I’d prefer you didn’t ask “why,” but I follow a WR250X/R owners Facebook group for laughs. Facebook being what it is, the group is mostly the kind of characters who layer thousands of dollars in aftermarket silliness on their WR’s and whine about how their loud pipes are still not attracting the attention they deserve. Think South Park’s “rub, rub, rub, rub . . . “ in blue, white, or black with lots of emoticons. One of the kids on the list posted this picture of his near-new, grossly overpriced ($1,000) Graves “Works Exhaust System” which had burned through on a short ride from home to work. (Maybe that’s the “works” part of the exhaust?”)
Manufacturers, especially the big four, have taken exhaust systems to some pretty incredible levels over the last two decades. Stainless steel headers and mufflers, electronically variable tuned systems, and at-or-below automotive noise levels are common. In other words, factory exhaust systems do exactly what they are supposed to do and deliver the broadest powerband practical for the engine (which has been tuned, mechanically and electronically, for the factory exhaust). When I was a kid (before the moon was born and when the sun was little-tiny), I did a lot of exhaust mods to my 2-stroke bikes. I cut and welded pipes, fashioned my own expansion chambers, tacked on “silencers” to get past state park spark arrester requirements, and did a lot of seat-of-the-pants “dyno tests.” After a few years of that foolishness, mostly I learned that my “engineering” was turning a moderately peaky motor into a narrow-band engine that worked in such a tiny powerband that the bike was practically unrideable. If I could keep the bike on the pipe, which was a pretty difficult task on most of the tracks I rode, it was definitely faster (also on the rare occasion that my “tuning” worked). If it fell off of the pipe, I was sometimes stranded on the track with a fouled plug or crawling along hoping to burn off the excess fuel before the bike stalled.
About the time I decided to spend less time on exhaust system experiments and more time on jetting, blueprinting the intake and exhaust ports, and suspension improvements, the kid I was tuning for started winning. Eventually, he (we) won the Nebraska state 125 expert class (A series) motocross series two years running on Suzuki RM’s. He and I both rode RMs in our respective classes (I was never more than a B Class rider) and we rode on bone stock exhaust systems as did the Yamaha riders who ended up owning the top two classes and four out of five top places in each class two years later.
So-called dyno chart data like the stuff published by Graves for their fragile and over-priced pipe is information more aimed at the ego of the buyer than useful information. Dyno test results are notoriously unrepeatable and not created equal, “It all depends on the dyno and how it is reading and computing the power / load ratings. There are ‘inertia’ dyno's that give a particular number, and there are ‘load’ dyno's that give an entirely different number. Why do they do this you ask? If you took a typical DynoJet dyno (chassis dyno) and ran a car on it, it would give you a given number based on inertia and the spool-up speed of the drums. If you took that same car and put it on a Mustang Dyno or SuperFlow ‘load’ dyno, you would get a much lower number simply because they are measuring power under different circumstances (under a load).” An acquaintance who has owned a dyno since the 80’s and has tested and tuned hundreds, if not thousands, of bikes over his years as a master mechanic once told me that a 10-20% variance between tests is common and that any results demonstrating anything less than a 20% real improvement will mean nothing to the average expert class racer.
With that in mind, the maximum of 16% shown by the Graves’ data should pretty much be ignored as marketing babble. Unfortunately, to get real data you have to run multiple tests and average the results. “While we're at it, let me explain something about engine dynos. People say dynos don't lie. Well, they don't... but the people entering the acquisition data DO, and if they fudge the numbers here or there (barametric pressure, relative humidity, air density, air temperature, the amount of water being fed to the water brake, etc.) then the dyno can only compute what it is told to compute. So in reality, no... the dyno won't lie, but it will give bogus readings if the data isn't accurately fed into the machine.” Any information provided by anyone’s marketing department is more than likely bullshit. Keep that in mind before you toss your hard earned cash into the aftermarket toilet bowl.