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Old 11-02-2009, 12:54 AM   #1
xXManwhoreXx
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Turbo compressor maps and calculating turbo compressor sizes

What you need to do is find you airflow rate based on : your desired boost (PSI) engine size and engine redline. you need to find your Airflow rate at two points first point is at redline second point is usually at 75% of redline or where ever the compressor hits full boost, whichever is at a lower RPM. All you need to know is basic math, add, subtract, multiply and divide. you'll deal with different units of measurement (lb,cfm,psi) lets start:

Basics you will use these numbers later on
PRESSURE RATIO = (14.7 + desired PSI) / 14.7 = PR
convert CFM to LB per MIN = CFM x 0.070318 = LB/MIN (@ sea level and 112 degrees*)
LITERS TO CUBIC INCHES DISPLACEMENT = # of L's x 61
CFM = Cubic Feet per Minute

now using my car as an example:
2.2L (134.2 cid), 7400 Redline, desired boost is 10 PSI

#1 CALCULATING AIRFLOW RATE AT REDLINE and 10 PSI
cid = Cubic Inches Disp.
VE = Volumetric efficiency in percent
.5 = (given) 4-stroke engine fills cylinder only on one-half the revolutions
1728 = converts cubic inches to cubic feet

Airflow in CFM = (cid x rpm x .5 x VE%) / 1728 = CFM no boost
CFM x PR = CFM under boost


so for my Lude:
PR = (14.7 + 10 ) / 14.7 = 1.69
Airflow = (134.2 x 7400 x.5 x .95) / 1728 = 272.99 CFM (no boost)
272.99 x 1.69 = 461.21 CFM under 10 PSI

now since most compressor maps have their flow rates in LB/MIN we need to convert CFM to LB/MIN. A cubic feet of air (length+width+height) weighs different at diff. Altitudes and different temperatures. to simplify it well just assume we are at sea level and the air temp is 112 *. the conversion number is 0.070318.

so for CFM TO LB/MIN = CFM x 0.070318 = LB/MIN
461.21 x 0.070318 = 32.44 LB/MIN

so here we have:
BASIC ENGINE FLOW RATE = 272.99 CFM
ENGINE FLOWRATE with 10 PSI = 461.21 CFM (32.44 LB/MIN)
PRESSURE RATIO (PR) = 1.69

so now we know our flow rate in LB/MIN
a 2.2L with 10 PSI of boost at 7400 RPM (redline)
flow rate = 32.33 pounds of air per minute (LB/MIN)
PR = 1.69
with me still? this is POINT NUMBER ONE. now we do the same for 75% of redline and 50% of redline.

#2 CALCULATING AIRFLOW RATE AT 75% RPM RANGE

now we will find the airflow rate at 75% redline = 5550 RPM (0.75 x 7400 = 5550)

(134.2 x 5550 x .5 x .95) / 1728 = 204.74 CFM no boost
204.74 x 1.69 = 346.02 CFM under 10 PSI

346.02 x 0.70318 = 24.34 LB/MIN

BASIC ENGINE FLOW RATE = 204.74 CFM
ENGINE FLOWRATE with 10 PSI = 346.02 CFM (24.34 LB/MIN)
PRESSURE RATIO (PR) = 1.69

#3 CALCULATING AIRFLOW RATE AT 50% RPM RANGE

As you may already know the point at which the compressor reaches full boost is largely determined by the Turbine side. but I usually calculate a third point just incase. I plot this third point on the compressor map this way if boost comes way early (50% redline) I know where I lie on the map.

50% of redline = 3700 RPM (0.50 x 7400 = 3700)
(134.2 x 3700 x .5 x .95) / 1728 = 136.50 CFM no boost
136.5 x 1.69 = 230.69 CFM under 10 PSI
239.69 x 0.070318 = 16.23 LB/MIN

BASIC ENGINE FLOW RATE = 136.5 CFM
ENGINE FLOWRATE with 10 PSI = 230.69 CFM (16.23 LB/MIN)
PRESSURE RATIO (PR) = 1.69

GATHER YOUR DATA

we have just figured out our engine's airflow rate at 3 RPM points (redline, 75% and 50%) you only need 2 at the minimum but you can use as many points as you want.

POINT 1 (7400 RPM)

FLOWRATE with 10 PSI = 461.21 CFM (32.44 LB/MIN)
PRESSURE RATIO (PR) = 1.69

POINT 2 (5550 RPM)

FLOWRATE with 10 PSI = 346.02 CFM (24.34 LB/MIN)
PRESSURE RATIO (PR) = 1.69

POINT 3 (3700 RPM)

FLOWRATE with 10 PSI = 230.69 CFM (16.23 LB/MIN)
PRESSURE RATIO (PR) = 1.69

these are the three points you will start referencing onto the different compressor maps, ideally you want all three to be within the highest percentile in the compressor map.

using this map of a T04E 60 trim first let me explain all the numbers on the map
1-left side, pressure ratio
2-bottom side, airflow rate (LB/MIN on this map)
3-dotted line on far left side of "ovals", surge limit
4-numbers on far right, 46020, 69640, 83972 etc, compressor RPM (yikes)
5-78%,75%, 74%, compressor efficiency, this is related to the temp of air, a low number (60%) means that the compressor is heating the air more a high number (78%) means the air is not heated as much when it is compressed.


NOW all you have to do is take the each RPM point and put them on the compressor map, use the airflow rate and PR wherever they intersect is where the point will go. ideally you want all points to be in the highest Comp. Eff. %, especially the redline and the 75% redline points.

go ahead and give it a trry, but thats pretty much how you select compressors if any of the points lie:
-in a low Comp Eff %
-in the surge limit
-too high a Comp. RPM
you need to look at a different Compressor map cause the one your looking at is.
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