You’ve probably seen the claims: more airflow, more horsepower, reusable design, and better throttle response. On paper, that sounds great. But what’s left out is the long-term effect on your engine — and even worse, your mass airflow sensor (MAF), which most European vehicles depend on for precise fuel metering.
Let’s break it down.
What’s Supposed to Be the Benefit?
Increased Airflow
Oil-soaked filters like K&N are less restrictive than paper filters. In theory, more air equals more power.
Reusable Design
Clean and re-oil every 50,000 miles instead of replacing equals long-term savings.
Slight Performance Gains
Sometimes you get a very minor bump in throttle feel on high-revving naturally aspirated engines or tuned setups.
The Downsides That Don’t Get Advertised
Contamination of the Mass Air Flow Sensor
This is the number one issue. On modern European engines — BMW, Audi, Mercedes, VW — the MAF is extremely sensitive. Excess oil from the filter, especially if over-oiled during cleaning, gets pulled onto the MAF wire, slowly contaminating readings and eventually causing drivability issues like hesitation, lean or rich codes, or limp mode.
Minimal to No Real Gains on Stock Cars
Most engines are not airflow-limited by the stock paper filter. Especially on turbocharged cars where boost pressure is regulated — any small gain in flow is quickly offset by the ECU dialing it back. On dyno tests, the real horsepower gain is often less than 2 or 3 hp, if any.
Poor Filtration Compared to OEM Paper Filters
To allow more airflow, oil-soaked filters trade off filtering efficiency. That means more fine dust and dirt can get into the engine — not ideal for long engine life, especially in dusty environments like Colorado.
Oil Service and Cleaning is Often Done Wrong
Most users apply too much oil after cleaning. This increases the risk of fouling the MAF. And if they don’t oil enough, the filter doesn’t capture enough debris.
Long-Term Engine Wear
Over time, slightly worse filtration adds up. It may not destroy the engine, but increased bore wear and degraded oil quality can show up in long-term ownership.
Poor Air Intake Design and Hot Air Draw
Another major issue rarely considered is the actual path the new intake system uses to pull air in. Most factory systems are carefully engineered to source cool air from outside the engine bay — often through fender ducts or dedicated scoops. Aftermarket high-flow kits, on the other hand, often sit exposed under the hood, pulling in hot air from right next to the exhaust manifold or turbo. That can easily raise intake air temperatures by 30 to 60 degrees or more, especially at a stop or in traffic. The engine then compensates by pulling timing or enriching the fuel mixture to protect itself. So instead of gaining the promised 3 or 4 hp, in the real world you might be losing 10 or 15. It ends up looking and sounding like an upgrade, but functionally it’s a downgrade in many cases.
When is It Worth Using One?
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If you’re running a standalone ECU and tuned velocity stacks or ITB setup that needs max airflow
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On a race car that is regularly torn down and rebuilt
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If you’ve relocated the MAF downstream or converted to MAP-only operation
What I Use and Recommend at Elevated Auto-Tech
For most modern European cars — stock or tuned — I stick with high-quality OEM paper filters. They flow plenty, protect better, and avoid all the MAF headaches.
If a customer really wants to go aftermarket, I usually recommend dry high-flow filters like AEM DryFlow or S&B Dry, which offer better filtration and don’t risk MAF contamination. Even then, the gains are minor unless the entire intake tract is redesigned.