How Fuel Injection Transformed The Two

Two-stroke engines used to be ubiquitous in dirt bike racing, thanks to their lightweight construction and horsepower. Packed in a dirt bike frame, they make a motorcycle playful in the corners and plenty fast when the throttle's wide open. Manufacturers kept plugging away at roughly the same recipe for decades until Yamaha released its YZ400F four-stroke dirt bike, which won its first race (1997 Las Vegas Supercross) with relative ease. This achievement marked the beginning of the end for widespread use of two-stroke engines in professional racing.

Following Yahama's maiden success, other manufacturers were forced to follow suit, developing four-strokes of their own. The all-new motocrosser was the Japanese brand's response to California's increasingly stringent environmental regulations—which two-strokes could never comply with. However, these futuristic bikes brought many more advantages than just running cleaner. Four-strokes provided more usable power and torque (especially lower in the rev range), making them much easier and more forgiving to ride. As brands scrambled to develop this newfound technology, two-stroke development went mostly dormant. Thankfully the big-foreheads on the KTM engineering team wanted to give these old-school motors a shot in the arm with fuel-injection.

Its new range of fuel-injected two-strokes aims to give the engines a new lease on life, making the bikes easier to ride, easier to wrench on, and easier on the environment. Traditional two-stroke motors are much more polluting than four-strokes because of the inefficiency of the carburetor along with burning oil within the cylinder head. Two-strokes have to run on a mixture of oil and gas to keep the engine properly lubricated. Thankfully, fuel injection solves most of this inefficiency, making the new bikes run cleaner than their carbureted counterparts, lowering fuel consumption while simultaneously widening the traditionally microscopic powerband.

That said, these Austrian two-stroke machines aren't fuel-injected in the same way as your car. Under the hood, most modern engines feature direct injection, where fuel is squirted directly into the combustion chamber. KTM very nearly released direct-injection bikes in 2015, but ultimately found the technology too complicated to apply to dirt bikes. "The thermal stress on components like the piston and conrod along with the difficult air-fuel mixture preparation are a no-go for a motocross bike," says Florian Bretterebner, head of off-road engine development at KTM. "A direct-injected bike would be heavier, less performant, and a lot more expensive than the system we chose."

The system they initially went with was Transfer-Port Injection (TPI), a much better alternative for the brand's 2018 lineup of enduro bikes—steadily perfecting the technology up to the recent 2023 model-year bikes. TPI bikes use fuel injectors within the transfer port located between the crankcase and the cylinder. However, the newest Throttle-Body Injection (TBI) bikes use an evolution of roughly the same concept, with fuel injectors within the throttle body instead of the transfer port. KTM says this new design allows for a much more homogenous air-fuel mixture, which leads to improved combustion.

Carburetors use the vacuum created by the intake stroke of the engine to force air through their ports and passages. As the air flows through, it simultaneously forces fuel into the combustion chamber. Fuel is then fed through tiny passages called "jets," which control how much gas flows into the combustion chamber—most carbs feature two jets, with the main jet controlling the fuel mixture at mid to high RPMs and the pilot jet controlling from idle to the middle of the rev range. While carburetors are elementary in function, tuning them properly is a long and arduous job that only veteran mechanics have perfected.

"Once you have a two-stroke 95 percent of the way there, that last 5 percent is really what's difficult to get down," says Joe Adragna, an amateur motocross racer who rode two-strokes throughout his childhood.

While a carburetor allows you to adjust the air-fuel ratio, it's more of a set-it-and-forget-it type of device. Fuel injection (or throttle-body injection in the case of KTM's two-strokes) is continuously variable. The process is now controlled automatically via an onboard engine management system that ensures a near-perfect air-fuel ratio all of the time. With more usable power and higher fuel efficiency, these engines feel much closer to a four-stroke when twisting the throttle open.

"We saw the benefits on the four-stroke side when we made the switch from carb to EFI more than 14 years ago," says Bretterebner. "Some of these benefits apply even more on a two-stroke engine, which is more sensitive in terms of air-fuel ratio."

With fuel-injected two-strokes running almost flawlessly out of the box, riders can spend more time riding and less time tinkering with the carburetor. While solving needless bike work is great for weekend tinkerers, it's paramount for professional racers like Taylor Robert, who don't always have immediate access to a mechanic to keep their bike in tip-top shape.

"Not having to worry about that as a professional racer takes a whole different element out of it," says Robert, who has competed on every generation of KTM's fuel-injected two-stroke dirt bikes since 2018. "I can just go out and ride without having to sit and tinker with my bike."

Fuel-injected two-strokes also pay big dividends for hard enduro riders like Robert in races such as Red Bull's Sea to Sky event, which starts on the beach at sea level and ends on a mountain at nearly 9,000 feet. "The cool thing about fuel injection is that the bike runs well the whole time no matter where you are on the course," says Robert; just like us humans, an engine at elevation needs much more air to continue running properly. While this would require a complete re-tune, KTM's fuel-injected two-strokes have an engine management system that can continuously maintain a nearly perfect air-fuel ratio. "I’ve done that race on a carbureted bike, which was barely running by the time I got to the top," says Robert.

Along with running properly at different elevations, Robert also mentioned that these fuel-injected two-strokes deliver on their promise of making the bike ride like a four-stroke. "The early TPI bikes had a flatter power curve, but didn't have a ton of top end," says Robert. "For hard enduro, you want the power to hit low in the RPMs where you have good traction and you’re not spinning your wheels." The TPI power curve was fine for hard enduro riding, which requires quick hits of throttle (and clutch control) to successfully navigate the obstacles on course. However, this condition could prove problematic for motocross, which involves much higher speeds—and therefore more time spent at wide-open throttle.

That's where the throttle-body-injection (TBI) bikes come into their own, keeping the linear power delivery of TPI while eliminating the previously anemic top-end performance. "You now have that raw power when you get to a fast straightaway or a race start, for example," says Robert. Sure, KTM can't promise a holeshot every time you dump the clutch on a TBI bike, but you’ll have access to more power at wide-open throttle—which will help you get there. "I don't want to say these bikes have made the sport easier, but they’ve helped elevate the sport," says Robert.

It's unclear whether this technology will trigger a two-stroke revival, but KTM remains committed to teaching these old dogs new tricks. And that's not entirely surprising, as quite a few casual riders and even professional racers still prefer the old-school feel and sound of these engines. This is especially evident in enduro racing, where KTM's own Manuel Lettenbichler won the 2022 FIM Hard Enduro World Championship on a fuel-injected two-stroke. At the end of the day, the Austrian manufacturer says it will keep developing these bikes as long as there is sufficient demand from buyers.

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