The XR1200 Intake

Air Box

There is a lot of discussion about the XR1200's airbox and intake, and whether Harley designed the airbox to be effective, or just for show.  I did a couple of simple tests using the dyno, to see whether the airbox contributes to  performance, decreases performance, or could be easily modified to improve performance.  Since my engine is cammed, ported, and modified, and since I had previously modified my airbox to use a larger air filter, this data will not be the same as that seen on a stock engine - but the observations about airbox performance should also apply to a stock XR1200.  But, it is noted that the changes seen on the dyno with my bike may be more pronounced than for a stock XR1200.

To the right is a look at the XR1200 air intake, as it sits below the right side of the fuel tank.  I have an aftermarket carbon fiber air scoop installed, with a larger air intake opening.  Behind that is an oversized air filter, which I custom-made from a larger K&N car filter. (The cosmetic tank cover is removed in this picture.)

To explore the effect of the airbox, I took the bike to the dyno and compared baseline dyno runs with the airbox, to those with the airbox removed, and then replacing the airbox with velocity stacks of different lengths.  Since intake design is a science in itself, I first used a velocity stack length calculator to see the theoretical difference that intake runner length makes.  Ultimately though, for my first tests I simplified my approach by cutting a couple of different tube lengths in the range of the calculations, and then tried them on the dyno to see whether they actually affected performance.

  • First, I took a look at removing the airbox entirely and letting the throttle body draw air without having to pass through the air filter.
  • Next, I took a look at the intake manifold length, and based on calculations from a velocity stack length calculator, tried adding a couple of  short extension tubes to see their effect.
  • And finally, I added a long tube like is often used when changing to a large cone filter or used in a bigger airbox.
For a look at the difference velocity tubes make on the XR1200, click here.
For a Summary of the effects of using intake tubes on the XR1200, click here.
For a look at the effects of individual tube lengths on TQ and HP, click here.