Modifying the Stock Airbox

Early on I decided to modify the stock airbox to eliminate some internal obstructions and install a larger air filter.  The modification is fairly easy with a Dremel tool or grinder, but the air filter is a custom piece so this mod is not for everyone.
Plus, I did this at a point where I was testing out a 88" engine build in my frame and letting more air in just seemed to make sense.  I did not though have the time to do before and after dyno runs to characterize any performance change which this mod might produce.  So, these pictures will show what I did -  but I do not have data to show whether the mod improved performance, or what kind of improvement might have been made.

First, the Air Filter

Before anyone made a K&N style air filter for the XR1200, I made a few prototype versions.  In addition to the stock size air filters, I made an over-sized variant for the XR1200's air box.  To see the difference in the size of the air filter itself, click here.

The Airbox

I whipped out this mod quickly in between some other work, and did not think to capture "before" pictures. 
To enlarge the air filter, one simply cuts out the enclosure for the solenoid for the airbox valve used in the international models of the XR.  A Dremel tool or angle grinder will eaxily cut out the plastic bits.  If one leaves the trailing edge of the solenoid enclosure, there is an easy straight line for the larger filter.  Removing the internals not only allows for a larger filter but should improve air flow.
Once the opening is enlarged in the airbox, pieces of the removed plastic can be epoxied in place to shape an extended mounting frame for the larger filter.

Box bottom - trimmed

Box Bottom - alt view

Here is a closer look at the internal area where the air valve solenoid enclosure was removed.
Also, you can notice that I pulled out the breather tubes from the valve covers, and have covered the openings with aluminum tape.  The outside of the box also has black electrical tape covering the aluminum, to blend in the color.

Box bottom - closer look

With the airbox opening enlarged and reshaped, here is the larger filter in place and the airbox ready to be closed and installed.

Filter in place

Here is the larger filter fully installed in the modified airbox.

Filter Installed

And finally, the modified airbox reinstalled on the engine.

Airbox on bike


Intrinsically I like the idea of enlarging the air filter and removing the obstruction to the air flow caused by the mount for the air valve.  Does it actually make a difference in performance?  Sorry, I have no data to prove this.  Perhaps some day I'll get my hands on a stock airbox, and then I'll run some comparison dyno numbers...

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