What happens if the air intake hose is broken?

Here's what happens if the air intake pipe (or hose) is broken, cracked, or disconnected, incorporating your keywords:

1.  Unfiltered Air Entry: The primary purpose of the intake hose is to carry air from the air filter box to the engine. A breach allows unfiltered air containing dirt, dust, and debris to enter the engine. This causes:
    Accelerated Engine Wear: Abrasive particles scoring cylinder walls, damaging piston rings, and wearing bearings.
    Throttle Body/MAF Sensor Contamination: Dirt buildup disrupts airflow measurement and throttle operation.

2.  Unmetered Air Leak: Modern engines use a Mass Air Flow (MAF) sensor mounted before the throttle body, often integrated into the air intake pipe assembly. A leak after the MAF sensor (like a crack in the hose or a disconnected coupler) allows air to enter the engine without being measured by the MAF.
    Check Engine Light (CEL): The engine control unit (ECU) detects the discrepancy between measured air and expected conditions (based on oxygen sensor feedback), triggering a CEL. Common codes include P0171 (System Too Lean - Bank 1) or P0101 (MAF Circuit Range/Performance).
    Poor Engine Performance: The ECU calculates fuel injection based on incorrect air mass data, leading to:
        Rough idle (often very noticeable)
        Stalling, especially at low RPM or when coming to a stop
        Hesitation and stumbling during acceleration
        Reduced power and sluggish response
        Poor fuel economy
        Crucially for performance racing: This unmetered air completely disrupts the carefully calibrated Air/Fuel Ratio (AFR), killing performance, causing hesitation under boost, and preventing the engine from making optimal power. An Aluminum Intake Pipe upgrade is popular in performance racing partly for durability to prevent such leaks under high stress.

3.  Vacuum Leak (Manifold Applications): On naturally aspirated engines, the intake hose connects directly to the intake manifold. A leak here acts as a significant vacuum leak.
    Severe Idle Problems: Very high, erratic, or low/stalling idle.
    Performance Issues: Similar symptoms to unmetered air leaks (hesitation, stumbling, lack of power).

4.  Turbocharger Damage (Turbocharged Engines like Audi A4): This is a critical risk for turbocharged cars (most modern Audi A4 models). The audi a4 air intake hose connects the air filter to the turbocharger compressor inlet. A leak before the turbo allows:
    Debris Ingestion into Turbo: Unfiltered dirt and debris are sucked directly into the turbocharger's compressor wheel. These wheels spin at over 100,000 RPM. Impact with debris can chip, bend, or break the blades, leading to catastrophic turbo failure, imbalance, and potentially further engine damage from metal fragments. This is an expensive repair.
    Reduced Turbo Efficiency/Boost Leak: A leak after the MAF but before the turbo (less common location for the hose itself) can also reduce boost pressure and efficiency.

5.  Altered Engine Sound: A large intake leak often creates a loud hissing or sucking noise, especially noticeable under acceleration or at idle.

Impact on Performance Racing & Aluminum Intake Pipes:
In performance racing, any intake leak is disastrous. It destroys carefully tuned AFRs, causes unpredictable power delivery, hesitation under load, and risks engine/turbo damage.
This is why durable aftermarket intakes, like Aluminum Intake Pipe systems, are popular upgrades. They are less prone to cracking, collapsing under suction, or heat soak compared to stock rubber/plastic hoses, providing more consistent airflow and reducing the risk of leaks under high-performance conditions. However, any intake system can leak if connections fail or pipes crack.

In Summary: A broken air intake pipe causes unfiltered air intake (causing engine wear), unmetered air leaks (causing poor running, CEL, and terrible performance), potential vacuum leaks, and on turbocharged cars like the Audi A4, a significant risk of turbocharger destruction. Immediate repair is essential to prevent damage and restore normal engine operation and performance. For performance racing applications, ensuring the integrity of the intake system (whether stock or an upgraded Aluminum Intake Pipe) is paramount.