Curious - How Much Difference Will Altitude Make In Performance?
07-27-2009, 04:47 PM
#61
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what does this mean? It doesnt really take any "longer" to compress. It just takes force to compress it, through a turbo or via the pistons' travel. How many molecules are compressed is the question
07-27-2009, 04:52 PM
#62
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Well, there are hills i climb that dont give the power loss of altitude. not even close. in fact, most hills that climb to the altitudes discuss here, dont hurt the performance as much as the altitude they finaly reach. 
12 seconds 60-100mph with my old 240rwhp 4.7 US euro. sea level 7 seconds.
The grade starting to the sierras is probably 5% . hard to say where the trade off is. Ill have to do some tests this weekend. better rolling dyno as you can do it at lower speeds like 60-80 and still have a good amount of time sampling.
(less room for testing error)
12 seconds 60-100mph with my old 240rwhp 4.7 US euro. sea level 7 seconds.
The grade starting to the sierras is probably 5% . hard to say where the trade off is. Ill have to do some tests this weekend. better rolling dyno as you can do it at lower speeds like 60-80 and still have a good amount of time sampling.
(less room for testing error)
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, 06:11 PM
#63
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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
07-27-2009, 08:07 PM
#64
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yes, compression ratio doesnt change and pressure (statically and dynamically) will be lower. (PSI).
how you produce the same hp at lower compression? (or psi).
I guess with boost, you effectively are raising the compression ratio and along with the displacement. (at the cost of some higher tempuratures due to compression)
are we kind of saying the same thing?
how you produce the same hp at lower compression? (or psi).
I guess with boost, you effectively are raising the compression ratio and along with the displacement. (at the cost of some higher tempuratures due to compression)
are we kind of saying the same thing?
You mean the compression RATIO is the same. The compression (PSI) itself is going to be lower. And with my blower, the boost is lower. And as a result, yes, HP is lower. Detonation is affected by the compression. If you produce the same HP at a lower compression you are less likely to detonate.
07-27-2009, 08:33 PM
#65
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I think we are getting 'compression' and 'cylinder pressure' mixed.
MK, I referenced 'bara' for manifold pressure in my comment, meaning absolute pressure. Used a piezo load cell with a 0 PSIA reference so it would read correctly at all uusable altitudes. Made accurate datalogging possible.
If only it was just force. It is an interactive process, where more time with the same 'force' or more 'force' with the same time, or a combination, will do the trick. The compressor is less likely to reach choked condition (sonic through the smallest region) with the lower inlet density and same rotor speed. However, the rotor takes less energy to spin with the lower density inlet, so with the same energy you will see higher velocities in the first stages. Works for turbine, doesn't work for belt-driven compressors since their speed is determined by the drive configuration. This isn't the place for a turbine compressor dynamics lesson; suffice it to say it isn't as simple as just 'adding more force'.
MK, I referenced 'bara' for manifold pressure in my comment, meaning absolute pressure. Used a piezo load cell with a 0 PSIA reference so it would read correctly at all uusable altitudes. Made accurate datalogging possible.
what does this mean? It doesnt really take any "longer" to compress. It just takes force to compress it, through a turbo or via the pistons' travel. How many molecules are compressed is the question
07-27-2009, 08:41 PM
#66
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Its been a while since ive plugged in the numbers, but it looks like it is pretty obvious that at higher altitudes its pretty easy to go fast. If you use the drag calculator, its near 380hp just to drive the drag at 200mph, but at 180mph (the air resistance of 200mph at 6000ft),it would drop down to near 200hp.
If you had 400hp to start, you might only have 320hp at 6000ft, so you would have a lot of extra power to go even faster if the rolling friction was only about 30hp. Not surprising that so many records are set at altitude. So if this is true, could the holbert car with its original motor making 300rwhp, have run 200mph if it attempted it on tarmac at 6000ft?
If you had 400hp to start, you might only have 320hp at 6000ft, so you would have a lot of extra power to go even faster if the rolling friction was only about 30hp. Not surprising that so many records are set at altitude. So if this is true, could the holbert car with its original motor making 300rwhp, have run 200mph if it attempted it on tarmac at 6000ft?
07-28-2009, 01:01 AM
#68
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I thought you would have appriciated the barometric equivilance to air density comment (or some actual emperical test results at 6k feet vs sea level, etc.)
Oh well, there are still those that mock what they dont understand.
(or some actual emperical test results at 6k feet vs sea level, etc.)Oh well, there are still those that mock what they dont understand.
07-28-2009, 01:08 AM
#69
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I saw that after I read it again.
without getting into the turbo turbine vs belt driven turbine discussion, supersonic is determined by velocity and tempurature, not density. counter intuitive, I know, and its been way to long to be able to explain why.
I think the point there was, we are probably assuming that the turbo is large enough and there is enough gas flow to produce any boost required to neutralized altitude.
mk
without getting into the turbo turbine vs belt driven turbine discussion, supersonic is determined by velocity and tempurature, not density. counter intuitive, I know, and its been way to long to be able to explain why.
I think the point there was, we are probably assuming that the turbo is large enough and there is enough gas flow to produce any boost required to neutralized altitude.
mk
I think we are getting 'compression' and 'cylinder pressure' mixed.
MK, I referenced 'bara' for manifold pressure in my comment, meaning absolute pressure. Used a piezo load cell with a 0 PSIA reference so it would read correctly at all uusable altitudes. Made accurate datalogging possible.
If only it was just force. It is an interactive process, where more time with the same 'force' or more 'force' with the same time, or a combination, will do the trick. The compressor is less likely to reach choked condition (sonic through the smallest region) with the lower inlet density and same rotor speed. However, the rotor takes less energy to spin with the lower density inlet, so with the same energy you will see higher velocities in the first stages. Works for turbine, doesn't work for belt-driven compressors since their speed is determined by the drive configuration. This isn't the place for a turbine compressor dynamics lesson; suffice it to say it isn't as simple as just 'adding more force'.
MK, I referenced 'bara' for manifold pressure in my comment, meaning absolute pressure. Used a piezo load cell with a 0 PSIA reference so it would read correctly at all uusable altitudes. Made accurate datalogging possible.
If only it was just force. It is an interactive process, where more time with the same 'force' or more 'force' with the same time, or a combination, will do the trick. The compressor is less likely to reach choked condition (sonic through the smallest region) with the lower inlet density and same rotor speed. However, the rotor takes less energy to spin with the lower density inlet, so with the same energy you will see higher velocities in the first stages. Works for turbine, doesn't work for belt-driven compressors since their speed is determined by the drive configuration. This isn't the place for a turbine compressor dynamics lesson; suffice it to say it isn't as simple as just 'adding more force'.
07-28-2009, 01:44 PM
#70
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No you could not run 200 mph, and that is a fact. Currently in ORR there are no cars under 500 rwhp that have hit 200 mph and a lot way over 500 that can not make it. Granted we are running with uncertain winds and variable temperatures and humidity, but in the real world it takes 500 hp (plus some aero mods) in our cars to reach 200 mph at 6000 ft.
07-28-2009, 01:57 PM
#71
Race Car
I was definitely able to feel the difference in power at SITM which is ~2,500ft higher in altitude than where I live. My boost gauge was also reading significantly lower when at WOT over there. That's a huge advantage of turbos, they can maintain the same boost level by just working harder (assuming they are not being maxed out at sea-level of course) at the expense of a little extra lag.
Dan
'91 928GT S/C
475hp/460lb.ft
Dan
'91 928GT S/C
475hp/460lb.ft
07-28-2009, 03:04 PM
#72
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This i believe, but trying to get the aero and rolling friction factors understood.
If the aero drag is 380hp at sea level, then at 6000ft, the aero drag would be 20% less, right? maybe its better to start at the emperical testing points, and work back from there. 1st of all, Marc Tomas did 200mph with only 420rwhp, right (your motor before it was really beefed up a bit to do his 209mph run) If a car over 500rwhp cant go 200mph, it might have a lot of drag or the wrong gearing I would think.
so, if we use, even best case , 440rwhp, as that was the most Devek ever had when they did the event, and work backward from 6000ft, we could say that the aero drag was 400rwhp and the rollign friction was 40hp. (ball park)
If that was the case, if we drop down to sea level, where the air is 20% denser
It would raise the hp requirement through the roof. 500hp or more, right. what is the sea level range for HP on our type of cars?
mk
If the aero drag is 380hp at sea level, then at 6000ft, the aero drag would be 20% less, right? maybe its better to start at the emperical testing points, and work back from there. 1st of all, Marc Tomas did 200mph with only 420rwhp, right (your motor before it was really beefed up a bit to do his 209mph run) If a car over 500rwhp cant go 200mph, it might have a lot of drag or the wrong gearing I would think.
so, if we use, even best case , 440rwhp, as that was the most Devek ever had when they did the event, and work backward from 6000ft, we could say that the aero drag was 400rwhp and the rollign friction was 40hp. (ball park)
If that was the case, if we drop down to sea level, where the air is 20% denser
It would raise the hp requirement through the roof. 500hp or more, right. what is the sea level range for HP on our type of cars?
mk
No you could not run 200 mph, and that is a fact. Currently in ORR there are no cars under 500 rwhp that have hit 200 mph and a lot way over 500 that can not make it. Granted we are running with uncertain winds and variable temperatures and humidity, but in the real world it takes 500 hp (plus some aero mods) in our cars to reach 200 mph at 6000 ft.
08-02-2009, 09:34 PM
#73
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*Bump*
Well, all you mountain folks...
What did your butt-o-meter say down in Dallas???
Well, all you mountain folks...
What did your butt-o-meter say down in Dallas???
08-03-2009, 10:06 AM
#74
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Maybe if your using two runs to average out... but I just need the downhill one to prove I "could" run 200 mph.
I still say your going faster, given the same horsepower, if your driving down hill - given a long enough stretch - headed down the hill - at a great enough angle - and using gravity..... I'll bet my 320 HP Euro could make 200 mph....
If an airplane was on a conveyor belt......
I still say your going faster, given the same horsepower, if your driving down hill - given a long enough stretch - headed down the hill - at a great enough angle - and using gravity..... I'll bet my 320 HP Euro could make 200 mph....
If an airplane was on a conveyor belt......
08-03-2009, 02:02 PM
#75
Rennlist Member
The difference in power was unbelieveable. The GT absoulutely came alive, it was a different car in Dallas compared to the 5500 ft I live at in the Denver area...