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Volumetric efficiency calculator

Enter displacement, rpm and the airflow you measured, either in cfm or in grams per second from a mass airflow sensor log. You get the volumetric efficiency and the theoretical airflow it is compared with.

Engine and airflow

Displacement

Enter a displacement greater than zero.

Displacement unit

Engine speed

rpm

Enter an rpm from 100 to 20,000.

Measured airflow in

Mass airflow

g/s

Enter an airflow greater than zero.

Air density for the conversion

kg/m³

Enter a density from 0.3 to 3 kg/m³.

Only used for g/s. The default, 1.225 kg/m³, is the standard atmosphere (ISA) at sea level and 15 °C. With it, VE is measured against standard air. Enter the density of the air the engine actually breathed to measure against that instead.

Four-stroke engines only. The result is only as good as the airflow reading: log at wide open throttle, at a steady rpm.

What the inputs mean

Displacement and rpm: set the theoretical airflow, the volume the engine would swallow if every cylinder filled completely on every intake stroke.
Measured airflow: cfm from a flow meter, or grams per second from a mass airflow (MAF) sensor log. Log at wide open throttle and read the rpm at the same moment.
Air density: turns grams into cubic feet. The default, 1.225 kg/m³, is the ISA standard atmosphere at sea level and 15 °C.

The formulas

theoretical cfm = CID × rpm ÷ 3456
cfm from g/s    = g/s ÷ 1000 ÷ density × 35.315 × 60
VE (%)          = measured cfm ÷ theoretical cfm × 100

A four-stroke cylinder fills once every two revolutions, so the engine draws its displacement every two turns. Cubic inches per minute divided by 1,728 gives cubic feet, and 1,728 × 2 = 3,456. The carburetor cfm calculator uses the same relationship the other way round. 35.315 is the number of cubic feet in a cubic meter.

Worked example

A 2.0 L engine (122.05 ci) logs 110 g/s at 6,000 rpm.

Theoretical airflow: 122.05 × 6,000 ÷ 3,456 = 211.9 cfm.
Measured: 110 ÷ 1000 ÷ 1.225 × 35.315 × 60 = 190.3 cfm.
VE: 190.3 ÷ 211.9 = 89.8%.

The same answer in mass: the theoretical flow is 2.0 L × 6,000 ÷ 2 per minute at 1.225 kg/m³, which is 122.5 g/s, and 110 ÷ 122.5 = 89.8%.

With a flow meter instead: a 350 ci engine at 5,500 rpm has a theoretical airflow of 557.0 cfm. A measured 470 cfm is 84.4%.

Which density to use

Against standard air, an engine breathing hot or thin air shows a lower VE, because it takes in fewer grams. That is useful for comparing runs made on different days. To see how well the cylinders fill with the air they actually get, enter that air's density: density = pressure ÷ (287.05 × temperature), in pascals and kelvin. At 101.325 kPa and 30 °C that is 1.1644 kg/m³, and the example above becomes 94.5%.

Related calculators

Work out the static ratio first with the compression ratio calculator, and the pressure ratio of a boosted engine with the boost calculator. The engine set also covers dynamic compression, rod ratio, BMEP and turbo airflow and compressor map points.

Questions

How do you calculate volumetric efficiency?

Divide the airflow the engine actually takes in by its theoretical airflow, displacement × rpm ÷ 2 for a four-stroke. A 350 ci engine at 5,500 rpm has a theoretical 557.0 cfm; measuring 470 cfm gives 84.4%.

How do I get VE from a MAF log?

Convert grams per second to cfm with an air density, then divide by the theoretical cfm. A 2.0 L engine logging 110 g/s at 6,000 rpm flows 190.3 cfm at 1.225 kg/m³, against 211.9 cfm theoretical: 89.8%.

Can volumetric efficiency be over 100%?

Yes. Tuned intake and exhaust pulses can pack in more than the swept volume, and a turbo or supercharger does it by pressure. At 150 g/s, the 2.0 L example at 6,000 rpm would read 122.4% against standard air.

What does 3456 mean in the airflow formula?

It is 1,728 cubic inches per cubic foot times the 2 revolutions a four-stroke needs to fill every cylinder once. CID × rpm ÷ 3456 gives the theoretical airflow in cfm.