Two-stroke engines used to be ubiquitous in dirt bike racing, thanks to their lightweight construction and power. Packaged in a dirt bike frame, they make a motorcycle that’s playful in corners and very quick when the throttle is wide open. Manufacturers kept 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 the widespread use of two-stroke engines in professional racing.
After Yahama’s inaugural success, other manufacturers were forced to follow suit, developing their own four-strokes. The new motocross was the Japanese marque’s response to California’s increasingly strict environmental regulations, which two-strokes could never meet. However, these futuristic bikes brought many more benefits than just running cleaner. Four-strokes provided more usable power and torque (especially lower in the rev range), which made them much easier and more forgiving to drive. As brands scrambled to develop this new technology, two-stroke development mostly lay dormant. Fortunately, the big heads at KTM’s engineering team wanted to give these old-school fuel-injected engines a shot in the arm.
Its new range of fuel-injected two-strokes aims to breathe new life into engines, making bikes easier to ride, easier to grip and kinder to the environment. Traditional two-stroke engines are much more polluting than four-stroke engines due to the inefficiency of the carburetor along with burning oil inside the cylinder head. Both strokes must run on a mixture of oil and gas to keep the engine properly lubricated. Fortunately, fuel injection addresses most of this inefficiency, making the new bikes run cleaner than their carbureted counterparts, reducing fuel consumption while expanding the traditionally microscopic powerband.
More from Popular Mechanics
That said, these Austrian two-stroke machines aren’t fuel-injected the same way your car is. Under the hood, most modern engines feature direct injection, where fuel is injected directly into the combustion chamber. KTM almost launched direct-injection bikes in 2015, but ultimately found the technology too complicated to apply to dirt bikes. “Thermal stress on components like the piston and connecting rod, together with the difficult preparation of the air-fuel mixture, is something impossible for a motocross bike,” says Florian Bretterebner, head of off-road engine development from KTM. “A direct injection bike would be heavier, less efficient and much 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 line of enduro bikes, steadily refining the technology up until the recent 2023 model year bikes . TPI bikes use fuel injectors inside the transfer port located between the crankcase and cylinder. However, newer throttle body injection (TBI) bikes use an evolution of roughly the same concept, with fuel injectors inside the throttle body instead of the transfer port. KTM says this new design allows for a much more homogeneous air-fuel mixture, which leads to better combustion.
Carburetors vs. Fuel injection
Carburetors use the vacuum created by the engine’s intake stroke to force air through their ports and passages. As the air passes, it simultaneously forces the fuel into the combustion chamber. The fuel is then fed through small passages called “jets”, which control the amount of gas flowing into the combustion chamber; most carbs have two jets, with the main jet controlling the fuel mixture at mid and high RPM and the pilot jet controlling from idle to idle. half the rev range. Although carburetors have an elementary function, adjusting them properly is a long and arduous job that only veteran mechanics have perfected.
KTM’s latest fuel-injected 125cc two-stroke engine.
“Once you have two-strokes 95 percent of the way, that last 5 percent is really what’s hard to get off,” says Joe Adragna, an amateur motocross racer who rode two-strokes in his childhood.
Although a carburetor allows you to adjust the air-fuel ratio, it is more of a set-it-and-forget-it device. Fuel injection (or throttle body injection in the case of KTM two-strokes) is continuously variable. The process is now automatically controlled by an on-board engine management system that ensures a near-perfect air-fuel ratio all the time. With more usable power and higher fuel efficiency, these engines feel much closer to a four-stroke when the throttle is opened.
“We saw the benefits on the four-stroke side when we made the carb-to-EFI switch over 14 years ago,” says Bretterebner. “Some of these advantages apply even more in a two-stroke engine, which is more sensitive in terms of air-fuel ratio.”
With fuel-injected two-strokes that work almost perfectly out of the box, riders can spend more time riding and less time fiddling with the carburetor. While getting rid of unnecessary bike work is great for weekend enthusiasts, it’s paramount for professional riders like Taylor Robert, who don’t always have immediate access to a mechanic to keep their bike in shape.
“Not having to worry about that, as a pro racer brings out a whole different element,” says Robert, who has raced every generation of KTM fuel-injected two-stroke dirt bikes since 2018.” I can just get out and go without having to sit and play with my bike.”
Fuel-injected two-strokes also give hard enduro riders like Robert huge benefits in races like 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 great thing about fuel injection is that the bike runs well all the time, no matter where you are on the course,” says Robert; just like us humans, a revving engine needs a lot more air to continue to function properly. Although this would require a complete readjustment, KTM’s fuel-injected two-strokes have an engine management system that can continuously maintain a near-perfect air-fuel ratio. “I did that race on a carbureted bike, which was barely running when I got to the top,” says Robert.
In addition to running properly at different elevations, Robert also mentioned that this fuel-injected two-stroke lives up to its promise of making the bike feel like a four-stroke. “The early TPI bikes had a flatter power curve, but they didn’t have a ton of top end,” says Robert. “For hard enduro, you want the power to come in low at RPMs where you have good traction and don’t spin the wheels.” The TPI power curve was fine for hard enduro riding, which requires quick throttle hits (and clutch control) to successfully navigate course obstacles. However, this condition could prove problematic for motocross, which involves much higher speeds and therefore more time spent on wide open throttle.
That’s where Throttle Body Injection (TBI) bikes come into their own, maintaining TPI’s linear power delivery while ditching the previously anemic top-end performance. “Now you have that raw power when you hit a fast straight or a race exit, for example,” says Robert. Of course, KTM can’t promise a holeshot every time you throw the clutch on a TBI bike, but you’ll have access to more power with the throttle wide open, 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.
Whether this technology will spark a two-stroke renaissance is unclear, but KTM remains committed to teaching these old dogs new tricks. And that’s not entirely surprising, as many 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 with a fuel-injected two-stroke. After all, the Austrian manufacturer says it will continue to develop these bikes as long as there is sufficient demand from buyers.
Matt Crisara is a native Austinite with an unbridled passion for cars and motorsports, both foreign and domestic, and as the automotive editor of Popular Mechanics, writes most of the automotive coverage in digital and print. He was previously a contributing writer for Motor1 after interning at Circuit Of The Americas F1 Track and Speed City, an Austin radio station focused on the world of motor racing. He earned a bachelor’s degree from the University of Arizona School of Journalism, where he raced mountain bikes on the varsity club team. When not working, he enjoys sim racing, FPV drones and the great outdoors.
