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My Z32 TwinTurbo 2+2

In this section, we will look into three engine efficiency topics: Volumetric Efficiency, Thermal Efficiency and Mechanical Efficiency.

Volumetric Efficiency

     Actual CFM     

= Volumetric Efficiency 

Theoretical CFM


Volumetic Efficiency or VE, I will be using from this point, varies depending on temperature and pressure.

From that, we know a normally aspirited engine will have VE of 100% or less. And force inductioned engine will have VE of 100% or more.

The actual calculation of VE is done by ECU using measured amount of intake air, with Mass Air Sensor measuring at intake pipe or Speed Density measuring inside the intake manifold (close to intake port of the engine).

  • Theoretical CFM

theoretical cfm = rpm x displacement / 3456

  • Engine Flow Demand
Engine Flow= (engine displacement) X (volumetric efficiency) X (engine speed) X (manifold pressure)
You can see the key to increase engine flow is to increase engine VE (volumetric efficiency). Reduce intake charge temperature is the easiest way to help increase engine VE. This is where air/air, air/fluid intercooler and water injection come into play.
Assuming VE at 100%, 1 atmosphere pressure, we have the following table for a 3.0L engine flow( CFM) at various rpm and pressure point.
  • Influence of the altitude above Sea Level on the Volumetric Efficiency

Influence of the elevation above Sea Level on the Volumetric Efficiency

Atmospheric pressure as given by average barometer reading

Height above sea level, ft. Atmospheric pressure, in. of mercury Atmospheric Pressure, in PSI (approximate) Relative volumetric efficiency
0 29.92 14.7 1.000
1,000 28.85 14.2 0.965
2,000 27.82 13.7 0.931
3,000 26.82 13.2 0.892
4,000 25.85 12.7 0.865
5,000 24.92 12.2 0.833
6,000 24.00 11.7 0.803
8,000 22.17 10.7 0.742
10,000 20.34 8.7 0.681
12,000 19.30 6.7 0.645


  • Engine Flow Measurement

Most engine control systems in production today utilize either speed-density sensors or air-mass sensors to measure engine air flow. Speed-density is very popular because of its low cost and high reliability. Speed-density systems typically calculate engine flow rate based on engine speed, intake manifold pressure and temperature. Some systems use barometric pressure sensors and inlet air temperature sensors to improve flow calculation accuracy for varying ambient conditions. Fig. 1 illustrates a typical engine with both speed-density sensors and an air-mass sensor.

fig001.gif (6059 bytes)

Figure 1: TBI Engine with Air-Mass Sensor and Speed-Density Sensors*


eed-density systems calculate an air flow rate that approximates the flow rate at At the intake ports, mdot_ao. Air-mass sensor systems usually measure the air flow rate near the throttle and thus approximate mdot_ai. During throttle transients (or whenever the manifold pressure fluctuates) the flow rates in and out of the manifold are different from one another, i.e., . This is illustrated in Fig. 2 for a rapid throttle transient.

fig002.gif (5644 bytes)

Figure 2: Air flow rates during a rapid throttle transient

The large in-rush of air during the throttle opening is often referred to as "manifold filling." During this time the flow rate into large manifolds can be several hundred percent higher than the flow rate at the ports.

Speed-density systems calculate air density in the intake using manifold pressure and temperature sensors and the perfect gas law. The air-mass flow rate at the intake ports, mdot_ao, is assumed to be quasi-steady and may be calculated as:

eq001.gif (2000 bytes)

The volumetric efficiency,eta_vol.gif (266 bytes), of the engine is usually mapped from steady-state air flow measurements and stored in the controller's Read Only Memory (ROM) as a table having inputs of manifold pressure and engine speed as shown in Fig. 3: