Of course, performance is always a prime consideration, but trimming weight was also a big issue for the 250 SX-F & 450 SX-F powerplants. In both cases they came up with engines that were more compact, lighter, and offered higher performance. Above you can see an example of the completed 250 engine (left), and artwork on the right that shows both the form factor of the new engine, and a ghosted overlay of the old engine. How did they make it more compact? The position of the crankshaft was raised 6mm, and it uses a 6mm shorter connecting rod. The clutch shaft position moved 11.1mm backward and 26.9mm up. That results in an engine that's 20mm shorter overall, and provides for centralized mass for improved handling.

Both engines have a redesigned Keihin Engine Management System with electronic fuel injection, and an all-new 44mm throttle body with a direct-connect design that eliminates internal linkage, for a more spontaneous response. It also has a new injector position, and separate systems for cold start and idle adjustment.

The Keihin Engine Management System also features a new lightweight ECU with faster processing that also includes a Launch Control system for more efficient starts. It's activated by a switch integrated in the mounting clamp of the front master cylinder, and it reduces the maximum engine RPM during starts. At idle speed, the rider switches between map one and two (or vice versa) to set the launch control map, and an EFI control lamp mounted on the upper triple clamp indicates when that it's active. The Map Select Switch also provides access to three available maps (soft, standard or aggressive).

There's also a new gear position sensor that allows for different engine maps based on the gear position, and the engine map that's selected. There's a KTM PowerPart User Setting Tool that can provide engine data with a laptop, and change the ignition and injection mapping with a few mouse clicks.

What else is new in the 250 engine? There's improved combustion chamber geometry, new cams that are 150 grams lighter, with new timing and a friction-optimized super-fine surface. The new finger followers have a 30% harder DLC coating and a reworked timing drive (friction reduced and shorter DID timing chain, new sprockets and chain guides). There are new valve springs and retainers for the four lightweight titanium valves (32.5mm intake, and 26.5mm exhaust), and flow optimized ports for maximum performance up to 14,000 rpm. You may have also noticed the new lateral engine brackets that are said to improved handling with less vibration.

KTM also reworked the cylinder to shorten it, and the engine now uses a new lightweight forged bridged box-type piston by CP that provides a 14.4:1 compression ratio, and is designed to work in conjunction with the new cylinder head combustion chamber.

There's also a new crank features a stiffer and shorter connecting rod, a plain big end bearing that features two force-fitted bearing shells run directly on the crank pin that KTM says assists with the maximum power, but also enhances the engine’s durability, allowing long crankshaft service intervals of 100 hours.

An all-new five-speed transmission features wider gears with a new surface treatment on the 2nd, 3rd and 4th gears for improved reliability in extreme conditions, while the shift mechanism has been reworked with a special low-friction coating on the shift forks to provide precise and easy shifting. An advanced gear lever design prevents dirt from blocking the folding shift lever tip.

A new CSS (coil spring steel) one-piece clutch with seven clutch plates (reduced from eight) and reworked inner hub and pressure plate provide better separation, improved oil supply for better cooling, reliability, and lighter weight. There's also a new lighter weight clutch basket design, new clutch springs that provide more consistent behavior under stress and over time, and a reworked Brembo hydraulic system with a smaller master cylinder for a lighter pull and controllable modulation.

KTM's electric start on the 250 features a new starter motor made by Mitsuba provides more efficient and reliable starting, and it's also lighter than before.

The starter is matched with a new super-compact and lightweight 1.1lb (495g) Lithium-Ion starter battery, which is said to provide reliable starts, and also reduce the weight by 2.2lbs!

KTM has also simplified the wiring harness to reduce weight, and improve maintenance, by centralizing the battery in a compartment above the airbox, which allows mechanics to remove the subframe without compromising the electronics.

The new design of the engine covers features an advanced surface design that provides reduced surface wear caused by the rider’s boots.

New WP flow optimized radiator design is said to feature a 10% boost in engine cooling due to the design of the radiators themselves, but also a new radiator guard design that improves air flow and also provides structural rigidity to the system.

For the exhaust, a new header pipe with resonator is designed to increase power with low noise levels, and the new silencer is 40mm shorter than the previous design for increased low end and quick throttle response. The shorter design and new mounting point allows the silencer to move 60mm closer to the center of the motorcycle.

Overall, the 250 engine is said to be 2.4lbs lighter than before.