Ok, I wanted to ditch the Harley Davidson looking side air filters. I originally was going to go with side mounted open element K&N's but decided I would rather have the "retro" look and see the huge jugs on this engine. So under tank forum thread searching was on...
First I tried the RA-0770 filters as mentioned in other threads. I don’t know if they have changed, but the 0.5" offset is not enough to get them onto the throttle bodies without MAJOR damage to the filter due to the frame tube locations. Looking farther down K&N's list I spotted their RA-077V which is the same filter but with a chrome top AND a 2.5" offset. It doesn’t "need" the chrome since it will be hidden, but I want the offset. They show up, same 0.5" offset. So I call K&N, get all the way to an Engineer just to find out it was a mis-print. They are the same as a RA-0770 with chrome tops, they have updated their online docs now. ;-(
Ok, this project is dead without a filter that actually fits with out cramming in against everything. None of the 65-67mm ID oval filters have an offset that will work inside the 109's frame. I turned to the round filters (originally wanted oval for the surface area) and find RB-0800. 67mm ID which is perfect. Measures 4" tall by 4" in diameter, has a 5deg. tilt to it which will help, and the surface area meets K&N guidelines for airflow up to ~7800rpm. http://www.knfilters.com/filter_facts.htm I know this is not a lot over the redline, but the goal was aesthetics not performance. The "front" filter easily fits but the "rear" filter sits a bit high, so I trimmed the flange down as far as I could with it still fully seating on the throttle body flange. Success!
That was actually the easy part, the crankcase vents into the air box and there are a LOT of sensors/hoses/valves mounted to it also and now they all don't have a home. The crankcase was solved during a junkyard run. A Ford crankcase oil separator (small block V8's) on top of a trimmed Chrysler oil breather/air filter hose (60s-80s 318/360 V8's) plugged perfectly onto the back of the bikes crankcase. For a bracket to hold the rest of the items, it took me probably 20 iterations in AutoCAD, mocking it up with cardboard each time, to get the design I wanted. Luckily I have access to a water jet cutting machine at work so I could actually make such an intricate design. I was also able to salvage some 1/16" stainless sheet from a scrap machine.
In the end, the effort was worth it. Now the cylinders are beautifully on display and it sounds AWESOME. I think I will be keeping the factory exhaust now! Beauty is, now all the sensors are in place, so no error codes and all I "used" from the factory parts was the rear cylinder PAIR hose, which I bought a replacement for. Now all the parts are sitting in a box if I ever want to un-mod it.
Here is a complete list of the parts for your entertainment:
Qty Part P/N Price
(2) Air Filter RB-0800 $30/ea
(1) PAIR Valve Breather 62-1370 $18
(1) Crankcase Breather 62-1100 $18
(1) Foot of 1/2" Hose $2
(1) Rear PAIR hose 18471-48G10 $40
(1) Ford Oil Separator E5TE-6A768-AC $1 (yard)
(1) Chrysler Crankcase Hose $1 (yard)
(1) 12x16" 1/16" Thick 316 Stainless $0
Total of ~$140, pretty cheap mod too!