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Old 07-03-2005, 02:11 PM
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VE, CFM from Injector Flow & RPM

To find the CFM and Pounds of fuel per hour:
you need to know the RPM, Pulse Width or Duty Cycle of the injectors and Air Fuel Ratio in addition to the Ci of the engine. The fuel pressure & lb/hr rating or injector rating. if using a Supercharger or Turbo you'll also need the Boost level.

Take Turbos turbocharged 406ci S10 for example. 5500 RPM,
"some low boost numbers from a part throttle run to 4 PSI. Injector PW was 15.0, intake pressure was 130kpa(4.1 PSI boost), RPM was 5500, Lambda was .78(11.5AFR), base fuel pressure is 50psi with 50lb injectors(..... rated at 42psi)"

RPM/60 seconds per minute = X revolutions per second
5500/60= 91.6666666 rev per second

1second/ X revolutions per second
1sec/ 91.6666666666= 0.0109090909090909

Multiply that by 2 because a four stroke intakes every other time
0.0109090909090909 x 2= .0218181818181818181 time to complete one 4 stroke cycle

Take that number and divide it into the Injector Pulse Width in seconds to determine the Duty Cycle {% of time the injectors are open}.
.015 sec / 0.0218181818181818181= 68.750 % duty cycle

Now multiply the Duty cycle by the Lbs/Hr rating of the injectors and then divide by 60 to convert to minutes and find the pounds of fuel per minute
68.75% x (50 lbs/hr x 8) /60= lbs per minute from 8 injectors= 4.5833333333333333 lbs/min of fuel @ 42 PSI

since the PSI has been changed from "stock PSI" & boost has been added we must figure that into the equation

PSI - boost pressure
50 psi - 4 psi {boost} = 46 psi

and that is divided by the injector psi rating
46 / 42 = 1.095238

now multiply that back into the factory rating to figure the actual lbs under boost.
1.095238 x 4.5833333333333333= 5.01984 lbs

Take that number and multiply by the AFR #
5.01984 lbs x 11.5= 57.728174 pounds of air per minute

Divide that by the weight of air, 0.0751632645624 lbs per cubic foot to figure actual CFM
57.728174/ 0.0751632645624= 768 actual cfm

Find the theoretical CFM
406/2= 203 CI per revolution.
203 x 5500 RPM= 1116500 ci per minute
1116500 /12 /12 /12 = 646 CFM required @ 0 lbs boost & 100% VE

Now divide the Actual CFM by the theoretical CFM, without a blower this number will be the VE. Multiply by 100 to find the this case 118.8%:
768/646 =1.18891

With a blower multiply that by the CI of the engine to determine how many actual CI of air it is moving
1.18891 x 406= 482 ci of actual air

now take your boost pressure and divide by 14.7 and add 1
4 /14.7 = 0.27210884 + 1=

multiply that by your Ci to find theoretical Ci
406 x (1 + 0.27210884)= 516 theoretical ci

divide the actual ci by the theoretical ci to determine %VE
482/516= 93.5% VE

It would be nice if somebody could work this on their car and see what they come up with.

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