Inline-four vs. V-Twin

[leno kim]

Inline-Four vs. V-Twin

There are lots of engine options for motorcyclists: Singles, Twins (in parallel, horizontally opposed and Vee configurations), Triples, inline-Fours and V-Fours, horizontally opposed flat-Sixes, rotaries and even jet turbines! Yet with all these options, the most popular engine types for motorcycles have been, and continue to be, V-Twins and inline-Fours. As the pinnacle of motorcycle technology has arguably swayed in favor of the 1000cc Superbike, let's look at these two engine configurations up close and do a little compare-and-contrast examinations.

V-Twin sportbikes, from the high-dollar Ducati 999S to the low-buck Suzuki SV650S, have been making a strong resurgence in recent times. Excellent midrange performance courtesy of relatively high torque values means easy access to engine power in the hands of the average rider.

Side profile of a typical dohc, inline-Four engine, in this case, the Suzuki GSX-R1000.

But what makes this so? It's not simply displacement, because in this comparison, we're only looking at one-liter engines. Instead, the reason V-Twins have relatively more torque than an inline-Four has nothing to do with what you'd expect, such as the engine's basic layout.

As CW Technical Editor Kevin Cameron explains, "For the most part, this is a function of valve area. The temptation in doing any sports or racing engine is to put into the head the largest valves that will fit. When this is done with a four-cylinder engine (250cc per cylinder in a one-liter Four), the result is more valve area per displacement than with a Twin of the same size (500cc per cylinder in a one-liter V-Twin). The result of this tactic is power concentrated at the top of the rev scale for the four-cylinder, and power concentrated in the middle for the Twin."

Why is that? It all has to do with intake-charge velocity. When the intake valves open, the downward movement of the piston creates a vacuum (unless you're running boost, but that's a story for another day) in the cylinder, drawing a fresh air/fuel mixture into the combustion chamber. Internal-combustion engines are nothing more than glorified air pumps; so because our one-liter inline-Four and V-Twin engines have the same displacement, they therefore flow a similar amount of air per crank revolution. And that means the volume of the incoming charge will be similar in either engine type.

A cylinder head from an Aprilia RSV Mille.

Because the charge volume is determined by displacement, that volume of air and fuel has to squeeze through the comparatively small carb throats/fuel-injection throttle bodies, intake runners, ports and valves of either configuration. Unlike rush-hour traffic, our orderly fuel and air molecules do this by traveling at high velocity. And because velocity increases as the cross-sectional area of a given passage decreases for a given volume, the cylinder heads of an inline-Four engine will have a naturally higher intake velocity than those of a V-Twin.

To take advantage of this fact, the engine designers strive to increase the intake velocity of the V-Twins to help them make more peak horsepower, while they try to decrease the intake velocity of inline-Fours so those engines can make more peak torque. What they end up with are two different engine platforms with very similar intake velocities.

"Every competent engine designer attempts to use the highest intake velocity he can get away with," says Cameron. "In earlier times, sharp port angles and sudden changes of section interfered with flow, so the engineers came to believe that very large ports were necessary. But as better shapes came into being, they found that smaller ports could be made to flow just as much--if not more--air as before. This has been the legacy of the recent 'flowbench' period of port development--that small, well-shaped ports can flow plenty of air and improve torque by raising the average velocity in the port."

The cylinder head from a Suzuki GSX-R1000.

We know, however, that Twins and Fours are not equal, and the reason is that there are still limitations that having more open valve area cannot solve. First, as intake velocity is increased, you begin to have a problem with the intake charge reaching supersonic speeds. A shock wave builds inside the intake tract and air begins to "back up" behind the wave. This severely impedes intake velocity and imposes the limit at which an engine can inhale. Additionally, an inline-Four has twice as many power pulses in any given time period than a V-Twin. What's more, Twins have more frictional losses in the valve train, thanks to needing two sets of slightly larger cams, two cam chains, stiffer springs pushing on larger, heavier valves moving through more lift and, of course, all the bearings necessary to support all the extra hardware.

To counteract the fact that a V-Twin only produces half as many power pulses per crank revolution, you could simply make the engine rev higher. But even if you were able to ignore the frictional losses (which increase by the square in relation to cam rotation speed--and remember, you have four cams pushing on the aforementioned bigger valves and hardware), you'd run into an even bigger issue, in fact the biggest issue: piston speed.

V-Twins require more valve gear. Here, you can see the dual timing-belt arrangement of a Ducati 999.

"What really determines how high an engine can rev is its peak piston acceleration, reached at TDC on every revolution. This is typically something like 7000 gs right now. Above that, there are problems with piston and ring reliability. Peak piston acceleration is directly proportional to stroke length, and to the square of rpm. In Superbike racing's previous 1000cc Twin/750cc four-cylinder formula, this difference worked in favor of the Twins, even though the formula had been carefully set up so that the difference in displacement would be offset by the smaller engine's shorter stroke and ability to rev higher. But in fact, the Fours could not rev as high as that formula assumed, because they reached that maximum piston acceleration sooner than thought."

From an engineering standpoint, torque alone doesn't make an engine configuration viable. That's why packaging has played such a strong role in the success of the V-Twin. Not only are V-Twins narrower, but contrary to popular belief, they offer greater flexibility in fore-aft weight distribution due to their broader polar moment. Inline-Fours, on the other hand, are wider than V-Twins, obviously, but longitudinally shorter. In a front-to-rear plane, at least, that shortness is helpful in achieving the "mass centralization" goals that most sportbike manufacturers currently ascribe to. But the width of inline-Fours means they are generally harder to flick side-to-side than their V-Twin counterparts.

The engine on the left, from an Aprilia Mille, shows how the weight is spread on a V-Twin: compact laterally, but spread out longitudinally. The Suzuki GSX-R1000 motor on the right, however, shows the exact opposite: wide laterally, but compact longitudinally.

But wait! There's more. By utilizing a shorter stroke, with smaller valves in smaller bores, Fours can rev much higher than Twins can. And because a smaller bore size means a smaller combustion chamber, the mixture is likely to burn more completely because the flame front has a shorter distance to travel; and that, in turn, allows the use of higher compression ratios. All of these factors equate to an engine that produces more peak horsepower than a Twin. No matter what sort of engine format you like, more power pulses per revolution, with more revolutions available over a given time period, equal more power.

Then there's the subjective feeling that V-Twins provide. Producing one large power cycle per crank revolution, and staging those pulses at staggered intervals, not only yields a character that can be felt and heard, it punches all the right feel-good buttons.

Does that mean inline-Fours are better? Absolutely not. As Cameron puts it, "In simple terms, the four-cylinder should slaughter the Twin in terms of power, but when can you use it? For a few seconds at the end of the straights? Meanwhile, the strong midrange of the Twin can allow its rider to get a better jump off most turns, which gives an advantage most of the way down the next straight."

What all this means is that, unless you're involved in a serious racing program, it doesn't really matter what motor layout powers your bike. Whether it's a torquey V-Twin or a high-revving inline-Four, the bottom line is that a rider's emotional preference is, as is often the case, the only real measuring stick that matters.

-- Calvin Kim