A Brief Introduction to Two-Stroke Dirt Bikes
In a recent article on street legal two-strokes, we talked about the reasons why you’d want to ride a two-stroke on public roads. To paint a fuller picture of why you’d want a two-stroke in the first place, let’s explore the history and functionality of the two-stroke engine and take a gander at where it’s headed next. It’s all in an effort to answer one of 2018’s most popular questions: are two-strokes a dying breed?
Why a Two-Stroke Engine Is Amazing
As you may know, there are four stages in the combustion cycle: intake, compression, power, and exhaust. In a four-stroke engine these steps are achieved as the piston completes two rotations through the cylinder. A precise balance of pressures and vacuums is maintained by using valves to meter airflow, injectors to deliver fuel, and timing chains or belts to keep everything in sync. The resulting homeostasis allows for a smooth power curve and precise control over emissions, two of the main reasons why most of today’s road-going car and motorcycle engines are four-strokes.
A conventional two-stroke engine doesn’t have any of that, yet it weighs less and makes more power.
Well, it has the same four-stage engine cycle, but it does it all in just one rotation of the piston. A two-stroke engine uses intake and exhaust ports along the cylinder wall to manage intake and exhaust gases, creating an organic exchange of pressures and vacuums without the use of valves, fuel injection, or even an accelerator pump. And where a four-stroke might use turbocharging or supercharging to increase power, a two-stroke uses sound.
Ever wonder why two-stroke dirt bikes have those bulbous exhaust pipes? That big chamber reflects the sound waves created by the combustion process back toward the engine, briefly forcing the exhaust gases back into the cylinder. At high RPMS this process acts like a supercharger, and optimal gains are achieved when those returning sound waves hit the exhaust port during the compression stroke.
That’s why people say “on the pipe” to describes a two-stroke’s optimal power band. It’s an abrupt transition that catches many inexperienced riders off-guard by producing insane amounts of instant power.
So two-stroke engines make more power using fewer parts, they weigh significantly less, and thanks to their simplicity backyard engine rebuilds are so easy a child can do it. In fact, a two-stroke engine makes nearly 30% more power than an equivalent displacement four-stroke. It’s a win-win for adrenaline junkies and thrill seekers, so why aren’t they all over the place?
If you’d like to see a visualization of two-stroke and four-stroke engines check out this video.
Why They’re Uncommon Today
Know how we’ve had electric cars for over a hundred years, yet we’re only just now seeing them on the road? It’s kind of like that.
Back in the 70s, two-stroke road bikes like the original Kawasaki H2 were making everything else look like a bicycle with wheels. That was around the time people began to care about words like “smog” and “emissions” in unprecedented ways. The fact that you could watch the pollutants exiting the tailpipe of a two-stroke vehicle didn’t sit well with the burgeoning environmental movement, and the idea of smoke visibly exiting the exhaust was thought to be directly tied to decades of thick smog in cities like Los Angeles.
While that thinking wasn’t entirely wrong, anyone in 2018 will take one look at those pictures and realize there’s no way two-strokes were responsible for all that smog: four-stroke emissions were horrid as well.
The use of valves, cams, and timing systems in four-strokes made it relatively easy to fine-tune them for better emissions. Amid increasingly strict EPA regulations of the 70s and 80s, it was more economically viable for manufacturers to invest in four-stroke engines than to spend years and millions researching how to make a two-stroke engine achieve the same result.
Major brands began choosing four-stroke engines for motorcycles and passenger cars around the world, and by the mid-90s road-going two-strokes had all but died out. Despite the higher showroom cost of four-stroke motorcycles, the (largely non-riding) public embraced this shift because those noisy smoke-belching two-strokes were the reason they couldn’t see the Hollywood sign on their way to work every morning.
Not that their 8.2-liter Cadillacs or their 5.7-liter Corvettes had anything to do with it – which somehow made less than 200 horsepower each. Talk about inefficiency… not that I’m bitter or anything.
Luckily for us dirt bikers, EPA regulations don’t apply to off-road vehicles. Major brands like Honda and Yamaha continued to embrace two-strokes well into the 90s, and the sport of Motocross did as well. Hardly any four-strokes could compete against the super light and insanely powerful two-strokes in sanctioned racing, and two-strokes were winning championships left and right.
That all came to a head in 1998, when the AMA handed down a landmark ruling that four-stroke dirt bikes up to 450cc could compete against 250cc two-strokes to even out that 30% power advantage. In the following years the pros switched over to four-stroke bikes, and since the public tends to buy whatever the pros ride, two-strokes quickly went the way of the dodo.
Except one Austrian company didn’t get the memo. (KTM)
Where Two-Strokes Are Going Next
KTM was almost singlehandedly responsible for preserving the good name of two-stroke dirt bikes through the 2000s and 2010s. As great names like Honda started abandoning the technology, KTM doubled down by offering a full range of off-road two strokes for riders of any experience level. These days over half of KTM’s annual sales are two-strokes, and that’s a great sign.
If you’ve ever talked to an old-timey rider you’ve probably been regaled by stories of how “you might think your bike is fast, but a two-stroke – now that’s fast.” I used to think those guys were out of touch with modern bikes, but they’re spot-on: two-strokes are a different beast entirely.
That brings us to 2018, and over the past few years thousands of riders have discovered the same thing. The number of new two-stroke models has increased exponentially since 2016 with names like Gas-Gas and Beta investing big bucks into this segment.
The big news in 2018 is that KTM/Husqvarna has rolled out transfer port fuel injection, a technology that will allow for precise control over air-fuel mixtures and thus emissions, potentially leading to a new era of street legal two-stroke engines.
It’s an innovation that would be over 40 years old by now had the industry focused on two-strokes the first time around. And the road bikes of today might be totally different – faster, lighter, and easier to maintain. So why are we not more excited by KTM’s innovation? Because right now two-strokes are selling better than they have in years, so there’s a lot of incentive for the industry to keep riding the carbureted cash train until it stops making deliveries. Kinda feels like déjà vu, huh?
KTM, Husky, Honda, Yamaha, Beta, you know what to do: make the two-stroke engine better than ever.
-By Justin Dake