Stock bike tests are fine and dandy. After all, each and every one-off race machine starts life as a simple stocker before the tweaks begin and personalization and modification take over. In reality, we all could keep our bikes completely stock and roost around as happy as hippos. But we're not hippos, and we're not going to be happy until we mess with every little part of our dirt machines in search of that all-too-often-exaggerated improvement in looks, performance or longevity.In honor of the wrenching code entrenched into our dirt-riding DNA, we invited each manufacturer participating in this year's DR 24-Hour Torture Test to bring a second bike. A cool one. One that would display the gamut of goodies available to those who choose to accessorize. What we received from Honda, Yamaha and KTM will surely put a smile on your aftermarket-buying mug. In fact, when you look at them with a mod-loving frame of mind, even if you have a nearly stock wallet, you might think you're in heaven. But the all-important question remains: Are any of these beasts better than their stock brethren? It's another one of those yes-no answers, so saddle up to the parts counter and get ready for a ride.Whenever you modify something, especially a dirt bike, which is the product of countless hours of factory testing and stock-setting decision-making, you're likely stepping into a push-pull world. You can rarely get something without giving up something else. Top-end improvement is regularly worshiped at the sacrificial altar of low-end grunt. Arm-popping pull and acceleration are often given up for the ease of tractability and smooth throttle control. This is because stock equipment is designed to be very broad. Yes, each brand has its own character and is unlike any other, but, in general, the engineers at each of the factories have a range they build their bikes to suit and they usually nail it.Then the aftermarket comes in with its cowboy-shoot-'em-up types and blows all their hard work out of the water, which, of course, is fine and dandy with us and this test.We did, however, have to keep them corralled a bit, so a few rules were imposed prior to the bike submissions: One, each bike had to begin its life box-stock. Two, it had to have a final sticker price of no more than $9000 (including the base MSRP and all modifications and parts cost). Three, it had to pass a responsible 96-decibel sound limit and come equipped with an approved spark arrestor. Other than showing up for 24 hours of abuse, that's all the builders had to do before we tested the beans out the modified machines. Let's get to it.We'll begin with the Yamaha WR250F because, as a package, the White Brothers-tuned bike is by far the most extremely modified in this test. Starting from the inside out, WB teamed up with CP Pistons and Hot Cams to build a ripper of a motor. The 13.5:1 piston, combined with a set of Stage 1 YZ-F cams (intake and exhaust), definitely picked up the ponies of the blue bike. And when piped with the White Brothers E2 exhaust, the package was totally race tuned-literally. With a quick-building power, the WR really liked riding high in the rev range. It was clear that lugging it around through the bottom wasn't how it wanted to run. There, the bike wasn't jetted as cleanly and didn't pull like it did in the mid to top. We cleaned up the bottom a little with a fuel-screw adjustment, but the bike was still happier up top. It's important to note that this piston and cam setup runs great on pump gas-better on race gas but really good on the pump stuff.This bike was a grip-and-rip sort of ride, with power delivery on the abrupt end of the spectrum, for sure. For all-out racing or wide-open desert trails, this type of delivery is quite desirable and a lot of fun. The modifications aren't aggressive enough to warrant any longevity fears, and the fast-paced nature of race situations will marry well with the companies' simple motor upgrades. On tight trail sections, this type of power isn't the best. It simply goes too fast too quickly to allow your momentum to be used efficiently. You'll catch yourself overshooting corners and breaking tape more often with a motor like this if you ride a lot of twisties.