Jaaaaaaaaaaaymac

[Nikuk 1]

The math looks good to me, but can You superenginebuilderextraordinaire's check it for errors in theory.

To figure VE of an engine in a given application, You'll need to run a test of airflow. To keep it simple, run the engine @ 7k rpm while measuring the amount air drawn into the intake manifold (lb/min or cfm), and the temperature of the air (IAT).

Now, bear with me:

Density: t1 / t2 = d2 / d1

• t1 = temp for air of know density (32* F @ 0.0808 lb/ft^3) in Rankin
  
• t2 = IAT (in degrees of Absolute Rankin, F* + 459.67)
  
• d1 = density of air for known temperature (0.0808 lb/ft^3 @ 32* F)
  
• d2 = Density of intake air charge
  

AVF = Actual Vol. Flow Rate (calculated): Mfa/d2

• Mfa = Mass air flow actual (measured)
  
• d2 = air density actual (measured)
  

Calculating Theoretical Max Airflow (cfm): TAF = (ED*RPM*VE) / (ES*C)

• rpm = Operating engine speed
  
• TAF = theoretical airflow
  
• VE = Vol. Efficiency (100% theoretical )
  
• ED = Engine Displacement (in^3)
  
• ES = Engine Stroke (2-stroke = 1 / 4-stroke = 2)
  
• C = conversion factor from in^3 to ft^3 (1728)
  

Calculating Volumetric Efficiency: (AVF/TAF)*100

• VE = Vol. Efficiency (calculated)
  
• AVF = Actual Vol. Flow Rate (calculated) (ft^3/min)
  
• TAF = Theoretical Airflow rate (calculated) (ft^3/min)
  

So, we have our equip tested. We come up with: 17.14 lb/min --OR-- 243.8 cfm @ 7000 RPM with our 122 (in^3) engine. The IAT was reading 72* F.

Now, do the math:

• Density: t1 = (32+459.67) = 491.67; t2 = (IAT+459.67) = 531.67; d1 = 0.0808. So: 491.67/531.67*.0808 = 0.0747.
  
• AVF: 17.14/.0747 = 229.43.
  
• TAF: rpm = 7000; Displacement = 122. So: (122*7000*1)/(2*1728) = 247.11
  

So, 229.43/247.11 = 0.9284 or 93% VE @ 7000rpm.

That about right?

Thanks,

-Nick

[Jmac 24]

I get 92.829% ...

[Nikuk 1]

lol, I meant does the theorum look right to You? Given density et al.

Nick

[Jmac 24]

Oh, okay, I just did the math

Calculating air density is more complex than that ...

Here's the shortcut form that is mostly correct (but incorrect between -1 and 1 degree celcius) ... Only reason I use this is because the actual equation is really, really long ...

AIR = ((BAR*1000/(287.05*(TEMP+273.15)))+(100*(6.1078*10^((7.5*TEMP)/(237.3+TEMP)))*HUM/100/((461.495*TEMP))))

AIR = Air density in kg/m^3

BAR = Barometric Pressure in kPa

TEMP = Ambient temperature in *C

HUM = Relative Humidity in %

[Nikuk 1]

cool, I knew You had it somewhere.

that works out in my Excel.. nice. Overall it looks good. Thanks.

Now, off to the flowbench.