How much lower would a sea level compression result of 150 psi be at 5500 ft, e.g. Denver?
02-14-2008, 05:19 AM
#11
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No difference
The only way you would see a difference if you did the compression test at sea level and then transported the vehicle to altitude or vise versa. Or you had a really long hose on your gauge and your gauge was at sea level and the engine was at altitude. I think this was covered in Physics 102...
02-14-2008, 05:42 AM
#12
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Duh. What we are talking about are readings taken in Denver compared to San Diego. Same vehicle.
Different days. No hoses involved.
Wanna revise your response?
Wanna revise your response?
02-14-2008, 06:21 AM
#13
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May have found the answer to the ^^above^^ question.
From a Google search on "compression ratio":
"The answer is actually more complicated. The baseline compression is nothing more than the mechanical compression ratio multiplied by by normal atmospheric pressure of 14.7 lbs/sqr in. But when air is compressed it's temperature increases, then the air tries to expand inside a fixed volume, and the pressure rises a bit more."
Now I need a defintion of "a bit". ;>
"The answer is actually more complicated. The baseline compression is nothing more than the mechanical compression ratio multiplied by by normal atmospheric pressure of 14.7 lbs/sqr in. But when air is compressed it's temperature increases, then the air tries to expand inside a fixed volume, and the pressure rises a bit more."
Now I need a defintion of "a bit". ;>
02-14-2008, 06:49 AM
#14
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Re: But isn't Boyle's Law about volumes not pressures? Compression ratio is about volumes.
If compression ratio is about volumes, I think it is, you have answered your own question. The gas volume of the cylinder never changes, however with a turbo boosting, the density will change. No turbo effect, comp ratio should be the same at sea level and altitude.
02-14-2008, 07:03 AM
#16
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Damnit
I find myself agreeing with Dave again.
Its the same reason why 1 bar of boost at sea level is different (in psi) than 1 bar of boost at altitude.
atmospheric pressure at sea level = 14.7 psi
9.3:1 compression at sea level = (9.3 * 14.7) =~ 137psi
atmospheric pressure at 5500 ft = 12psi
9.3:1 compression at 5500ft = (9.3 * 12) =~ 111.6 psi
If you want constant numbers you could speak in terms of bar. EG
atmospheric pressure at sea level = 1 bar
atmospheric pressure at 5500ft = 1 bar
9.3:1 compression at sea level OR 5500 ft = (9.3 * 1) = 9.3 bar
Its the same reason why 1 bar of boost at sea level is different (in psi) than 1 bar of boost at altitude.
atmospheric pressure at sea level = 14.7 psi
9.3:1 compression at sea level = (9.3 * 14.7) =~ 137psi
atmospheric pressure at 5500 ft = 12psi
9.3:1 compression at 5500ft = (9.3 * 12) =~ 111.6 psi
If you want constant numbers you could speak in terms of bar. EG
atmospheric pressure at sea level = 1 bar
atmospheric pressure at 5500ft = 1 bar
9.3:1 compression at sea level OR 5500 ft = (9.3 * 1) = 9.3 bar
02-14-2008, 07:03 AM
#17
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Based on my Pratt and Whitney Aeronautical Vestpocket Handbook
PSIA at 5000ft is 12.23 at 6000 is 11.78 extrapolation yeilds 12 pisa
So 12 times 9.2 (think this is AAN compression ratio) you get
110 PSI
at sea level it would be 135..
Now that is stack pressure but dynamic should be higher but you could just expect a reduction in cylinder pressure of about 18 percent..
Hope that helps and
So 12 times 9.2 (think this is AAN compression ratio) you get
110 PSI
at sea level it would be 135..
Now that is stack pressure but dynamic should be higher but you could just expect a reduction in cylinder pressure of about 18 percent..
Hope that helps and
02-14-2008, 07:11 AM
#20
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BAR to PSI calculator that includes altitude...
<ul><li><a href="http://www.dragsource.com/index.php?navselect=calculators&calctoview=8"> I would like to see the calculations behind it....</a></li></ul>