WHAT YOUR ENGINE’S GUTS SAY ABOUT YOUR WINGS
There’s something visceral about piston engines. Maybe it’s the way they roar to life with a shudder and a cough, or the fact that they work not because of some modern wizardry, but because fuel goes boom in a metal tube. Whatever it is, any light aircraft pilot who’s spent a few hours behind a Continental, Lycoming, or Rotax, even a Jabiru for that matter, has probably wondered ‘what’s really going on in there?’. And I don’t mean in a general, “well, combustion happens” kind of way. I’m talking about the intimate details — the dirty stuff. The bore. The stroke. The displacement. So, if you’re not a fan of technical stuff, this is your chance to move on to the pretty pictures on another page. For those of you who are…
BORE VS. STROKE: THE ENGINE’S YIN AND YANG
Let’s kick things off with the basics: bore is the diameter of the cylinder, and stroke is the distance the piston travels up and down inside that cylinder. Think of bore as the width of your engine’s chest, and stroke as the length of its stride. The ratio between the two — unsurprisingly known as the bore-to-stroke ratio — has a massive influence on how your engine behaves.
If you’ve got a short-stroke engine with a big bore, you’ve basically got a lightweight boxer in your plane — wide, fast hands, lots of jabs. These engines tend to spin faster and make more power at higher RPMs. That’s great if you’re driving a race car, or a highrevving sport bike. Not always ideal for light aircraft where you want torque low down and a predictable, steady pull.
On the flip side, a long-stroke engine with a smaller bore is like a heavyweight wrestler — massive lung capacity and terrifying leverage. They deliver torque early, run at lower RPMs, and are often more fuel-efficient. That’s the sweet spot for aviation, especially for the kind of flying most of us do, short hops, long cruises, and easy maintenance. So, which one’s better? Well, like most things in aviation, the answer is “it depends.” But let’s get more specific.
THE GOLDEN RATIO? FINDING THE SWEET SPOT
For light aircraft engines, particularly those spinning props directly (i.e., not through reduction gearboxes), the theoretical ideal bore-to-stroke ratio tends to hover around 1:1. That means the bore and stroke are nearly equal, or the stroke is a bit longer. Most engines run greater than that.
Let’s put some real-world numbers to that:
- Lycoming O-320 (a staple of Cessnas and Pipers): Bore = 5.125 in, Stroke = 3.875 in. Ratio: ~1.32. That’s an over-square engine (shorter stroke than bore), tuned for low-end torque and steady RPM cruising. Perfect for direct-drive applications where prop speed is constrained by tip-speed limitations.
- Continental O-200 (hello, Cessna 150): Bore = 4.0625 in, Stroke = 3.875 in. Ratio: ~1.05. Nearly square, giving it a nice balance between torque and the ability to spin a bit faster without sacrificing too much low-end power.
- Rotax 912 ULS (a favourite of the ultralight and LSA crowd): Bore = 84 mm, Stroke = 61 mm. Ratio: ~1.38. That’s a very over-square design—short stroke, big bore. But here’s the kicker: the Rotax is geared at 2.43:1. It revs to 5800 RPM but thanks to a reduction gearbox, the prop spins at a gentle 2380 RPM. It’s using that high-revving, short-stroke efficiency and tempering it for prop use.
- Jabiru 2200 (homegrown Aussie engine): Bore = 97 mm, Stroke = 74 mm. Ratio: ~1.31. Again, an over-square layout. Like Rotax, it revs higher than a Lycoming or Continental, though without the same level of gearbox complexity.
These engines reflect different philosophies and different aircraft missions. Want bulletproof simplicity? Lycoming and Continental’s long-stroke, undersquare engines are your old-school, four-on-the-floor pickup trucks. Want lighter weight and higher RPMs? Rotax and Jabiru serve up newer-school, short-stroke precision.
THE DISPLACEMENT DANCE
Displacement is the engine’s lung capacity – it’s how much air/fuel mix it can suck in and blow up in a single revolution. It’s calculated as:
Displacement = π/4 × Bore² × Stroke × Number of Cylinders
So bore and stroke don’t just shape performance — they determine the very volume of your engine’s combustion. A bigger displacement generally means more torque, but also more weight and fuel burn. That’s why design compromises get tricky fast.
If you multiply all those numbers for, say the Lycoming O-320, it comes out to 319.8 cubic inches – rounded up you get (surprise, surprise) 320. Thus, the O-320.
Rotax just decided that 9 was a cool number and added the 12 to indicate 1.2 litres capacity – because, you know, metric. Ditto for Jabiru with their 2200 representing 2200 CCs or 2.2 litres.
You can’t just make an engine massive and expect it to be better. More displacement can mean more cooling needs, higher internal stresses, and bigger casings. That’s why some manufacturers go for smaller displacements but compensate with…
…TURBOS, GEARBOXES, AND OTHER NECESSARY EVILS
Let’s talk about turbocharging. You’ll find turbocharged variants in both Lycoming (TIO series) and Continental (TSIO, anyone?) offerings, along with the Rotax 914 and up. The idea is simple: cram more air into the cylinders than atmospheric pressure would normally allow, and you get more bang per stroke. That’s especially useful at altitude, where thin air kills power output faster than a political scandal. But there’s Turbos and then there’s Turbos. Some manufacturers use a turbo to “turbo-normalise”. As you climb higher, pressure and its life-giving oxygen reduces. That will reduce your power the higher you go. A turbo normalised engine can maintain the manifold pressure at sea level up to a given altitude. The Cessna T182s I flew years ago had that and were good up to maybe 20,000 feet – way beyond where a C182 would comfortably go. Rotax on the other hand, uses the turbo to get a lot more power out of a smaller powerplant.
Turbos can make a smaller, lighter engine perform like a much bigger one — especially handy for aircraft needing performance at high DA (density altitude) airports. But they bring complexity: wastegates, intercoolers, heat management, and stricter maintenance.
Then there’s gear reduction. Since props don’t like spinning faster than about 2700 RPM without turning into inefficient, noisy air blenders, you need to keep their speed down. Short-stroke, high-rev engines (like Rotax and some Jabirus) need reduction gearboxes to allow the engine to sing while keeping the prop calm.
The downside? More moving parts, more maintenance, and the occasional gear-chatter scare at idle.
A lot of aircraft engines have fuel injection. That’s what the I in an engine designation often stands for. E.g. IO540 or 912IS. It can promote more accurate fuel measurement and distribution to the cylinders at the sake of more complexity and cost.
SO… WHAT SHOULD A PILOT CARE ABOUT?
If you’re a homebuilder deciding what type of engine is best, this stuff gets real important, real fast. You’ve got to match your engine’s torque and RPM characteristics to your prop, your airframe, your budget and your mission.
If you’re flying a factory-built aircraft, it’s still useful. Understanding your engine’s bore/stroke tells you what to expect:
- Long-stroke Lycoming? Treat it like a diesel. Low RPM, predictable powerband, sip fuel and cruise.
- Short-stroke Rotax? Rev it out. Watch the gauges. Use the whole tachometer.
- Turbocharged? Respect the redlines like your life depends on it. Because, well, it kind of does.
Many of us learned to fly behind Rotax engines, which don’t require leaning. But if you switch to an engine where leaning is necessary, it’s important to get some extra training. Without understanding the correct process, you could easily damage or foul the engine.
And remember, engine design is compromise. You want more power? It’ll come at the cost of weight or complexity and extra fuel consumption. You want simple and reliable? You’ll probably give up a few ponies or gain a few kilos.
FINAL THOUGHTS FROM THE HANGAR COUCH
Wisdom, they say, is the combination of knowledge and experience. Understanding the nature of your engine is another way of understanding the capabilities and limitations of your aircraft. As such, it’s beholden to us as pilots to have at the least a broad understanding of what’s going on under the cowling. What this article highlights is the difference between the older style square bored engines and the more recent, high revving counterparts. So, hopefully we have taken another baby step on the path to wisdom. I’m torn. I love the no fuss, torque forever, of my 0-320. I also appreciate the technological approach of a Rotax and the utilitarian approach of Jabiru. We live in a broad church of aviation, so you will find all sorts of different engines out there. Figuring out which suits you will – as always – come down to mission, budget and personal preference.
