Math check = VE Calcs

NYSpeed Automotive
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

Volumetric Flow Rate: AVF = Mfa / d2

  • AVF = Actual Vol. Flow Rate (calculated)
  • Mfa = Mass air flow actual (measured)
  • d2 = air density actual (measured)

Calculating Theoretical Max Airflow (cfm): TAF = (EDRPMVE) / (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 shit 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, giterdone:

  • 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: (12270001)/(2*1728) = 247.11

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

That about right?

Thanks,

-Nick

2

ehhh

3

LoL, is that a no?

4

yeah dude, u lost me after the work MATH

5

:doh:

Instead of assuming all 4v motors are 95-105% efficient, I’m trying to work out a test to grasp ACTUAL efficiency. The difference in 105% & 92.5% can be HUUGGE BUFFALO, when it comes to tuning, boosting, etc.

So i believe that I’ve got something that works. Requires airflow numbers, rpm numbers, and IAT.

<shrug> that’s what I get for trying.