First...don't do it! It's too unpredictable from an engineering standpoint. Exhaust systems are designed for optimum performance across a wide range of engine conditions relative to a bunch of factors in the netherworld of physics.
A bunch of engineers (with a whole lot more smarts than I have) have evaluated these conditions and determined an optimum performance level for the average scooter stock exhaust system.
Photo courtesy of Yoshimura
Baffles are engineered to allow a certain volume of exhaust gases to pass to the muffler chamber and manage sound and back-pressure. Now, before some engineer calls me...the term back-pressure is biker jargon to explain the complexities of returning exhaust gases (reversion) back up the pipe, where, depending on their effect, can affect efficiency of exhaust flow. This all has to do with escaping pressure waves as they move into the exhaust muffler and are bounced off the walls of your baffles.
Some of these pressure waves bounce all the way back up the pipe (my word is: back-pressure) and apply a whole bunch of physics factors on things like resonance, flow efficiency, etc.
But we aren't visiting here to learn physics...we just want to know why you shouldn't start poking holes in your baffles, right?
When you start dickering around with these factors you're gonna create a problem somewhere else. Bikes of today are a lot more sophisticated with their carburetion and exhaust factors.
Most of the forum messages I've seen indicate that people are simply drilling or poking holes at the back plate of the muffler housing...please don't do this! If you have an old Cushman Eagle from the early 60's...poke all the holes ya' want, if it's a bike of today...get a grip...get a budget and get a set of engineered aftermarket exhaust. Period!
Second...once you alter the stock pipes, they are useless as "orginal equipment". Original equipment may (someday) be a selling point should you decide sell the scooter. Just ask any ol' HD rider.
Third...if you want to increase the noise or let the engine breathe easier, consider your neighbors and family first. Then consider the possible costs to adjust pilot jets or replace main jets. Anyone can just drill some holes and go on about their merry business.
Anytime you change a "tuned" condition within an internal combustion engine...you must compensate somewhere else (usually fuel/air mixture). Those are the facts! People who just go out and drill some holes, and don't take other factors into consideration, are usually people who have no understanding of the mechanics of an internal combustion engine.
Fourth...save your original OEM pipes and get a set of aftermarket pipes. If you can't afford 'em right now...get a budget and start saving for the set of your dreams. Try to find other riders in your area (via clubs or the Net) that you can ride with and behind of...to listen to pipes at idle and as a rider - behind their machine.
If you regularly ride with other riders and can't hear your own engine (20-30 ft. behind) when riding behind their bike...don't buy the pipes. You must always be able to tune-in to your engine's sound.
If the exhaust system annoys your dogs, your neighbors and (most importantly) your spouse...don't buy it! Drag pipes belong on the drag strip...not accelerating next to Aunt Bea's car on the freeway.
It may surprise some of you to know that (all other things being the same), you may actually have less horsepower available to the rear wheel with drag pipes than you would with stock pipes off-the-line. Drag pipes typically begin their efficiency at higher RPM's than most of us will ever ride at.
I agree that pipes should sound better than stock in the throaty, low sound spectrum. But, running straight pipes is offensive to most other scooter riders and cage drivers. Find a muffler system that will allow you the freedom to express your need for sound without creating an annoyance to those riding with or near you. Motorcyclists have a bad enough "rep" without you contributing to it.
Why would anyone spend (up to) $10,000 for a scooter and then take a drill to it over the issue of sound? Doesn't make sense. If you're gonna do it...do it right! Install a set of pipes designed for the sound capability you seek...don't start backyard mechanics on your new machine if you don't know what you're doing. If you have to ask someone how big or how many holes to drill...you don't know what you're doing - and chances are, neither does the person you're asking!
The aftermarket manufacturers (Cobra, Roadhouse, Jardine, etc.) have done testing on your bike and can help you rejet where necessary to compensate for the easier breathing of aftermarket systems. I get occasional email re this subject, only to find out that they've spent tons of bucks on bolt-on accessories (billet, billet, billet) and now they're balking at buying an exhaust system (which is critical to engine operation). Am I missing something here?
Submitted by Ronn Self