Bimmertech, it would be a waste to buy one because it's just gonna get dirty again after you clean and re-oil your air filter.
Years ago I chased a pinging/detonation problem and changed everything trying to figure out what was causing this. After cleaning it I had a new car,,or at least it felt like it.
What is causing this are pre-oiled filters that are soaked with oil. This clogs up the mass air meter and the computer goes nuts with crazy A/F ratios. Cleaning it is well worth the 20mins that it takes.
this shiet works. cleaned mine today, took it out for a sping, and wow... i raced around all day today, tore up the streets... if you havn't do it, do it.