Don’t let the percentage value confuse things.  That value represents the percentage of rated flow for a particular orifice @ 15” H2O.  I couldn’t achieve anywhere near 15” H2O because these elements don’t offer enough restriction to start pulling higher vacuums with this small vacuum source.  The shop vac just isn’t big enough to torture these filters.  But at the test pressures used, the percentages give us a very good picture of which elements perform better.  I think it’s safe to say that the K&N outperforms all other elements by a comfortable margin.  It also looks like the K&N filter oil performs a little better than canola.

It’s hard to say whether the other filter elements would come closer to the K&N as the flow went up.  The data seems to trend in that direction.  The spread is smaller @ 1” H2O than it is @ ¾”, but I’m almost certain the two curves will never cross.

While researching filter prices, I stumbled across another choice, the HiFlo Filtro.  It retails for $15, looks like a standard paper element, and seems to have some good reviews.  If I can get my hands on one, I will test it and post the results.
 
On 04/09/18 I posted some test results on various modifications to the airbox (see “Air Box Performance Tests”).  Those tests were performed with a simple u-tube manometer.  I intend to retest the airbox mods using the flow bench.   That way, we might be able to get some data that reflects actual flow, rather than simply a change in flow.

Hope this information is useful to you all.  Knowledge is power.

Best regards, Mike                  

Thanks Ohiomoto, but the airbox has to stay in the bike for the tests.  The seat actually forms part of the flow path into the snorkel.  I'm also finding that the decorative covers that hide the airbox make a significant difference.  More to come.

Regarding consistency of the data, I had problems with consistency because I was performing the tests using the floating pressure drop method (i.e. just closing all the bleeds and seeing which combination resulted in the lowest depression).  That method will tell you which combination offers the least resistance to flow, but the results are significantly affected by atmospheric conditions, and you can’t readily convert the results to a flow. You know its more or less, but you don’t know how much.  I’d go to recheck a combination and the result would be different.  So, as I said earlier in the post, I sat down and thought things out.  I changed my method to the standard pressure drop, where you do all the tests at a fixed pressure.  Once I started testing all combinations at the same pressure, the results were totally repeatable.  I could come back to the rig the next day under slightly different atmospheric conditions and get the same results.  My inconsistent data was the result of my method, not the filter absorbing water.  The inconsistencies were across the board, stock, poly-fil, K&N, I would get screwy data on all of the combos.

Regarding the pleats, those pleats expand the surface area of the filter media so that the area exposed to the flow of air is actually about 2-1/2 times greater than the filter frame.  So that would make the stock pleated filter about 2-1/2 times larger (in area exposed to flow) than the poly-fil inserted into the stock frame.  The opening in the frame is 4.75 x 4.31 for an area of 20.5 inches squared.  If you take the pleated media out of the frame and lay it out flat, it measures 4.75 x 17.25 for an area of 53.4 inches squared.
 
There are 14 pleats in the stock filter, and each pleat is 5/8” tall.  I used a long strip of paper 4.75” and folded it to form 14 pleats that were 5/8”.  I made sure it fit correctly in the test frame.  When stretched out and laid flat it measures 17-1/4”.  That works out to a lot more exposed surface.


Here is a shot of the paper mockup.

I’m not sure what affect humidity in the air has on the K&N.  I do know the poly-fil package talks to the fact that the polyester material doesn’t absorb water.  The K&N gauze filter is saturated with oil.  I don’t see there being a lot of room for moisture to get into the fabric with all that oil.  I do know that K&N has an excellent reputation, and until now, I’ve never heard any complaints about the hydroscopic properties of the filter media used in the K&N.

I'm looking for performance.  Some of you are looking for economy.  Some want simplicity and ease of maintenance.  This study should provide some good data to help you make an informed choice.

I bought a k & n and had to send it back because it does not fit. The top of the filter hits the backward sloping surface at the top of the airbox which pushes the filter away from where it is meant to seal.

I had numerous emails with all different people at K & N where they kept asking for more information then not replying back. FFIW the entire company are a bunch of deceptive lying scumbags. I'll post some photos if you like.

I have also discovered upon checking my old OEM filter that the pleats have a lot of sand in them even though it's just used on the road. How that sand gets in there is hard to see seeing how the intake is so protected, but you wouldn't want any of that in your engine.

I bought some special foam and the correct oil but I've not yet worked out the best way to fit it into the old gutted filter.

edit: see for yourself...

BTW, Batz, I ran a K&N in my old Harley.  I bought the bike new in 1990 and almost immediately installed an S&S Super E carb.  That carburetor kit came with a foam filter element that was pretty sketchy so again, almost immediately, I changed out that element with a K&N.  I rode that bike for 16 years, then I hung it up.  Sold it in 2015.  Still had the same K&N element.  The guy that bought it from me had me do some work on it about six months ago.  It still has the same K&N element, and it looks just fine, and there isn't a pile of dirt clinging to the mouth of the carburetor so I assume its still trapping the crud.  What can I say, that cheap cotton, you know it only lasts about 28 years.  I might also add, that in the 25 years I owned it, I don't remember ever even washing and reoiling it.  But I'm really old and tend to forget stuff like that.  Heck, I must have serviced it at least once or twice in 25 years.

Of all the money I spent on mine, I only regretted the K&N air filter.I wouldn't do that again.