I sold my 1997 E300 Turbodiesel W210? about a year ago, but I ran it with a K&N air-filter for about a year (25,000km) before that. I felt a good improvement. I thought that the electronic throttle (drive-by-wire) seemed to respond faster, and the engine made a lovely noise. About six months later I also had a "Superchip" fitted (www.superchips.co.uk). This made a huge difference. Engine performance was unbelievable. Claimed bhp increase of 15-20%, lifting bhp from 177 to about 210. It certainly felt like it. I drove a new 2001 E320CDI at the time, and my car felt faster than it. Just before I sold it I burned off the rear tyres doing US-style "doughnuts" in a carpark. I never saw a diesel that could do that. I have the photos to prove it.
I now run a 1990 R129 500SL and a 1991 W126 500SEL which I have K&N's in, and again I feel they make a difference and I love the sound.

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'66 W100 600 SWB
'05 W463 G55K AMG
'91 W126 500SEL
'93 W463 300GDL
'84 W460 280GES
'83 W460 300GDL
'66 W111 220SEb Coupe
'58 W121 190
'58 W180 220S
Have previously owned '99 SL600, '99 ML320, '79 450SLC, '79 450SEL 6.9 x 2, '68 600, '91 190E 1.8, '90 190E 2.0, '85 190, '90 500SL, '92 500SEL, '97 E300Turbodiesel, '91 300D, '72 280S, '82 300D, '81 300SD, '85 230E, '56 220a and I'm sure I'm forgetting some.