Drilling the stock Exhaust Cans

Let me preface this page with a major qualifier - I did not run a baseline set of dyno pulls on my bike before I drilled the exhaust.  Like many other riders, it just seemed to be intuitive to me that opening up the exhaust cans would let the engine breather free'er and let it build a bit more power.  I did not get a chance to run an unmodified stock exhaust system on my engine until late in my comparison.  The end results were that drilling the exhaust cans like this let the engine sound a little perkier during accleration - but caused it to lose at least 2 HP from about 5,200 RPM through redline without gaining any Torque. 
So, this write up shows two common ways to drill the XR1200's exhaust cans, but while the bike might sound a little peppier when a rider is on the throttle, doing so actually hurt my performance.

A look at the XR1200's Exhaust Cans

Drilling like this will not give a major sound increase, and obviously does not lose any weight.  And while, the drilled stock pipes that I have seemed to match the performance improvement of many of the aftermarket pipes available for the XR1200, it wasn't until late in the game that I realized that the performance of the unmodified stock pipes was better than the drilled ones.  But again, here are common approaches to drilling. 
Figuratively, the two dimensional drawing below shows the path that the exhaust gas takes as it moves through the stock exhausts cans. The gas flows down one perforated tube to the end of the exhaust can, then passes back through packing material to the front of the exhaust can, and finally exits out the outlet tube to the rear of the bike.

Exhaust Flow Diagram

I borrowed the below pic of the baffle inside the stock cans from JoeP on the XR1200Ownersgroup forum.  He split open his exhaust cans for a more major exhaust mod.  Having seen how much even minor changes in baffle length can make on the XR1200's power output, I'm not a big fan of slicing the stock cans - but then some guys like to make changes for power, some for looks, and some for sound.  Regardless, this is a good image of the actual baffle.
I drilled my baffles in two locations.  Some riders only use one of these approaches, and the size of the holes can be changed as the rider desires.
First, I pulled off the cans and drilled about a 1/2" opening where the arrows point.  This allows some exhaust gas to bypass the full routing of the internal baffle and exit more directly through the exhaust cans.

Just the Baffle

While the above picture shows the baffle removed from the exhaust can, I did not disassemble the exhaust cans to drill the holes in the baffles.  The below shows the drilled hole and the main path of the exhaust, in a still assembled exhaust can.  This is what I mounted on the XR.

Drilled Hole

Rotating the exhaust can 180 degrees, here is a look down the main path of the exhaust baffle.  You can just see the edge of the drilled hole inside the can, at 9 o'clock at the edge of the opening.

Main Path

Next, I drilled 3/8" holes in the ends of the cans.  When I drilled the holes, I had pulled the cans off and marked the centers of the holes to mirror the exhaust outlet.  For a better cosmetic appearance, it would be better to mark the hole centers while the exhaust was still assembled, so that the holes line up.
Some people drill these holes larger.  The thought is that this bleeds off exhaust gas half way through its flow inside the exhaust cans.

Drilling the Exhaust

In reality though, all that drilling seems to do is negatively affect the harmonics of the exhaust system flow, slow down velocity, and ultimately hurt performance.  It was a neat experiment, but I would not recommend this as a performance enhancer.

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