Short answer
NOACK volatility is the percentage of an oil’s weight lost when a sample is held at 250 °C for one hour while air is drawn across it, run under ASTM D5800 or the European CEC L-40 method. A lower number means less of the oil boils off. ILSAC gasoline standards cap it at 15%, API CJ-4 capped it at 13%, and many automaker specifications set their own limits.
How the test works
A weighed sample is heated in a crucible to 250 °C (482 °F) for one hour with a controlled flow of air pulling vapor away. Afterward the sample is reweighed, and the loss is reported as a percentage of the starting weight.
ASTM D5800 includes more than one procedure, so data sheets sometimes note which one was used, and results from different methods should not be compared directly.
Why volatility matters
The lightest fractions of an oil evaporate first in hot areas such as piston ring zones and turbocharger bearings. Losing them adds to oil consumption, and the oil left behind tends to thicken over the drain interval.
Evaporated oil also travels through the crankcase ventilation system into the intake, where it can contribute to deposits, and it carries some additive components toward the exhaust aftertreatment.
Where limits appear
ILSAC standards for gasoline oils, and the API Resource Conserving categories that match them, limit NOACK loss to 15%. API CJ-4 set a 13% limit for heavy-duty diesel oils. European ACEA sequences and automaker specifications carry their own limits, some tighter than the API and ILSAC values.
Reading a NOACK number
Base oil choice has the largest effect on volatility, and lower-viscosity oils are harder to formulate with low NOACK values. When comparing data sheets, compare oils of the same grade and make sure both list the same method; NOACK is one property among many and does not on its own rank overall oil quality.
Frequently asked questions
Is a lower NOACK number better?
For volatility, yes: a lower percentage means less evaporation under test conditions. It is still only one property, so the specification the engine maker requires comes first.
Does NOACK explain oil consumption?
It is one factor. Ring and valve-guide wear, PCV system condition and turbocharger seals usually matter more in an engine that is using oil.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].