TERM-RESTRICTION
Restriction
Air Intake
Also: pressure drop · ΔP · intake restriction · air restriction
Abbrev: ΔP · inH₂O · mbar
Canonical Definition
The pressure drop (ΔP) across an air filter element at a given airflow rate, measured in millibar (mbar) or inches of water column (inH₂O). Restriction increases as the filter element loads with contaminant. Service limits are reached when restriction exceeds the engine manufacturer's threshold — typically 25 mbar for naturally aspirated engines and 37.5–62.5 mbar for turbocharged engines. Measured per ISO 5011 and SAE J1539.
Engineering Context
Restriction directly reduces engine volumetric efficiency and power output. A 1 mbar increase in intake restriction reduces naturally aspirated engine power by approximately 0.3–0.5%. For turbocharged engines, restriction after the compressor inlet reduces compressor efficiency and increases exhaust back pressure, creating a compounding power loss. Restriction indicators (vacuostatic alerts) signal when service limits are approached.
Related Technologies
Protection Systems
Knowledge Center Articles
KC-00 Governance Metadata
| Permanent ID | TERM-RESTRICTION |
| Category | Air Intake |
| Status | Published |
| Version | 1.0 |
| Last Reviewed | 2026-07-05 |