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RK-3909-1
$153.99 USD
MSRP $268.99 42.8% Off
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K&N RK-3909-1

Custom Assemblies > Intake System-Harley Davidson
  • Designed to increase horsepower & torque
  • Dyno-tuned velocity stack
  • Washable and reusable
  • Excellent air filtration
K&N's RK Series was developed to get more horsepower from your Twin Cam® Harley-Davidson® engine. Built around the legendary K&N High-Flow Air Filter, the this RK Series assembly includes an innovative powder coated billet aluminum backplate that eliminates external breathers and features a built-in, dyno-tuned velocity stack. The result is a significant airflow increase to the engine without sacrificing filtration. This RK Series assembly retains a factory appearance by utilizing the stock air cleaner cover. The RK-3910-1 comes with a larger air filter and longer spacers than the RK-3909-1.

[More Information on High-Flow Air Intake Systems for Harley-Davidson Motorcycles]

Product Specifications

Estimated Horsepower Gain6.05 HP @ 5206 RPM
Air Filter ColorRed
Air Filter Height1.625 in (41 mm)
Air Filter Inside Diameter6.188 in (157 mm)
Air Filter MaterialCotton Gauze
Air Filter Outlet ShapeNot Flanged
Air Filter ShapeRound
CARB EO NumberD-269-35
Clamp MaterialNo Clamps Required
Clamp StyleNo Clamps Required
Clamps IncludedNo Clamps Included
ColorRed
Filter MaterialCotton Gauze
Inlet Diameter2.25 in (57 mm)
Mounting Hardware IncludedYes
Neck Flange2.25 in (57 mm)
Overall Height1.688 in (43 mm)
Product Box Height4.88 in (124 mm)
Product Box Length11.75 in (298 mm)
Product Box Width9.5 in (241 mm)
Product StyleMotorcycle Intake System
Replacement FilterE-3972
Size7 Inch
Street Legal In All US StatesSee Application Link Above
StyleOval
Top Material/FinishBlack Powder Coated
Top StyleMetal
TUV Approved (Europe)No
Weight3.3 lb (1.5 kg)
Horsepower gains based on similar vehicle dynamometer test. Results will vary.

Customer Feedback

Be the first to review part RK-3909-1
Dear sirs:

All I can say is most people won't believe this but then it's their loss...in miles per gallon that is. I Own 2 Harleys, a 1979 HD Sportster that several years ago I put a K&N air filter on and felt like it doubled it's performance, and a 1999 FLSTF Fat Boy. I didn't rest there, as a year ago I put a K&N on my 2001 Dodge Ram Quad cab Cummins turbo diesel and also felt the increase in performance not to mention the scream of that turbo that I had never really heard from the factory.

Recently two of my buddies and I went on a motorcycle trip to Stecoah, N.C. Before going on the trip I changed all my fluids and serviced the whole bike. I decided to buy a K&N air filter and pitch the stock one. They were riding a 2003 Honda Goldwing, a 2005 Harley Davidson Roadking and I took my 99 Fat Boy. Prior to installing the K&N my Fat Boy got between 40 & 43 MPG. As we went from fuel stop to fuel stop my mileage was as follows: 43 MPG, 47 MPG, 46 MPG, 48 MPG and 52 MPG EVERY tank after that. My buddies only mustered 44 -46 MPG. Most people would ask what about the weight and the speed. Well both my saddle bags were packed, I had a bag with 20 lbs. on my luggage rack, plus my TBags weighed in at about 50 lbs. The weight we carried was all about the same. The motorcycle weights were different, but not enough to warrant the major difference in MPG. Also mine is carbureted and theirs are fuel injected. As far as the speeds, we were cruising on the interstate at 70 to 80 MPH through a whole fuel run which was about 200 miles. Even when I was on the tail of the dragon, which is 318 turns in 11 miles, my mileage stayed at 52 MPG. All I can say is after over 1700 miles on that trip the K&N definitely paid for itself with the increase of 9-12 MPG.

So everyone has a choice. you can pay at the pumps or spend that money on the LAST air filter you will EVER buy.

Ronnie E.
Glen Rock, Pennsylvania
K&N has a great web site. I used their vehicle search to find the correct filter for my Harley Davidson 103. I really like the nut on the end of the filter as it makes installation and removal easy, no special tools required. An overall excellent experience Fast shipping as well.

Harold Brooks
San Antonio, Texas
Noticed right away that car had more power. Increased fuel mileage almost two and a half mpg. I always replace the stock filter with a K&N. I have 2 Pickups and 3 cars and a Harley Davidson Street Glide all have K&N filters.

Terry Hagg
Wheatland, Louisianna