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Old 12-07-2012, 06:48 AM   #1
Noswal OP
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What is the K&N Filter Mod?

I know this subject has been talked about a lot, and I also know that there are, or appear to be, two distinct schools of thought when it comes down to K&N (or replacement) filters vs. OEM- and I'm really not trying to start a discussion with regards to that (you either do or don't).
But- I'm interested in this, partially because my particular bike has holes drilled in the backside of my airbox, so there is a direct exposure there to the atmosphere and the elements- rain, dirt, etc...
In reading about this subject (and I've done a bunch) I came across a post in one of the threads (can't even recall who the author was) and he (or she) stated that if modified, the K&N outperformed the factory (paper) element. There was no follow-up given though, as to what that mod was. I'm hoping, whoever that was that wrote that (or anyone else who might know) could give me a clue. I do know that supposedly they filter best when they're left a little dirty.
Lastly, along this same subject, I did read that if I did the switchover to the K&N, that rejetting isn't needed.
The bike's a '75 R90s.
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Old 12-07-2012, 07:14 AM   #2
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The K&N filter mod (as I know it) applies to the later, flat airbox and involves trimming either the filter or the starter cover so the filter seats/seals better against the box itself.

With your bike, you already have the airbox mod I know about, drilling holes for more airflow, which, if this were the early 1970s, would probably start a debate equal to the "K&N, good or bad?" question.
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Old 12-07-2012, 10:07 AM   #3
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That was probably me and I was talking about the later square filters that need to be modified to fit right. Of all the people I have worked with and whatnot, I think I was the first to think of getting that flat filter to fit right. I am sure someone else thought of it too but AFAIK . . . .
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Old 12-07-2012, 12:09 PM   #4
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As far as i know a 90S should have holes in the airbox. Just as the later R100 has. I have two bikes with a replacement K&N filter. An R60/6 and an R100RT. There is no performance gain with the round replacement filter. Form, fit and function are the same as the original paper filter. The gain is in the reduced maintenance and the saving on new paper filters.
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Old 12-07-2012, 02:37 PM   #5
supershaft
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Quote:
Originally Posted by halflive View Post
As far as i know a 90S should have holes in the airbox. Just as the later R100 has. I have two bikes with a replacement K&N filter. An R60/6 and an R100RT. There is no performance gain with the round replacement filter. Form, fit and function are the same as the original paper filter. The gain is in the reduced maintenance and the saving on new paper filters.
Just because there is SO much idiotnet 'data' out there that claims they don't filter as well as paper, I am going to repeat my opinion of the most important gain you get from using K+N's over stock paper filters: They filter a LOT better! I know of all the 'scientific' tests that prove the contrary but I know what I see. I see K+N's keeping air boxes and snorkel tubes a lot cleaner than paper filters. Some people have come back and have said that it is the dirt that you can't see and feel that K+N's don't filter well. Well, I am sticking with what I see and feel. Besides, you can look at the photos of those 'scientific' tests being done and see that the K+N's are being tested while perfectly clean with not near as much oil as they should have or barely any oil at all. Well, I can tell you that too: K+N's don't filter well lacking oil.
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Old 12-07-2012, 10:37 PM   #6
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Quote:
Originally Posted by supershaft View Post
Just because there is SO much idiotnet 'data' out there that claims they don't filter as well as paper, I am going to repeat my opinion of the most important gain you get from using K+N's over stock paper filters: They filter a LOT better! I know of all the 'scientific' tests that prove the contrary but I know what I see. I see K+N's keeping air boxes and snorkel tubes a lot cleaner than paper filters. Some people have come back and have said that it is the dirt that you can't see and feel that K+N's don't filter well. Well, I am sticking with what I see and feel. Besides, you can look at the photos of those 'scientific' tests being done and see that the K+N's are being tested while perfectly clean with not near as much oil as they should have or barely any oil at all. Well, I can tell you that too: K+N's don't filter well lacking oil.
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Old 12-08-2012, 05:46 PM   #7
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Not to set this thread adrift, but SS mentioned a point I've seen posted more tha once and more than one place - that K & Ns pass more particulate matter than paper filters.

I have not seen definitive proof either way. I ran K & N on the Ducatis, and have one on the R 80 ST.

To hedge my bets on the ST, I bought a sheet of Uni Filter foam and cut out a prefilter that fits across the top of the K & N. I can sleep well knowing that I have top notch filtering. AND, the prefilter and filter are cleanable and reusable. The Uni Foam didn't even force the need for idle mix nor needle adjustment.

Works for me.
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Old 12-07-2012, 04:18 PM   #8
mendoje
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This is the K&N Filter Airhead Mod



I took a Dremel grinder and grooved the front of the filter body, where it contacts the rear edges of the starter cover. At the bottom its about 3/16 - 1/4" deep, tapering to zero at the top, Basically you want it deep and wide enough to not contact the starter cover, therefore not warping the filter.
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Old 12-07-2012, 08:45 PM   #9
supershaft
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That's it! I do it with an Exacto blade but the effect is the same. You can get little light and put it into the hole in the starter cover for the air pump vacuum lines and then go around to the other side and make sure that the front bottom lip of the filter is over and in front of the box all the way across the box.
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