On many bikes it can be done without re-jetting the carbs or remapping, other bikes not so much. You don't want to drill multiple small holes, you want one larger hole. I've drilled out the exhaust successfully on two different bikes, a Bonneville and a Thunderbird. The first was my 2013 Bonneville. It was an EFI bike, the carbs on the bike are just shells (fake) they just house the EFI. The Thunderbird had 3 carbs. The size you drill depends on the engine size, the exhaust type, and of course the sound that you want to achieve. On the Bonneville, it had reverse cone exhausts with a very small exhaust ports at the rear (see first photo). I had to drill the rear out at 2-1/4" then I drilled out the inner wall of the exhausts at 1-1/8" and then the baffles came out. You can see the size difference in the rear exhaust ports in the before and after photos of the Bonneville. It made the sound much deeper in tone and gave the bike a nice rumble.
The Thunderbird had different exhaust ends and a much larger stock opening at the ends (see photo below). So, I didn't have to drill the large opening at the end of the exhausts. All I had to do was drill a 1-1/8" diameter hole thru the center of the exhausts, as seen in the 2nd below photo. Again with great results. A much deeper tone and rumble. If I really layed on the throttle, it was loud, but normal riding, it was a much nicer deeper tone without being too loud.
Large rear opening, stock exhaust
1-1/8" hole drilled thru
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2016 Moto Guzzi Griso 1200 SE
Last edited by Zebraranger; 08-28-2016 at 10:15 PM.