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KTM is using fuel injection to revamp its two-stroke engines

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Two-stroke engines have been ubiquitous in dirt bike racing, thanks to their lightweight construction and horsepower. Packaged in a dirt bike frame, it makes the motorcycle playful in the corners and very fast when the throttle is wide open. Manufacturers kept plugging in roughly the same recipe for decades until Yamaha released its YZ400F four-stroke dirt bike, which won its first race (the 1997 Las Vegas Supercross) with relative ease. This achievement marked the beginning of the end for the widespread use of two-stroke engines in professional racing.

After Yahama’s initial 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 possibly comply with. However, these futuristic bikes brought many more advantages than just a cleaner run. Four-strokes provided more usable power and torque (especially less in the rev range), making them much easier and more forgiving to ride. With brands scrambling to develop this new technology, two-stroke development has been mostly idle. Luckily, the big forehead on the KTM engineering team wanted to give those old engines a shot in the arm with fuel injection.

Its new range of fuel-injected two-strokes aims to give engines a new lease of life, making bikes easier to ride, easier to operate and easier on the environment. Conventional two-stroke engines are more polluting than four-stroke engines due to the inefficiency of the carburetor combined with the oil burning inside the cylinder head. Two-strokes must run on a mixture of oil and gas to keep the engine properly lubricated. Fortunately, fuel injection solves most of the inefficiencies, making the new bikes run cleaner than their carburetor counterparts, reducing fuel consumption while simultaneously expanding the conventional microscopic powerband.

ktm 300 exc at Red Bull Erzbergrodeo

Marcus Berger

However, these two-stroke Austrian machines are not fuel injected the same way your car is. Under the hood, most modern engines feature direct injection, where fuel is pumped directly into the combustion chamber. KTM almost launched direct-injection bikes in 2015, but ultimately found the technology too complex to apply to dirt bikes. “Thermal stress on components like the piston and conrod along with difficult air-fuel mixture preparation is a no-go for a motocross bike,” says Florian Bretterebner, KTM’s Head of Off-Road Engine Development. “A direct-injected bike would be heavier, lower performing, and much more expensive than the system we chose.”

The system they used initially was Transfer- Port Injection (TPI), a much better alternative to the brand’s 2018 lineup of enduro bikes—continuously improving the technology through to the latest 2023 bike models. TPI bikes use fuel injectors inside the transfer port located between the canister Utility and roller. However, the latest Throttle-Body Injection (TBI) bikes use an evolution of much the same concept, with the fuel injectors inside the throttle body instead of on the transfer port. KTM says this new design allows for a more homogenous mixture of air and fuel, resulting in improved combustion.

Carburetor vs fuel injection

Carburetors use the vacuum created by the engine’s intake stroke to force air through their ports and passages. When the air flows, it simultaneously pushes the fuel into the combustion chamber. The fuel is then fed through small passages called “jets”, which control how much gas flows into the combustion chamber – most carbs have two jets, with the main jet controlling the fuel mixture at mid to high rpm and the pilot jet controlling from idle to mid-rev range. While the carburetor is basic to the job, setting it right is a long and arduous task that only seasoned mechanics have mastered.

ktms 125cc fuel injection two stroke

KTM’s newest 125cc fuel-injected two-stroke engine.


“Once you get 95 percent of the way there, the last 5 percent is what’s hard to get off,” says Joe Dragna, an amateur motocross racer who rode two-strokes during his childhood.

While a carburetor allows you to adjust the air-fuel ratio, it is 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 automatically controlled via the integrated engine management system which ensures a near perfect air-fuel ratio at all times. With more usable power and higher fuel efficiency, these engines feel much more like a four-stroke when the throttle is open.

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

With a fuel-injected two-stroke that performs nearly flawlessly, riders can spend more time riding and less time fiddling with the carburetor. While a solution to unnecessary bike work is great for weekends, it’s crucial for professional racers like Taylor Robert, who don’t always have immediate access to a mechanic to keep their bike in top shape.

“You don’t have to worry about that since the pro 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. riding without having to sit down and mess with my bike.”

And fuel-injected two-strokes pay big dividends for hard enduro riders like Robert at races like Red Bull’s Sea to Sky event, which begins on the beach at sea level and ends on a mountain at about 9,000 feet. “The great thing about fuel injection is that the bike runs well all the time no matter where you are on the track,” says Robert. Just like humans, an engine at altitude needs more air to keep running properly. While this would require a complete reset, KTM’s fuel-injected two-strokes have an engine management system that can continuously maintain an almost perfect air-fuel ratio. “I did this race on a carbureted bike, which was barely running by the time I got to the top,” says Robert.

ktm 300 exc at Red Bull Erzbergrodeo

Marcus Berger

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 much of a top end,” says Robert. “For a hard Enduro, you want the power to go down at rpm where you have good traction and you don’t spin your wheels.” The TPI power curve was fine for hard enduro riding, which requires quick hits of the throttle (and clutch control) to successfully navigate obstacles on the track. However, this condition can be problematic for motocross, which involves much higher speeds—and therefore more time spent on wide-open throttle.

This is where throttle body injection (TBI) bikes come into their own, maintaining TPI’s linear power delivery while eliminating the previously anemic over-the-top performance. “Now you have that raw power when you get to a highway or the start of a race, for example,” says Robert. Sure, KTM can’t promise to hit punctures every time you throw the clutch on a TBI bike, but you’ll have access to more power when you open the throttle wide—which will help you get there. “I don’t want to say these bikes made the sport easier, but they did help elevate the sport,” says Robert.

It is unclear if this technology will lead to a two-stroke revival, but KTM remains committed to teaching these old dogs new tricks. Which isn’t 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 Manuel Lettenbichler won a race 2022 FIM Hard Enduro World Championship On a fuel injected two-stroke engine. At the end of the day, the Austrian manufacturer says that it will continue to develop these bikes as long as there is enough buyer demand.

A shot to the head by Matt Crisara

Matt Crisara is an Austin native with an unquenchable passion for cars and motorsports, both foreign and domestic, and as the Automotive Editor for Popular MechanicsHe writes the majority of auto coverage across digital and print. He was previously a contributing writer for Motor1 after his training at Circuit Of The Americas F1 Track and Speed ​​City, an Austin radio broadcaster focusing on the world of motor racing. He received his bachelor’s degree from the University of Arizona School of Journalism, where he raced mountain bikes with the university’s club team. When he’s not working, he enjoys racing sims, FPV drones, and the great outdoors.

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