Have a look at..
Look back at old carburetor filter housings on things like Cortinas and what you generally have is a nice large round filter and a housing breathing through a tiny snorkel. The aim here is to reduce induction noise which it does very nicely at the cost of approx 5% of the engine's output. K&N started to gain their reputation back in the 70's from bolt on systems that gave you back that
5% but it was the removal of the restrictive housing that gained the power and not the type of filtration media used. You also generally had to suffer extra noise and carb icing in winter to go along with the extra power.More modern cars, especially FI ones, have better designed systems with larger internal cross sections but still scope for improvement. In flow testing I generally found it was the bottom part of the air box containing a flat panel filter that caused the major part of the restriction. Fiesta XR2i is a case in point. Drill a couple of extra 1" diameter holes in the bottom of the box and the flow goes up considerably. The filter itself causes no restriction.
2) K&N type oiled filters do NOT filter as efficiently as a new paper element and do NOT remove particles down to such a small size. In addition they rely on a careful procedure to clean and re-oil them that is time consuming and prone to error. Too little oil and their filtration efficiency drops, too much and the excess gets sucked into the induction system where it can harm MAF sensors and other items. I've compared the wear on race engines with K&Ns and paper filters and you see more bore wear and valve/seat wear when K&Ns are used. Whether that is partly down to improperly oiled filters is irrelevant - that's part of the equation that the average owner has to face.The average garage mechanic or DIY servicer can't really screw up a paper element filter change. It's fast and foolproof. Servicing a K&N properly is time consuming and error prone. It isn't a cost effective solution when labour charges have to be included in the equation.
3) Cone type FI filters can certainly improve power by removing large sections of the OE trunking that cause flow restrictions. The filtration media isn't the critical element here - it's the change in the flow path. A big cone paper element would be even better than a big cone K&N type one. What sells modern replacement filter systems is mainly looks. Chrome, stainless steel, carbon fibre look materials. I have no objection to people wanting to alter the appearance of the engine bay or gaining power where such systems genuinely do so. My own approach though is not to pay £200 for a bit of fancy looking 'fluff' that contains about £10 worth of product.Employ some large diameter proprietary trunking, locate the input end at a high pressure area served by cold air and slap a big truck paper filter element on the end and you'll do a better job for a tenth of the cost.
Dave Baker - Puma Race Engines