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Air Filters

     
     First off, my comments are sure to precipitate urges to flame and 
     defend. Chill. Your rebuttals will probably not change my opinion. 
     Channel the Perry Mason energy somewhere else. 
     
     Second, I'm not a hidden front for a filter manufacturer.
     
     Third, this is a long read. Skip if you're in a hurry.
     ~~~~~~~~~~
     
     'Man of the sea' section:
     
     I used to motorcross, back in my pimple days. I also worked my way 
     thru college as a motorcycle mechanic. I was completely immersed in 
     motorcycles (street and dirt) for almost a decade. In that time, I 
     changed and cleaned a lot of air filters. Paper, oiled gauze (K & N 
     typifies this bunch), oiled foam, dry foam (some people would forget 
     to add the oil), no air filter (the ultimate free-flow dice roll).
     
     There were basically two camps for the performance motorcycle 
     airfilter market - Oiled foam and oiled gauze (K & N). In my own 
     motocross bikes, I always ran oiled foam. The ultimate filter for 
     filtering and service was dual-stage foam filters. The outer 'sock' 
     was a course filter that removed the majority of particulates. This 
     outer filter could alone be changed when it got clogged. There was an 
     inner foam filter that finished the job with a much finer celled foam. 
     
     In a day of racing, the filter would usually get changed twice. 
     Sometimes just the outside sock, other times the whole pair. Off road 
     racing, when it's dusty, is a challenge to an airfilter. Back in the 
     shop, I would often get to service peoples race bikes that used K & N 
     filters. Since it was my job to find things wrong with motorcycles as 
     a mechanic, my fingers would always probe intake boots (downstream of 
     air filters). In almost all of the bikes that I would work on that 
     used K & N's, I could feel grit in recesses of air boots or stuck in 
     the oily throat (2-strokes use premix) of the carbs. Maybe it got 
     there when changing filters...or with a different type of filter, but 
     oiled gauze filters always seemed to let more particulates downstream 
     than an oiled foam filter. If you can feel it, just think what got 
     thru that's too small to feel. Oiled foam filtered bikes always had a 
     cleaner intake tract (except for that time I failed to tighten the 
     filter &/#$%@!) Notice I said oiled foam. I'd often get bikes in that 
     had dry foam filters. Usually when quizzed, the customer was just 
     trying to get rid of all that oily mess and just cleaned the filter 
     without reoiling. Ignorance is bliss...
     
     With all that said. You're probably thinking that this ancient wisdom 
     doesn't apply to BMWs and the road. But remember, I quote the great 
     Forrest Gump, "air is air, and particulates are particulates."
     
     GET LOST...
     
     Go away and read the FAQ article from the mining engineer, if you 
     don't believe me about oiled gauze and poor filtering. It's a good 
     read even if you do agree with me.
     
     I'll start condensing my thoughts now to keep it down to novel size. 
     
     Race Face...
     
     K & N filters were designed for the 'race' environment. They are a 
     great racing filter! I think their best application is for ROAD 
     RACING. They flow great and filter out the majority of the big stuff. 
     They aren't really that effective at filtering the small stuff. They 
     don't have to be the ultimate sub-micron filter for most race 
     applications. K & N's are much better than an open or screened intake. 
     Race motors don't get the chance to wear out, due to ingesting years 
     of micron sized particulates. Race engines don't usually wear out 
     their rings from getting blasted with abrasive intake tract 
     particulates. Race engines usually break rings due to too many 
     RPMs...not wear. Oiled foam filters better and has nearly the same 
     flow rate.
     
     Off-Road use - you already know my opinion there.
     
     Street use - The lowly paper filter is best. Why? It filters the best. 
     Really ask yourself, "where are 99% of my miles driven?" "Do I plan on 
     keeping and driving my car for over 100-200K miles, or am I just going 
     to trade it in early before it would make any difference?" Just think 
     how many cubic feet (or liters) of air your engine pulls thru itself 
     over the course of it's life (that would be a nice reply for those 
     with a handy calculator...hint, hint).
     
     The Problem...
     
     The problem with most street stock setups is - the airbox is usually 
     more of a maze than a straight shot. A lot of the performance gain 
     from aftermarket filters, is due to a less restrictive airbox (or no 
     airbox) which naturally flows better. You can't blame the factory too 
     much. What happens when you go thru that deeper than expected puddle 
     and suck water into the engine? Wet paper doesn't flow very good. The 
     airbox has to be well protected. And due to a lack of room in that 
     congested engine bay, the poor engineer that has to design an airbox 
     isn't usually given much to work with, the intake is probably going to 
     be inherently restrictive.
     
     A lot of the performance gains from aftermarket filters, is due to an 
     increased surface area of the replacement filter. All things equal, 
     increasing filter area increases air flow. 
     
     The hidden greed surfaces...
     
     What I WANT for a street setup - an aftermarket filter that has larger 
     surface area than stock, is less restrictive in path (yet somewhat 
     protected), is paper or duel-stage oiled foam, resonance tuned, and 
     costs under $100.
     
     I'm about to finger-cramp soon, so hang in there.
     
     One more testimonial involving the 'F' word... as in Ford.
     
     A friend of mine has a ~'88 Ford van. My Ford van is a '79. Both are 
     351 Windsor. We both drive a lot of dusty New Mexico highway miles 
     chasing wind (we windsurf). BMWs don't have enough room to haul the 
     prolithic amount of 'boardhead' gear...even the touring models.
     
     He uses that really expensive AMSoil and a dual oil filter setup and 
     has his oil analyzed (like aircraft owners do, see *** below). I just 
     use premium mineral 20W50 and 1 filter changed at 3-4K.
     
     His van just got completely rebuilt at around 150K (burning oil, 
     low/uneven compression...). My van is still running fine with the 
     original engine (not rebuilt) at 245K miles.
     
     He used (not present tense) a K & N. I use OEM paper (usually Fram).
     
     ***Looking back, all oil analysis reports showed higher than expected 
     silica (AKA - airborn dust). If you'd like to call the 'F___ van' 
     person I'm referring to, he'd love to 'talk' about his experience. 
     E-mail for an address or phone.
     
     One final comment. I operate a Scanning Electorn Microscope (SEM) for 
     a living. From my job experience of being immersed in a microscopic 
     world, I've learned there are a lot more things you can't visually see 
     floating in the air and sitting on surfaces that you can imagine. Lots 
     of things you can't see with the naked eye can cause big damage that 
     IS visible with your eyes and often thins-out the wallet.
     
     Barry Ritchey
     '90 325iX (paper, 106K)
     '86 Nissan Stanza 4WD Wagon (paper, ~136K)
     '79 Ford Van (paper, ~245K)
     '57 body (none, 39 yr)
     CCA# 134069
     ;^)