in better performance when near the ocean, it defintely will. All of my comments are relative to "bigs'" question relative to where he lives now, Provo Utah and where he will be, Arlington Texas. The altitude difference and the resultant 3hp decrease per 1000 feet of elevation is not enough to override other variables such as temp/air density/humidity in a way that make the car perform notable more powerful.
Bill - I certainly am no expert in the workings of our cars, especially of the LH at different altitudes, but if it funtions like most other mid 90s FI systems it is doing a lot of metering and adjustment. Just consider for a moment how aircraft compensate for air density by going more rich/lean depending upon altitude to get optimum performance. Why planes get best performance (fuel burn and speed) at higher altitudes as opposed to sea level. This is why I mentioned how George's car, even yours running at 150mph, at higher altitude must consider the aerodynamic efficiencies due to thinner air as opposed to just higher engine hp output. Your cars move at speeds commensurate and indeed faster than most modern production NA personal aircraft (Cirrus/Cessna) where the difference between flying just above the ocean as opposed to 5,000 - 6,000 feet asl makes a significant different in final speed and performance. The faster you go at higher altitude the greater the aerodynamic benefit of thinner air. I don't know but there must be some crossover between hp decline per 1000 feet of elevation versus aerodynamic effect as elevation increases. Perhaps, just perhaps, a car like George's, has a higher final top end speed at 5,000' than at sea level due to the aero advantage of thinner air. Tough to know unless you can assert consitency of all environmental variables.
07-27-2009, 12:45 PM
#49
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MK should also add the one of the early inspirations for his E-Ram system was power loss at altitude in the old car. I can remember a bit of discussion in the way-back-when period, after a trip over the Sierras and the significant power loss experienced in the NA car.
Two cars before the 928, I drove an early (CIS) Saab 900 Turbo, with some slight modifications, between home in the OC to alternate home in the Sierras at 8000 ft. Even with the compressor, there was a very noticeable difference in throttle response. It was also slower to come up on boost. Howvere, once on boost the performance was comparable when I corrected the gauge reading to accurately reflect actual inlet pressure at altitude. The wastegate was mostly spring-operated, with a little bit of pressure bias from the intake and an in-cabin dial-a-limit system. At 2 bara actual inlet pressure, the peak engine performance was equal regardless of outside air density. It takes longer to get inlet pressure at altitude, but once achieved the end results were the same as at sea level. There really is a good case for pressurized intakes.
Two cars before the 928, I drove an early (CIS) Saab 900 Turbo, with some slight modifications, between home in the OC to alternate home in the Sierras at 8000 ft. Even with the compressor, there was a very noticeable difference in throttle response. It was also slower to come up on boost. Howvere, once on boost the performance was comparable when I corrected the gauge reading to accurately reflect actual inlet pressure at altitude. The wastegate was mostly spring-operated, with a little bit of pressure bias from the intake and an in-cabin dial-a-limit system. At 2 bara actual inlet pressure, the peak engine performance was equal regardless of outside air density. It takes longer to get inlet pressure at altitude, but once achieved the end results were the same as at sea level. There really is a good case for pressurized intakes.
07-27-2009, 01:11 PM
#50
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Not that I would know anything about this topic, but that seems to make perfect sense ... thinner air = longer to compress = once compressed, equally efficient at any altitude.
MK should also add the one of the early inspirations for his E-Ram system was power loss at altitude in the old car. I can remember a bit of discussion in the way-back-when period, after a trip over the Sierras and the significant power loss experienced in the NA car.
Two cars before the 928, I drove an early (CIS) Saab 900 Turbo, with some slight modifications, between home in the OC to alternate home in the Sierras at 8000 ft. Even with the compressor, there was a very noticeable difference in throttle response. It was also slower to come up on boost. Howvere, once on boost the performance was comparable when I corrected the gauge reading to accurately reflect actual inlet pressure at altitude. The wastegate was mostly spring-operated, with a little bit of pressure bias from the intake and an in-cabin dial-a-limit system. At 2 bara actual inlet pressure, the peak engine performance was equal regardless of outside air density. It takes longer to get inlet pressure at altitude, but once achieved the end results were the same as at sea level. There really is a good case for pressurized intakes.
Two cars before the 928, I drove an early (CIS) Saab 900 Turbo, with some slight modifications, between home in the OC to alternate home in the Sierras at 8000 ft. Even with the compressor, there was a very noticeable difference in throttle response. It was also slower to come up on boost. Howvere, once on boost the performance was comparable when I corrected the gauge reading to accurately reflect actual inlet pressure at altitude. The wastegate was mostly spring-operated, with a little bit of pressure bias from the intake and an in-cabin dial-a-limit system. At 2 bara actual inlet pressure, the peak engine performance was equal regardless of outside air density. It takes longer to get inlet pressure at altitude, but once achieved the end results were the same as at sea level. There really is a good case for pressurized intakes.
07-27-2009, 01:12 PM
#51
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07-27-2009, 02:22 PM
#52
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when they tell you barometer is at 29.92 in denver, its adjusted to sea level.
Yes, absolulte atmospheric pressure IS density (with a temperature factor a well).
You abolutely feel the 12% hp gain at 4k feet, maybe not at 2k feet as Ive done that test before as well. but at 6000 feet, your car feels like it is pulling a trailer.
mk
Yes, absolulte atmospheric pressure IS density (with a temperature factor a well).
You abolutely feel the 12% hp gain at 4k feet, maybe not at 2k feet as Ive done that test before as well. but at 6000 feet, your car feels like it is pulling a trailer.
mk
You're right that the barometric pressure is adjusted for altitude, so recalcing
the power loss yesterday, Bigs would have 10% more power in Dallas then
Provo based on air temp, humidity, and absolute barometric pressure...
07-27-2009, 02:49 PM
#53
Drifting
JB was correct in his statement.
Think about it... attitude will win over altitude air density every time. In other words so long as the slope is significant, performance while going downhill will always be better than when going uphill regardless of altitude air density.
Think about it... attitude will win over altitude air density every time. In other words so long as the slope is significant, performance while going downhill will always be better than when going uphill regardless of altitude air density.
07-27-2009, 03:05 PM
#54
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H'man: Thinner air lowers compression. We flogged the crap out of my car at 5000-6000 feet before the last race and it never pinged, so we advanced the timing a whole bunch. Still didn't ping. As soon as I got back to down to sea level, I had to take all the advance out or it would ping like crazy.
07-27-2009, 03:24 PM
#55
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Bill--
That was with no SC, right?
----
Folks who live 'at altitude' in places like Denver will notice that the octane rating on their local "premium" gas is lower than similar premium gas at lower places. Detonation is a function of cylinder pressure among other things. Amazingly, cylinder pressures (and horsepower...) drop off as altitude goes up, reducing the potential for knocking/pinging noises from detonation. A fill-up on Denver gas might leave you at a disadvantage after you drop down off the plateau there. Raising the octane level at altitude has the effect of slowing combustion in an already under-pressurized cylinder, making performance worse instead of better. So your tank of low-altitude higher-octane premium is less than a help when you actually do get to higher altitudes. Gotta love flame physics!
That was with no SC, right?
----
Folks who live 'at altitude' in places like Denver will notice that the octane rating on their local "premium" gas is lower than similar premium gas at lower places. Detonation is a function of cylinder pressure among other things. Amazingly, cylinder pressures (and horsepower...) drop off as altitude goes up, reducing the potential for knocking/pinging noises from detonation. A fill-up on Denver gas might leave you at a disadvantage after you drop down off the plateau there. Raising the octane level at altitude has the effect of slowing combustion in an already under-pressurized cylinder, making performance worse instead of better. So your tank of low-altitude higher-octane premium is less than a help when you actually do get to higher altitudes. Gotta love flame physics!
07-27-2009, 03:29 PM
#56
Rennlist Member
Ok, when I dyno'd last in Vegas (2550 ft) Rocket pulled very strong. When we moved to Alamogordo (4400 ft), she seemed to feel winded at first. But when the cooler temps kicked in with little humidity, she ran like a freight train (i.e., wheel spin @50mph).
Big's, we'll soon find out what sea level feels like
Big's, we'll soon find out what sea level feels like
07-27-2009, 03:33 PM
#57
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Dr. Bob: No this was in June with my SC. Good point about octane ratings and altitude. I didn't happen to notice what we were getting for premium up in Ely. Whatever it was, the car had zero detonation at altitude even with advanced timing.
Bill--
That was with no SC, right?
----
Folks who live 'at altitude' in places like Denver will notice that the octane rating on their local "premium" gas is lower than similar premium gas at lower places. Detonation is a function of cylinder pressure among other things. Amazingly, cylinder pressures (and horsepower...) drop off as altitude goes up, reducing the potential for knocking/pinging noises from detonation. A fill-up on Denver gas might leave you at a disadvantage after you drop down off the plateau there. Raising the octane level at altitude has the effect of slowing combustion in an already under-pressurized cylinder, making performance worse instead of better. So your tank of low-altitude higher-octane premium is less than a help when you actually do get to higher altitudes. Gotta love flame physics!
That was with no SC, right?
----
Folks who live 'at altitude' in places like Denver will notice that the octane rating on their local "premium" gas is lower than similar premium gas at lower places. Detonation is a function of cylinder pressure among other things. Amazingly, cylinder pressures (and horsepower...) drop off as altitude goes up, reducing the potential for knocking/pinging noises from detonation. A fill-up on Denver gas might leave you at a disadvantage after you drop down off the plateau there. Raising the octane level at altitude has the effect of slowing combustion in an already under-pressurized cylinder, making performance worse instead of better. So your tank of low-altitude higher-octane premium is less than a help when you actually do get to higher altitudes. Gotta love flame physics!
07-27-2009, 04:38 PM
#59
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not really. thinner air lowers mass flow, compression is still a volume function. your engine will move 500cfm at 6000rpm regardless if it is open or closed throtttle, 10,000ft or sea level. the question is what the density of that CFM is. 
you didnt ping becasue you had less density, and reduced your hp by 20%! same as if you found a throttle position to lower the mass flow. the engine doesnt know altitude, all it knows is molecules of air and temp.
mk
you didnt ping becasue you had less density, and reduced your hp by 20%! same as if you found a throttle position to lower the mass flow. the engine doesnt know altitude, all it knows is molecules of air and temp.
mk
H'man: Thinner air lowers compression. We flogged the crap out of my car at 5000-6000 feet before the last race and it never pinged, so we advanced the timing a whole bunch. Still didn't ping. As soon as I got back to down to sea level, I had to take all the advance out or it would ping like crazy.
07-27-2009, 04:43 PM
#60
Rennlist Member
Thats right . It was the early impedus for the eRAM.
anyway, there is also absolute pressure with turbos and manifold pressure.
absolute pressure will make the turbo work over boost to get the total manifold pressure where you want it regardless of altitude. Most turbos just work on manifold pressure which has to be added to ambient pressure So, at Tahoe, you might be at 11psi ambient and running 11psi boost. at sea level you would run 1psi boost as welll, but ambient is 14.5psi. add them together for total boost. 22psi vs 25.5psi which is significant.
anyway, there is also absolute pressure with turbos and manifold pressure.
absolute pressure will make the turbo work over boost to get the total manifold pressure where you want it regardless of altitude. Most turbos just work on manifold pressure which has to be added to ambient pressure So, at Tahoe, you might be at 11psi ambient and running 11psi boost. at sea level you would run 1psi boost as welll, but ambient is 14.5psi. add them together for total boost. 22psi vs 25.5psi which is significant.
MK should also add the one of the early inspirations for his E-Ram system was power loss at altitude in the old car. I can remember a bit of discussion in the way-back-when period, after a trip over the Sierras and the significant power loss experienced in the NA car.
Two cars before the 928, I drove an early (CIS) Saab 900 Turbo, with some slight modifications, between home in the OC to alternate home in the Sierras at 8000 ft. Even with the compressor, there was a very noticeable difference in throttle response. It was also slower to come up on boost. Howvere, once on boost the performance was comparable when I corrected the gauge reading to accurately reflect actual inlet pressure at altitude. The wastegate was mostly spring-operated, with a little bit of pressure bias from the intake and an in-cabin dial-a-limit system. At 2 bara actual inlet pressure, the peak engine performance was equal regardless of outside air density. It takes longer to get inlet pressure at altitude, but once achieved the end results were the same as at sea level. There really is a good case for pressurized intakes.
Two cars before the 928, I drove an early (CIS) Saab 900 Turbo, with some slight modifications, between home in the OC to alternate home in the Sierras at 8000 ft. Even with the compressor, there was a very noticeable difference in throttle response. It was also slower to come up on boost. Howvere, once on boost the performance was comparable when I corrected the gauge reading to accurately reflect actual inlet pressure at altitude. The wastegate was mostly spring-operated, with a little bit of pressure bias from the intake and an in-cabin dial-a-limit system. At 2 bara actual inlet pressure, the peak engine performance was equal regardless of outside air density. It takes longer to get inlet pressure at altitude, but once achieved the end results were the same as at sea level. There really is a good case for pressurized intakes.
Last edited by mark kibort; 07-27-2009 at 08:09 PM.