Aftermarket filters often reduce hp and airflow by being an inappropriate design (or more truely, not designed at all, just assembled out of large aluminum tubing). Aerodymanics are as important inside the intake system as they are for airplane wings, any odd bit of pipe won't necessarily work better.

Witness the threads in the vintage forum where an aftermarket intake system serious compromised intake effeciency on a 76 230/8 -- black smoke out the back, no power, etc. Cured by restoring it to factory. Also, a member ran his 280 SE 4.5 on a dyno with and without an air filter while he had the chance - hp was down about 5% without the air filter and housing......

K&N filters are fine for racing on dirt tracks where you end up with half the filter covered in dust, but do not do a good enough job on the street. If you DO gain any hp, it will only be at full throttle/high rpm, and where do you drive like that? Not often, I'd bet!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!