Automotive Calculations: airflow, units and extrapolation tools

Automotive Calculations (airflow, units and extrapolation)

The Automotive Calculations window is a set of small calculators for chiptuning estimates — converting air-flow figures, sizing airflow for a power target, working out volumetric efficiency and density ratio, converting common units, and extrapolating map values. Open it from Modules › Automotive Calculations (Figure 1), or from its toolbar button; each menu entry opens the same window on a specific tab.

Every calculator works the same way: type the input values and press Apply to see the result. Many results also have a copy button so you can paste the figure into your notes or a map. These figures are estimates to guide tuning decisions, not exact measurements.

Figure 1

Air Flow Estimation

The first tab (Figure 2) groups five related calculators:

Mass Air Flow — enter a MAF reading and choose its unit (mg/stroke, g/s, kg/h or CFM); with engine RPM and the number of cylinders the tool fills in the other units and an estimated power figure in HP and kW.
Potential Air Capacity — enter displacement (cc), RPM, a pressure ratio (1 for naturally aspirated, higher for boost) and the volumetric efficiency, then pick the output unit. It estimates the air the engine can draw using CFM = (CID × RPM × VE) ÷ 3456, scaled by the pressure ratio.
Power to Airflow — enter a target power (HP) plus VE and density ratio to estimate the airflow needed to support that power, in the unit you choose.
Volumetric Efficiency — divides a measured airflow by the potential airflow to give VE (VE = measured CFM ÷ potential CFM).
Density Ratio — works out the charge density ratio across an intercooler from the pressure ratio and the inlet/outlet temperatures (entered in °C).

Figure 2

Simulation

The Simulation tab (Figure 3) plots a Potential Air Capacity curve on a chart. Enter the displacement, the RPM range (minimum and maximum), the pressure ratio and a VE range, choose the output unit (mg/hub, g/s, kg/h, CFM, HP or kW) and press Plot to see how the estimated airflow changes across the rev range.

Figure 3

Unit Conversions

The Unit Conversions tab (Figure 4) handles the conversions you reach for most while tuning:

Displacement — cubic centimetres (cc) to cubic inches (CID) and back.
AFR Ratio — turns an air mass and a fuel mass (in mg) into an air/fuel ratio shown as x.x:1.
Power — horsepower (HP) to kilowatts (kW) and back.
Torque — newton-metres (Nm) to pound-feet (ft·lb) and back.
Pressure — bar, psi and kPa in any direction.

Pick the "from" and "to" units, type a value and press Apply.

Figure 4

Extrapolation

The Extrapolation tab (Figure 5) is a small grid for linear extrapolation — extending a known trend beyond its last point, which is handy for filling map cells past the logged range. Set the number of columns, then enter two known axis points and their values in each column, plus the target axis point in the Result row. Press Calculate and each column is filled in using:

result = ((target − a1) × (v2 − v1) ÷ (a2 − a1)) + v1

where a1/a2 are the two known axis points, v1/v2 their values in a column, and target is the axis point you want. Use Clear All to reset the grid, tick Round the result for whole numbers, and Copy results to the clipboard to paste them elsewhere.

Figure 5

A fifth Help tab inside the window gives a short reference for these calculators.

Tip — Worked example for Potential Air Capacity: a 1998 cc engine at 6000 rpm, VE entered as 0.95 (i.e. 95%) and a pressure ratio of 1 gives about 200 CFM — roughly 130 HP. Enter VE as a fraction, not a percentage.