“There’s no replacement for displacement!”
That’s the way the old engine saying goes. If you ask me, the phrase is misleading, however. Forced induction, “the bottle” (nitrous oxide), or sky-high compression are all good examples
of ways to obtain lots more power out of an engine without increasing the size of it. Even regular ol’ naturally aspirated engines can have huge differences between
two mills of similar displacement. But why?
Displacement is just the size of the piston’s work area — no more and no less. Perhaps the saying ought to be “All things being equal, an engine of greater size will make more power,”
even though that doesn’t sound nearly as good. And things are almost never equal, as I am about to explain.
Just a number
Remember high school geometry? Good. You’ll recall that πr² is the formula for figuring out the area of a circle. (That’s the footprint of a cylinder. Cylinder bore is synonymous with diameter.
Remember that d/2 yields the radius of a circle.) Once you calculate the area, you can multiply it by the swept height (the distance between the top and bottom of the piston’s travel), and
you’ve got the displacement of the cylinder. Since the cylinders in an engine are nearly always the same size as each other, multiply that number times the number of pistons jumping up
and down and voilà, you have calculated the engine’s displacement. It’s literally a calculation of space in which a piston can travel, not dissimilar from measuring your office.
But bigger engines are nastier and require better riders to tame them, right? Not necessarily, though that can be true. Displacement is just one number. Other numbers are critical in
determining how “hot” an engine is — the number of cams, the bore-to-stroke ratio, the number of cylinders and the compression ratio are all examples of numbers that can change a
little and vastly affect an engine’s power output.
Also remember that listed horsepower figures are the highest ones a powerplant produces. Peak horsepower tells nothing of an engine’s powerband — the engine’s character and tractability.
Comparing peak power numbers relative to the whole dyno chart is a little bit like comparing a picture to a video. The picture may show a 12 o’clock wheelie, which is a snapshot in time.
The video, however, may tell the story of a looped motorcycle.
Displacements are not plucked from a vacuum
Outside forces have affected the design of every engine ever developed: cost, layout, space, weight, tax and emissions guidelines, customer whim, and racing classes, to name a few.
For instance, there was a point in time when 600 cc and 1000 cc sportbikes were quite popular in America. As replicas, they followed the size restrictions for production-bike racing,
but if you weren’t going racing, the displacements weren’t necessarily desirable.
Let’s examine The Law, since I mentioned it. In Great Britain, for instance, an A1 motorcycle license limits a rider to 125 cc. Similarly, in Japan, there is a difference between the
“futsuu” and “oogata” licenses riders may obtain. The oogata license permits a rider to ride machines over 400 cc in displacement but is much more difficult and costly to obtain than
the futsuu. In both areas, machines coming in just under those displacements are far more popular than they might be if were there no governmental restrictions.
Closely related is customer preference. Air-cooled narrow-angle V-twin engines, desmodromic valvetrains, and flat engine designs, for instance, are all well loved by some riders.
Despite their shortcomings from a design or engineering standpoint, their respective fans expect them, so the compromise of a particular design is often a foregone conclusion.
Smaller bikes often use narrower engines (singles and parallel twins and V-twins) to keep weight low. Larger bikes often need engines with multiple cylinders to produce the requisite
power to move a bike package that is heavier. All of these designs, layouts, and technologies have their own characteristics, strengths, and drawbacks.
Piston size, speed, and other engine metallurgy come into play, as do vibration issues. You don’t see 1,000 cc single-cylinder engines because they quake and shake and are generally
unpleasant to ride. Sure, the engineers could add a balancer, but that adds weight, width, and complexity… why not just toss another cylinder on there instead, make the pistons smaller
and less violent, and make more power to boot? There’s lots of theory behind engine design and parts selection; books can and have been written about the topic, but it’s equal parts art
and science. Just recognize that the layouts and sizes aren’t just made-up numbers pulled like a rabbit from a hat.
Generally speaking, it is easier to make more power with more cylinders. If one has two engines of similar displacement, the engine with more cylinders will generally produce more power.
For proof of this, look to the AMA Supersport rulebook, for example. Liquid-cooled twins in the 600 class may displace up to 855cc. Liquid-cooled triples are limited to 680 cc, and liquid-
cooled four-valve four-cylinders are capped at 640 cc to keep an even playing field. As you can see, displacement is certainly not the only number of import.
Let’s examine the Kawasaki Ninja 650 (649 cc) and the Kawasaki Ninja ZX-6R (636 cc). “Logic” (conventional wisdom, in this case) would dictate that the 649 cc engine should edge out the
636, but that doesn’t happen for a few reasons. One of the big reasons for this is the difference in the engine layouts and number of cylinders. The 650 is a twin and the ZX-6R is an inline four.
The pistons in the ZX-6R are smaller than the 650’s, and the crank’s throw is shorter. This raises the speed at which the engine can rev by reducing mean piston speed (which lowers the load
placed on the crankshaft.) The engine can spin almost twice as fast, so it can spin the crankshaft almost twice as many times as the twin in the same amount of time.