Usually, when you think of a single-cylinder engine, you don’t think of anything that spins particularly high. Afterall, when there’s only one cylinder to do all the work, it only makes sense that without help, the upper RPM ranges would be out of reach. However, these X engines in the article below, as well as a few more in the video, go against the grain and prove that the mono-piston engines are still capable of reaching (and making power at) some astounding engine speeds.
Ducati Supermono 500cc
Images of the Ducati Supermono are probably what first danced through your head when you thought “high-revving single cylinder engine.” While it’s peak power of 75 horsepower at 10,000 rpm is quite impressive, what is even more impressive is the dual connecting rod setup which helped balance the engine’s operation, and take away most of the distinct vibration caused by a single-cylinder four-stroke engine.
With a 100mm (3.937 inches) bore and a 70mm (2.756 inches) stroke, it actually displaces 549.8cc with an 11.8:1 compression ratio. Besides a four-stroke operation, the Ducati engine featured a whole hose of other impressive features, like being water-cooled, having Weber electronic fuel injection, and using a desmodromic dual overhead cam valvetrain.
This engine really doesn’t behave like what you’d expect of a single-cylinder four stroke, and that’s what makes this example so cool, especially when you consider the street version strays from rules compliance and used a better-flowing head, 104mm piston with titanium rods and a billet crankshaft, and made almost 90 horsepower — over 150 horsepower per liter.
KTM 390 Duke 373cc
While the previous engine is almost 30 years old at this point, the high-revving four-stroke single-cylinder engine is alive and well in the 20th century as evidenced by the popularity of the 10,000-plus-rpm Duke engine. With an 89mm (3.504 inches) bore and 60mm (2.362 inches) stroke, it only measures in at 373cc of displacement, but makes its peak power of 44 horsepower at 9,500 rpm.
The little engine is fairly modern with a dual overhead cam, four-valve cylinder head configuration, electronic fuel injection, and when you look at its power vs. displacement numbers, it easily surpasses the 100 horsepower/per liter mark in stock form. Once the aftermarket does its thing, both the power numbers and the RPM numbers jump considerably, with 11,000 rpm engine speeds not uncommon.
KTM RC250GP 250cc
Possibly the highest-revving and highest power-per-liter modern single cylinder engine in production, is the four-stroke KTM RC250GP. With a huge overbore ratio from an 81mm (3.189 inches) bore and tiny 48.5mm (1.910 inches) stroke, total displacement comes out to 249.9cc. With a peak power rating of 55 horsepower at 13,500rpm (with the rev limit set at an even 14,000 rpm prior to rules changes), that gives the engine a rating of 220 horsepower per liter.
This is achieved through a 14.5:1 compression ratio, chain-driven ofur-valve, dual overhead camshaft arrangement, multi-point electronic fuel injection, and a variable capacitive discharge ignition system. The technical behemoth of this list, the RC250GP engine also holds the honor of being one of the most successful powerplants of its type in competition, as well.
Additionally, the video above covers several other engines including a multi-fuel diesel engine and a few 50cc engines from the 1960s (The Suzuki RM 64 and the Honda RC110, specifically) which spun upwards of 14,000 rpm, but only made power in the single-digit range, due to the incredibly small displacement.