Bore, Stroke, and Engine Performance

Torque, revs, cooling, breathing: Bore and stroke influence it all.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World.Robert Martin

During my time spent with Kawasaki’s H1R two-stroke triple in 1971, I made the acquaintance of the late Larry Worrell, better known as “Mississippi Round Man.” He was a respected and experienced engine specialist, two-stroke or four. Some years later I again encountered him again after Honda’s 1980–1982 thrust into the air-cooled Superbike fray. He described three 1,000cc engines they had built and tested at Honda, based on the three crankshafts available to them, each with a different stroke. What fascinated me was that the engine with the smallest bore required the least ignition timing for best torque, the next-largest needed more, and the largest-bore needed the most.

Flame Speed Is Determined by Turbulence

Some would explain these differences by saying that the larger the bore, the longer it takes for the flame to cover the distance from the spark plug to the cylinder wall. But flame speed is not a constant: The vigor of the fuel-air charge turbulence in the cylinder is the determining factor. When a correct fuel/air mixture is perfectly still, the flame speed is inches per second—too slow for any useful engine. What actually spreads the flame is turbulence in the cylinder—energy left over from the high-speed intake process—and the greater the turbulence, the faster the flame travel.

This brings up another question: How fast does this initial charge motion (turbulence) decay in combustion chambers of differing proportions? As we make the bore bigger, the combustion chamber’s shape changes from that of a hockey puck in a small-bore/long-stroke engine to something more like a pancake in engines with larger bores and shorter strokes. Air motion decays more slowly in the hockey puck and more rapidly in the pancake. This rapid decay causes flame speed to slow down in wide, vertically thin chambers, so they require earlier spark timing.

Turbulence of the air-fuel charge in a cylinder is the main determinator of flame speed during combustion. A compromise between bore and stroke leads to the best performance.Suzuki

Who cares how much ignition timing an engine needs? The engine does. The longer it takes for the charge to burn, the longer the piston crown and cylinder head are exposed to flame temperature, and the more heat is lost to them from the combustion gas. The result is some loss of power.

The Trend Toward Bigger Bores and Shorter Strokes

But hasn’t the long-running trend been away from long-stroke/small-bore engines to higher-revving short-stroke/big-bore designs? In general, yes it has. That trend exists for two major reasons:

  1. The shorter the stroke, the lower the piston acceleration at a given rpm, allowing shorter-stroke engines to generally operate at higher revs, and therefore produce more power.
  2. Higher engine speeds require more intake- and exhaust-valve area to fill and empty the cylinder in the shorter time available. (See this related story on valve curtain area.) Since the valves are in the cylinder head, if they need to be bigger we may have to increase the bore to make room for them.

The Compromise Between Acceleration and Top-End

Now I’m confused. One minute you’re saying smaller bores are better because they burn more quickly, and the next minute you’re saying the opposite: that we need a short stroke to lower stress on reciprocating parts at higher rpm, and a bigger bore to accommodate bigger valves. Which is it?

“It” is about compromise, not an either/or choice. Here’s an example. When Yamaha had a romance with its five-valve-per-cylinder design (resulting in its “Genesis” series), it built the FZR750. Tuners went nuts with that engine. Everyone wants maximum acceleration, and the main controlling variable for acceleration is compression ratio. So when the tuners gave their modified FZRs a bunch of compression, they indeed accelerated hard. But because the slow combustion in the resultant “pancake” combustion chambers couldn’t keep up with rpm, the builders had to advance the ignition a bunch—like 50 to 60 degrees BTDC. This meant that as the bike accelerated, it ran out of breath on top—the extra heat loss from that early ignition timing was killing top-end performance.

“It” is about compromise, not an either/or choice.

OK, now I get it: We have to open up the combustion chambers and leave room for the rapid charge motion that translates to higher flame speed and shorter ignition timing. So those Yamaha tuners did, and while the resulting engines provided socko top-end, the lowered compression ratios meant they had squirrel-hormone acceleration.

Yamaha wasn’t the only manufacturer to step into this bear trap. In 1988 Suzuki decided its GSX-R750 needed a performance upgrade, so they shortened its 48.7mm stroke and increased its 70mm bore to 73 x 44.7mm. The resulting engine was such a disappointment that two years later they went back to the original 70 x 48.7mm layout. Kawasaki did something similar but stuck with its bigger bore and shorter stroke of 71 x 47.3mm, taking two years to equal the performance of the long-stroke engine it replaced.

Breaking the Compromise

If you read about an updated motorcycle, you’ll often come across language like, “combustion chambers have been reshaped.” Those five words often poorly summarize big, expensive R&D programs which use sophisticated research tools such as laser Doppler velocimetry to boost combustion-chamber turbulence through step-by-step shape changes. Back in the 1920s, when Englishman Harry Ricardo was first researching combustion and turbulence, he took a tiny anemometer (those things with whirling cups that weather stations and drag-race teams use to measure wind velocity) and actually placed it inside an engine’s cylinder to measure how fast inrushing fresh charge was whirling. Many years ago I was shown a similar rig at Ducati.

Laser Doppler velocimetry sounds complicated, but its idea is simple: mix tiny flakes of something into the air stream, then bounce laser light off of them to measure how fast they are going.

Smaller Pistons Are Easier to Cool

In the early days of motorcycling, small bores were the usual choice. Why? The bigger the bore, the harder it is to cool the piston. This is because the main heat path from the piston crown runs radially out to the cooler cylinder wall, and the bigger the bore, the more distant the wall. Not only can an overheated piston cause an engine to knock or ping, but overheating also weakens the piston itself, causing possible dome sag or even the dreaded “hole-in-one.”

The dreaded “hole-in-one.”Mark Lindemann

Since pistons were fragile, builders wisely shied away from trouble. Only very gradually through the 1930s did bores cautiously grow bigger, made possible by replacing hot-running iron cylinders and heads with aluminum counterparts having much higher heat conductivity.

The first two-stroke to win an FIM roadracing championship, Suzuki’s RM62 in 1962, used a 50cc engine with a tiny 40mm bore. That small bore size made it easier to cool its hard-working piston. Only gradually were two-strokes of larger bores made to cool well enough to win world titles.

Does a Long Stroke Really Increase Torque?

One of the deathless engine myths states that long-stroke engines make big torque. The degree of truth here comes from the heat-loss argument given above, but do consider the geometry. As we make the bore bigger and stroke shorter, yes, we are shortening the “lever” by which pressure on the piston translates to torque at the crankshaft. But (keeping cylinder displacement constant) the larger bore allows combustion pressure to act on a larger area, and consequently exert a larger force on that shorter crank arm. Do the arithmetic and you will find that the two exactly cancel—as the crank arm is made shorter, the piston area grows in exact proportion, resulting in zero change in torque.

What really persuaded early builders to associate torque with small bore/long stroke is that small-bore engines have smaller valves, and intake air reaches best cylinder-filling velocity at a lower rpm.

Is There a Heat Limit to Cylinder Displacement?

In thinking about piston cooling, the old squared/cubed relationship rears its ugly head. As we increase a cylinder’s displacement, we are increasing the heat being released by the cube of dimension, but we are increasing the piston’s area of contact with the bore (through which cooling takes place) only as the square of dimension. That suggests that as we make cylinders bigger and bigger, we may get to a point at which heat release outruns heat transfer—the recipe for chronic piston overheating. Pratt & Whitney, known for its air-cooled aircraft radials, kept making bigger and bigger nine-cylinder engines, with correspondingly larger bores. It hit chronic piston trouble at a bore of 159mm (6.25 inches), so the company limited its later designs to 146mm and instead increased power by adding more cylinders.

The way out of this compromise is to stop relying on piston-to-bore contact alone for cooling, and to add piston-cooling oil jets located in the crankcase, aimed up at the undersides of the piston crowns.

Why Modern Auto Engines Have Longish Strokes

Through the 1960s, the trend toward higher rpm and shorter strokes affected car and motorcycle engines alike. But in recent years, car engines have moved the other way—toward equal bore and stroke dimensions, or “undersquare” designs with strokes a bit greater than bores. Why?

In the present era, reducing greenhouse-gas emissions (in this case, the carbon dioxide that along with water makes up most of IC engine’s exhaust) requires improving fuel economy. That calls for proportioning cylinders to have the least practicable combustion-chamber surface area through which heat loss from hot combustion gas can take place. In other words, smaller bores.

Heat loss is only one concern; another is the total length of piston-ring crevice, which also diminishes as bore size decreases. Piston-ring crevice volume is the space between the top piston ring land and the bore, and above and behind the top piston ring. At peak combustion pressure, a small but significant amount of unburned mixture is forced into this crevice. As cylinder pressure falls, that unburned mixture expands into the cylinder to become unburned hydrocarbon (UHC) emissions in the exhaust. The need to lower UHC emissions therefore exerts pressure on designers to reduce bore diameter.

Will a mad rush to electric soon sweep all of this away? Consider the following: Owners currently replace their vehicles at about 6 percent per year. The internal combustion engine appears to be with us for a while longer.

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