Four pistons hurtling up and down at up to 15,000 times a minute. Exhaust screaming. Air intakes howling. Whatever you think of four-cylinder sports bikes, you can’t deny they do their job pretty damned well.
The in-line four is now the most popular of motorcycle engines, used in awesome sports bikes like the Yamaha R1, Suzuki GSX-R1000 and Honda FireBlade. And that’s no accident. The configuration is efficient, smooth and can be made to produce phenomenal power outputs without sacrificing reliability or rideability. It defines the road-going superbike, despite the best efforts of Ducati to oust it in favour of the V-twin. Yet it’s also a great touring engine.
Let’s look at Suzuki’s GSX-R1000. It produces 150bhp at 10,500rpm, which gives it a 176mph top speed, yet it is uncannily easy to ride. You could get off a 125 and straight on to the GSX-R. Dealing with the dynamics of riding the bike wouldn’t be a problem – though getting to grips with the phenomenal acceleration might be more tricky.
Only a few years ago, engines in high states of tune – though not even as potent as the GSX-R1000’s – would have been a nightmare to ride, and would need a rebuild after, say, 10,000 miles. But fours are now so developed that after 30,000 miles of really hard riding, including racing, there is often no significant sign of wear at all. MCN’s sister magazine Bike proved as much with a number of high-mileage stripdowns.
History of the in-line four
So how did this dominance of four-cylinder machines come about? In recent history it was undoubtedly thanks to the Japanese, but it was a Brit who started it all. He was Sir Henry Capel and he built the world’s first four-cylinder motorcycle in 1899.
Then the story gets even more unlikely, because the four-cylinder, now the unashamed hooligan’s engine, was latched on to by a Belgian firm in 1904, with the bizarre result that the nation more famous for beer and chocolate than motorcycles came to play a pivotal role in the development of the four-cylinder machine.
The FN, as it was called, was the first practical four-cylinder engine, even though the quartet of cylinders were arranged along the length of the bike, rather than across the frame as we’re used to. The first ones revved to a heady 1800rpm, which was good for about 35mph. Woah, hold back there!
FNs didn’t even have a clutch or gearbox for years, so there was plenty of room for the long engine in the frame. It was the same for the Yanks’ first four-pot, the Henderson, although that bike was so long the wheelbase was 65in (a foot longer than a typical modern bike) and the pillion sat in front of the rider. Maybe you’re beginning to see why the Japanese now dominate the bike market…
You could almost jump 40 years now before the next big step in four-cylinder bikes. Except that in 1935 a water-cooled, supercharged, double overhead camshaft four-cylinder engine, developed by Italian firm Rondine with the encouragement of short-arsed dictator Mussolini, was fitted across the frame of a Gilera race bike. It produced 60bhp at 8500rpm, which was way higher than almost any other engine had been known to rev to at the time, and propelled the little Gilera to 155mph.
In 1937, the Gilera took the world motorcycle speed record, at 170mph, and after World War Two it lapped the Isle of Man TT course at 101mph – without its supercharger.
And then the Japanese arrived. Honda started to show off its abilities in the early 1960s with 125 and 250 four-cylinder racing engines that revved high, with reliability thrown in as an unexpected bonus. The engines were impressive, but not astounding, because racing had conjured up all sorts of mad motors, like Moto Guzzi’s fantastic V8 in the late 1950s.
Anyway, we Brits weren’t taking the little Hondas too seriously, because about the biggest bikes they did were the 250s. We were busy exercising our hairy chests on manly 650 twins, thanks all the same.
But in 1968, Honda launched the CB750. It was a four-cylinder road bike, with near-perfect manners, at the same price as the antiquated British twins. Oops. The four-cylinder superbike had arrived, and it has never gone away. The 1973 Kawasaki Z1 demonstrated the four’s performance potential and the 1977 shaft-drive Yamaha XS1100 showed how smooth the configuration could be.
The real performance wars started in 1984, when Kawasaki unleashed the GPz900, which would clock more than 150mph. Suzuki retaliated with the first of a long line of GSX-Rs, and then Honda got it spot-on with the 1992 FireBlade. By that point, we were talking 123bhp and 166mph.
We know what’s happened since. The FireBlade is now up to 149bhp, but Yamaha’s 148bhp R1 eclipsed it three years ago by being lighter and feeling sharper. However, for 2001, the GSX-R1000 is the king.
Advantages and disadvantages
Want to know why so many top sports bikes are all four-cylinders? All right, here goes… The basic point of the four-cylinder engine across the frame of a motorcycle is that it’s the most efficient power-producing unit that will actually fit comfortably, without seriously compromising handling or rider comfort. Sure, you can have six cylinders, but they take up so much room that the bike becomes cumbersome.
Or you could have four cylinders arranged in a V, or in a square, but it starts to get difficult to squeeze in fuel systems and exhausts. It works for some, like the Honda VFRs, but not for lithe, compact sports bikes.
So, four cylinders in-line it is. They give an obvious route forward for the exhaust pipes, and enough room behind the bank of cylinders for the fuel systems. With noise regulations so strict now, even the airbox is a cumbersome beast, simply to reduce the sound of air being drawn in. Then there are all the electronics needed to control ignition and fuel injection if it’s fitted, as much to control emissions as to boost the power. It all has to go between the engine and the rear suspension.
The width of the engine is a problem, though. As well as the four cylinders next to each other, there also has to be room at the side for the chain that drives the camshafts at the top of the engine. Until a few years back, these engines were air-cooled, so there were fins around the edges to dissipate the heat into the airstream. Now water-cooling is favoured because it’s more efficient and predictable and, crucially, it makes for a quieter engine. But it means that water passages have to be arranged around the cylinders.
All the same, four-cylinder engines are narrower now for given capacities than they’ve ever been, mostly thanks to ditching the old pressed-in cylinder liners in the aluminium alloy engine blocks. Now the aluminium cylinder bores are electro-plated with incredibly hard wearing coatings that survive tens of thousands of miles.
Lower down in the engine, the traditional bulge around the generator has been minimised, too. Generators are smaller, but they satisfy a higher electrical demand than ever. Farther back, gear clusters are neatly stacked to minimise the size of the gearbox, and once-bulky starter motors have shrunk in size.
Why go to all this trouble to squeeze in four cylinders? Because four cylinders give lovely, smooth power, that’s why. For a given amount of power, an engine generally has to be a certain cubic capacity. Say that it needed to be 600cc. You could have one cylinder, of 600cc. Or two at 300, or three at 200. Or four at 150cc.
The single-cylinder 600 would have belting great pistons, which would have to travel a long way up and down the bore to displace 600cc. But the four-cylinder can get away with relatively tiny pistons, which will be light, and so less prone to causing vibrations. And while two are travelling upwards, two will be going down, so much of the vibration they inevitably cause will be cancelled out.
The only way these four little cylinders will achieve big power, though, is to use high revs. Big pistons in motion have plenty of energy to keep the engine turning, but their weight stops it from turning fast. The four-cylinder’s smaller pistons don’t have that energy, which is one of the reasons why four-cylinder engines don’t have such low-down pulling power (torque) as big V-twins.
They are also so smooth in operation that the exhaust and intake noises can lose a bit of character, especially compared with the distinct sound of a single or twin-cylinder thudding away. You can’t just make the exhaust louder to up the excitement factor, either, because then it would break noise regulations. Remember that the four-cylinder is already causing more mechanical noise than a single, for example, because there are more parts moving around inside.
So now manufacturers are having to direct air trumpets (from the air intakes) up at the rider to give a flavour of the noise that the bike would be making without all the silencing paraphernalia. And still four-cylinders get labelled bland sometimes.
Want some more downsides? Well, the more parts there are in an engine, the more it’s going to cost to make and the harder it will be to fix. And despite the efforts to keep four cylinders narrow, their width can still sometimes compromise ground clearance.
But when we’re talking about 140bhp from a one-litre engine, we can probably forgive the four-cylinder these shortcomings.
Can power outputs from these amazing engines increase any further without compromising reliability? The short answer is yes. Mechanical improvements may be hard to come by, but the secret of the GSX-R1000 is its fuel injection. It’s not better than carburettors as such, but it’s the only way the top-end sports bikes can now achieve higher power.
Carburettors have been in development for a century. As you’d expect, they’re pretty good now. The Yamaha R6, with its faultless power delivery, proves that perfectly. But when engines start to go above 35bhp per cylinder (in other words, 140bhp for a four-cylinder), as they have lately, carburettors can’t cope because the air they suck in travels too slowly through the carburettor at low engine speeds with wide throttle openings. Slow air won’t pull fuel into the mix, so the engine misfires.
Fuel injection squirts fuel into the air whatever the air speed, though there is a danger of it dropping out again. However, it means higher power can be achieved, and the GSX-R1000 is now using a system to speed up the air so the fuel doesn’t drop out, either.
Considering the Japanese lost a lot of ground in the 1970s and 1980s on fuel injection development because of various patents taken out by German firm Bosch and others, as well as legal agreements with the carburettor makers, there’s still a lot of scope for improved fuel injection over the next few years. That means higher power and even better rideability. As long as no-one (like the European Union or Government) intervenes.