There are issues with life in the 250 class, especially if you’re a western off-road rider. Unlike in the east, where horsepower is rarely a deciding factor, southwestern races almost always have horsepower-hungry sections. It might be deep sand, a long start run or a steep hill, but you can usually count on something that puts a 250 at a disadvantage to 450s in the overall results. And whether you’re in an official 250 class or not, everyone looks at the overall results.
That’s why we built this bike. The 2015 KTM 250XC-F was already the fastest bike in the 250 class, but with the  introduction of the 2016 version, we felt the need to go a step further. We weren’t talking about a big-bore kit that would make the bike an outlaw; we were talking about turning to the people who deal in Supercross-level horsepower.
Most 450s make around 50 horsepower. We’ve heard that a 250 can reach that too. The boys at Rocket Exhaust build motors in the Supercross world for guys like Kyle Cunningham and Michael Leib. They said that they could at least approach that figure with a KTM motor that would be torquey enough to race off-road. It sounded great to us, but there was a catch. Such a motor would have to run on race fuel—very expensive race fuel. That might be fine for a race to two, but it would get old quickly. The alternative was to make a convertible. It’s possible to build a motor that reaches 95 percent of that level with race fuel, but then you can flip a switch that enables it to run pump gas. The KTM is a great bike for this, because most tuners agree that it has the highest-quality components, so the final result should be reliable.
We started with our 2015 test bike, which had about 38 hours on the motor. The list of what we didn’t need to replace was surprising. We actually reused the original piston after getting fresh rings, and we used the stock cams and valves. The cylinder and head were modified to increase compression, and we did a fair amount of head work. Rocket installed its carbon/stainless exhaust system, which comes with two different tips for different noise regulations. The real work was on the digital front. Rocket imports Vortex ignitions from Australia, which gives them huge latitude in mapping the fuel delivery and spark advance—far more than with the KTM software.
Most of the mapping was tested and developed on the dyno. In our experience, this usually doesn’t end well. We’ve ridden many bikes over the years that looked great on the dyno but were terrible in the real world. As it turns out, no one rides a bike with the throttle all the way open, which is the way most dyno runs are performed. Fortunately, Josh at Rocket has developed a consistent methodology for doing partial-throttle dyno runs. It turns out that at a quarter throttle, a properly mapped motor can often produce more power than at wide open, particularly in the low-rpm ranges. After days of testing, it’s possible to develop a very complicated map that has optimum spark and fuel at each throttle opening. It takes longer to develop an ignition map than it does to port a head or build a motor.
The Vortex ignition allows you to select from 10 different maps. You can switch between any two of these with a handlebar switch. We had all 10 curves customized for different conditions. Nine of the settings were developed for VP MR Pro-6 race fuel; one was for pump gas. That last one will become very important to us later, because we like to ride a lot, and MR Pro 6 sells for $40 a gallon.
RocketpipewebOTHER PARTS
We finished off the motor with a Rekluse Core manual clutch and CV4 radiator hoses for greater durability. As for the rest of the bike, it was pretty much what you would expect for an off-road KTM. The biggest issue, as we’ve pointed out so many times, is the suspension. We went to high-desert, crusty-old-guy Earl Schuler at ESR for that. Over the years he’s been the go-to guy in desert racing for riders like Ty Davis and Paul Krause. He has his own way of approaching the WP4CS fork that involves rebuilding the compression stack, opening up the cartridge and giving the fork a low-speed compression damping adjuster, which it lacks in stock form. The shock isn’t redesigned to the same degree, but frankly it doesn’t need as much. What we like best about Earl is that he’ll work at the suspension until we’re happy. Other suspension tuners just argue with us about what we should like.
We used FasterUSA wheels with Dunlop MX52 tires and a Sunstar sprocket. Cycra plastic, a TM Designs chain slider and a TM Designs chainguide rounded out the package. We kept the stock handguards, handlebar and gas tank.
RocketKTMjumpwebHOW FAST IS IT?
Crazy fast. Even guys who normally ride 450s walked away in amazement. It was the rev ceiling that really blew their minds. The KTM revs just as high as stock, which is higher than any other motocross bike sold to the public, and much higher than any over-bore kit would allow. And, the power is just awesome. To think that this is just 95 percent of what’s possible is somewhat humbling.
KTM 250 veterans already might be accustomed to high revs and good peak power, but what really stands out on this bike is the throttle response. It jumps forward at the first crack of the throttle. A KTM 350 is sort of like that, but the 250 revs much more freely and has more of a let’s-go-now feeling. When you use the clutch exiting a turn, the motor responds more willingly than a 350 or 450. As far as dyno numbers, we came pretty close to meeting our 50-horsepower goal. The bike produced around 48 horsepower. People often point out that numbers can’t be compared from one dyno to another, but Dynojet’s current software is much more consistent than it was in the old days. And for reference, a stock Honda CRF250R makes 38 horsepower on the exact same dyno. Wow.
RocketKTMlengwebWhen we switched to the pump-gas setting on the Vortex CPU, we were pleased with the way it ran. On top, it was still very, very fast. The biggest loss was throttle response down low. It no longer had such crisp willingness to run, although it was still better than a stock KTM.
We’re not especially worried about the engine’s longevity, either. It has a stock piston, not something that was lightened to the point of frailness. It has the stock cam lift and duration, although the timing was altered slightly. The only real concern is funneling this much more power through the stock gearbox, but the same gearbox is used for the KTM 350XC, so how risky can that be? So for now, we’ll run it as hard as we want until we run out of gas. After that, we’ll ride it some more. Is it worth $40 a gallon? Maybe. It’s kinda hard to go back to bean dip once you’re used to caviar.


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