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Being a VW/AUDI gearhead for over 7 years, having first hand experience with 4 different intake systems on VAG cars and having seen tons and tons of dyno results with various intake systems I can safely say the following:
* AFE intake systems and AFE drop-in filter replacements are junk!
Way back when they first started selling intake products, they were selling foam filters which turned out to be massively restricting. A mechanical engineer in the VW/AUDI community tested over 13 different filters on a flow bench. The afe filters were always the most restrictive ones. Afe filtering efficiency is really low. People keep talking about surface area of the filter but in actuality, it is the actual flow capability per square inch, that gives you the true performance.
Air velocity is another very important factor when taken into the context of filtration. Most filters DO NOT perform linearly with different speed of the air. Paper filters for example give you best results for slow and mid range air speeds. At higher rpms and speeds where the air velocity in the intake is higher, paper filters will not perform as good because they become too restrictive. On the other hand, most aftermarket drop in filters will perform slightly worse at low rpm compared to paper filters. Their gains are mostly seen at higher air velocities.
K&N filters on the other hand achieve higher performance than paper filters because they are less restrictive. The price however is that more particles are able to pass through the K&N filter and thus enter the engine. Also, K&N filters clog up really fast (about 1 year) and can never be restored to original state EVEN with the recharge kits.
So basically, the trick is to find the best intake system, simply boils down to finding the best filter. By BEST I mean a filter that will give you adequate filtration and at the same time offer performance over the widest powerband.
After years of experiments and dyno testing, it has been determined that the best performing intake system OVER THE LARGEST POWERBAND is a stock airbox, gutted, and smoothed down and sanded down and a quality OEM filter.
That's just my $0.02
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