Volumetric efficiency calculator: is the MAF reading right?
A four-stroke engine fills half its displacement every revolution. From displacement, rpm, intake air temperature and barometric pressure you can calculate the airflow of a perfect engine; the MAF reading divided by that is the volumetric efficiency (VE).
- Air density
- 1.184 g/L
- Airflow of a perfect engine
- 71.0 g/s
- Volumetric efficiency
- —
How to use it in a diagnosis
- Do a full-throttle pull in 2nd or 3rd gear (safely, on a road or dyno) up to about 4,000–5,000 rpm.
- Record MAF (g/s), rpm, intake air temperature and barometric pressure at the same moment.
- Enter the values: a healthy naturally aspirated petrol engine usually shows 75–95 % at full throttle.
Questions mechanics ask
What is a normal volumetric efficiency?
At full throttle a healthy naturally aspirated petrol engine typically reaches 75–95 %. Turbocharged engines read above 100 % under boost, so this check works best on naturally aspirated engines or off boost.
What does low volumetric efficiency mean?
Either the engine really breathes less air (blocked air filter, restricted catalytic converter or exhaust, cam timing out, sticking intake flaps) or the MAF under-reads (contaminated sensor, air leak before the MAF is unlikely but a leak after it is common).
Why do I need intake air temperature and barometric pressure?
Air density depends on both. Cold air at sea level is denser than hot air at altitude, so the same engine draws a different mass of air.