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  #61  
Old 09-02-2004, 08:27 PM
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David Staten David Staten is offline
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Default havent talked to anyone yet..

the pressure change I believe is a logarythmic equation..

Anyways.. goin to go eat.. more later.. maybe.. or tomorrow..

Dave
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  #62  
Old 09-02-2004, 08:58 PM
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Results

cubic feet per minute engine volume 583.33

weight of cubic foot of air at sea level 0.0765716
Pressure Ratio Sea Level 1.09
Lbs of air a minute at Sea Level 48.5

weight of cubic foot of air at Altitude 0.012443
Pressure Ratio at Altitude 2.95
Lbs of air a minute at Altitude 21.4

OK, the numbers i know are for 25000 feet boost to 31.5 Hg
the numbers above are from MT's spreadsheet

to me you look at the turbo map(not MAP)

and you size the turbo for corrected 48 lbs per min airflow with a 2.95 pressure ratio.

my number from garrett were

44lbs per minute corrected
2.85 pressure ratio.

When mike gets his spreadsheet done, it takes many effeciencies into consideration and should be of great value

what am i missing here?????
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  #63  
Old 09-02-2004, 10:03 PM
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I would need to know your engine displacement in cubic inches, and the engine RPM to use.

Also I would want to know what barometric pressure you want to use for 25,000 ft in inHg. (I'm just doing a quickie estimate of 9.25 inHg).

Also I would turn down the -3.35% weight loss of air when esitmating over 20k feet. Maybe something like -2.85%.

Obviously this is not rocket scientist accurate... I'm just trying to get preliminary numbers before fine tuning the accuracy!
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  #64  
Old 09-02-2004, 10:04 PM
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HOLD EVERYTHING! Dust, aren't you using 2 turbos?
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  #65  
Old 09-02-2004, 10:25 PM
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Ok, I'm about to get back to the spreadsheet.

Here's the formula for pressure. Pressure is wholly based on temperature. Determine the temperature for your altitude and you can calculate the pressure.

p = 2116.224 * [ (T + 459.7)/518.6 ] ^ 5.256

where T = temperature in degrees F
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  #66  
Old 09-02-2004, 10:36 PM
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Hmmm... no variable for altitude?

So a warm day on top of a Rocky mountain ski slope in the Summer has a higher air density than the cooler valley below (during a temp inversion)?

I know temperature plays a part of air density, but I didn't think it was the only factor....

What about water where the further you go down, the higher the pressure, but the lower the temp? Okay, water is not a gas. Hmmm...
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  #67  
Old 09-02-2004, 10:52 PM
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Hey cool... it works!

http://www.usatoday.com/weather/wstdatmo.htm#us

I plugged in the different temperatures for the different altitudes and divided P by 70.80... or was it 80.70...

... and it worked!

New numbers to plug in the worksheet!
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  #68  
Old 09-02-2004, 11:39 PM
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Wait a minute... something's fishy in Denmark!

Did you just use regression analysis against the standard atmospheric table to come up with that formula?

I just looked up my local weather... 78 degrees F and 30.11 inHg. Now applying that formula I come up with 36.15 inHg.
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  #69  
Old 09-03-2004, 12:32 AM
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Here's the spreadsheet I have so far.

I have three columns. I put in numbers for a rotary at 10000 ft and dust's Continental at sea level and at 25000 ft. Standard temp and pressure for all.

All these numbers can be changed. If you're based at 2000 ft and it's 80 degrees with bp at 29.80, then that's what you enter. You also need to enter such things as volumetric efficiency, compressor efficiency, intercooler efficiency, pressure drops, and what pressure is desired at the manifold. The spreadsheet calculates pressure ratio, air density, air flow, etc...

There is a portion of the sheet that converts air flow to that used by the turbo maps. They are at:

http://64.225.76.178/main.htm

They are converted because the maps were derived at temp = 85 deg and pressure = 13.949 in Hg.

A turbo 13B at 10,000 feet can produce 10 psi of boost (over bp of 10.12 at 10k) with a pressure ratio of 2.42 but the corrected air flow is off the map. Real air flow is ok though. I don't know if this part is working though. I thought you used the corrected flow is what you use for the turbo map.

Note that horsepower calculations vary greatly with volumetric efficiency, brake specific fuel consumption, duty cycle and air/fuel ratio.
Attached Files
File Type: xls Turbo sizing.xls (45.0 KB, 36 views)
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  #70  
Old 09-03-2004, 12:38 AM
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Quote:
Originally Posted by MarbleTurtle
Wait a minute... something's fishy in Denmark!

Did you just use regression analysis against the standard atmospheric table to come up with that formula?

I just looked up my local weather... 78 degrees F and 30.11 inHg. Now applying that formula I come up with 36.15 inHg.
What I did was: based on a known temperature and altitude (let's say at ground) I derived the temperature at the altitude we want to examine. Then based on that new temperature, I calculate the pressure.
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  #71  
Old 09-03-2004, 01:12 AM
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I think the target should be
lb/min, actual

that way the intake manifold effeciency is taken into account


Absolutly great spreadsheet, buya donkee
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  #72  
Old 09-03-2004, 01:37 AM
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Quote:
Originally Posted by Dust
I think the target should be
lb/min, actual

that way the intake manifold effeciency is taken into account


Absolutly great spreadsheet, buya donkee

Sorry, my south african is rusty....
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  #73  
Old 09-03-2004, 10:17 AM
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Quote:
What I did was: based on a known temperature and altitude (let's say at ground) I derived the temperature at the altitude we want to examine. Then based on that new temperature, I calculate the pressure.
What's your formula for deriving temp at altitude based on ground temp?
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  #74  
Old 09-03-2004, 11:02 AM
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Quote:
Originally Posted by MarbleTurtle
What's your formula for deriving temp at altitude based on ground temp?
Ta = Tg- 3.56(A - Ag)/1000

where:

Ta = temperature at altitude
Tg = temperature at ground
A = altitude you want to calculate temperature
Ag = altitude at ground
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  #75  
Old 09-03-2004, 12:14 PM
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Hmmm...

I came up with Altitude*(-0.0035666666) + 518 - 459 = temp F at altitude.

But this only works up to 30,000 feet and you replace 518 with the 459.7 + actual ground temperature... uh... I mean air temperature at 0 ft elevation.

No we're getting somewhere!
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