“There’s no replacement for displacement!”
That’s the way the old 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.
The 636 also sports literally twice as many fuel injectors and valves as the 650. This means in addition to the higher engine speed, the 636 can also pack more air and fuel into each cylinder, enabling the higher engine speed. Because the pistons are smaller, the combustion chamber is also much smaller than that of the 650, meaning the flame has less distance to travel to fully ignite the air and fuel mixture. Effectively, the valve-to-bore ratio is increased, which means better breathing, which help sustain those higher engine speeds.
When we talk power, the ZX-6R makes about 115 horsepower at 13,500 rpm. The 650? Despite its increased displacement, it makes 65 horsepower at 9,000 rpm. Those are peak numbers, though — just a snapshot, remember? Let’s look at some dyno charts. (These are rough plots. I didn't strap down a bunch of Ninjas to an in-house dyno. Lance still won't buy me one.)
Note here the tradeoff of peak horsepower numbers. Power on the 636 builds very progressively, whereas the 650 is about as linear as one could hope for, and the torque relative to horsepower is much greater. The general takeaway here is that engines that make large power figures for their size often make it at rpm levels that can be difficult to access and even harder to consistently stay in. The 636 is making more power because it's spinning the engine almost twice as fast.
So which powerband is better?
Like many of the topics WTATWTA delves into, there’s not a right answer here. However, some people will find particular setups more suited to a particular task. The high-revving 636 engine? It produces blistering power for its size and weight, and the ride could be considered a bit violent. If you’ve ever experienced the phenomenon of turbo lag, riding a race replica offers a similar experience. Nothing, nothing, nothing... Pow holy-moly this thing is a rocket!
While the Zed six-three-six still makes more torque than the 650, the torque is a much lower figure relative to total power output and the engine has to be spun up very high to tap into the area where the big numbers happen. All of these factors have a lot to do with why the ZX-6R is such a success on the track. On the street, however, it’s difficult to remain within the boundaries of the law and really spin the engine out too much beyond the first few gears.
On the other hand, the 650, while lacking in peak numbers, produces power much lower in the rev range, and the power is more usable in a street scenario — running the engine all the way to redline in several gears is much more do-able. A better chunk of the bike’s power is accessible at lower engine speeds. You don’t get as much peak power, but a rider can tap into the power much faster.
Riding style plays a role here, too. A big engine can cover a lot of sins. It’s forgiving of being ridden in the wrong gear, and it makes a less skilled rider faster by being able to make up time in the (easier-to-manage) straight sections. A smaller engine may require more careful shifting and line selection to maintain speed, which can be a fun challenge for some and frustrating for others. Do you like to play windshield wiper with the tach or would you prefer to amble along? Do you enjoy the challenge of keeping a high-strung engine “on the boil” or would you prefer to let your mind wander and watch the scenery go by?
Engine displacement is but a single determinant of an engine’s temperament. If you still don’t believe me, ride an unfamiliar bike, and see if you can determine the displacement just from its behavior. It’s a little harder than you might think.