Motorcycle Engine Types: Which is Best?
By Alan Dowds
ENGINES: PART ONE
Most of us know what kind of engine we have in our bikes – the different characters of a single, twin, triple or four (or more!) cylinder motor are much more obvious than they might be in a car. You can even see the bloody thing – and counting the number of burny-hot pipes coming out of it will give even more clues…
But what’s best? Which motors are great and which suck? Well, like anything, they all have their strengths and weaknesses – so it’s more about which is right for the job. With that in mind – here’s part one of our guide to engine layouts, where we take a look at singles, twins and triples…
Being a single person is generally uncomplicated – and that applies to single cylinder engines as well. You can’t get any simpler than one cylinder, with a piston whizzing up and down, two or four valves up top, and a single con-rod attached to a two-bearing crankshaft. They’re cheap to run, compact, easy to make, and rule the roost when it comes to small, budget motors.
They have serious downsides when you try and get more power out of them though. Make the capacity larger, and you end up with a gigantic, heavy piston thrashing up and down a cavernous bore. Igniting the fuel/air mix gets harder, and you need a strong crankshaft and conrod to manage the forces from the big piston at high revs.
The big problem is vibration and balance though. Compared with multi-cylinder motors, a single piston moving up and down has nothing to cancel out its motion, so vibration quickly gets unmanageable. Clever balancer shafts can help – they have a rotating mass driven off the crank which can cancel out some of the vibes. But then that adds complexity, cost, friction – so it might make as much sense to add another cylinder at this point…
PROS: narrow, compact, cheap to make and run.
CONS: vibey, limited peak power
GREAT EXAMPLES! KTM’s 690 Duke engine, which actually features two balancer shafts, one in the head drive by the camshaft and one in the crankcases.
SHITTERS! The (recently discontinued) classic Royal Enfield ‘Bullet’ lump is an engine only a mother could love. Slow, (so slow), vibey, clunky and heavy, it’s a misery to ride.
Some of the most legendary engines around are V-twins – so they must be doing something right. Having two pistons moving around instantly gives you an advantage over a single, because you can cancel out the motion of one by the other. So if one is going up, the other can be moving down, and help to balance the whole affair out.
Arranging them in a V* shape helps massively, and with some fettling of the angle between the ‘V’, you can get a super-compact, powerful, smooth motor.
Having the capacity split over two cylinders brings more benefits – each piston is lighter, so you can raise the revs, and revs means power. There’s more space in the two combustion chambers for valves, so you can have bigger inlets and exhaust valves and ports, again boosting peak power.
So – more power potential, higher revs, excellent balance, and all still staying in a narrow profile like a single. What’s not to like?
*Ducati insists on calling its V-twins ‘L-twins’ which is obviously wrong, since the two arms of an ‘L’ aren’t the same length, are they? I accept a the arms of the letter ‘V” aren’t necessarily 90 degrees apart, but a 90 degree letter ‘V’ looks more like a ‘V’ than an equal-legged letter ‘L’ would look like an ‘L’ (it looks like half a square).
PROS: great balance, power potential, narrow profile.
CONS: can be long, exhaust and inlet packaging needs more work, and you need to machine two heads and barrels rather than one, increasing costs.
GREAT EXAMPLES! Ducati’s penultimate Desmoquadro big twin, the 1199 Panigale motor. The 1299 was a step too far for many…
SHITTERS! Controversial, but several of Harley-Davidson’s 45 degree V-twins have been really disappointing in terms of smoothness, peak power and economy. 70bhp from the 1,340cc Evo engine? Come on now.
Here, both cylinders are arranged next to each other, and they can go up and down together, a so-called 360 degree twin, or one can be at the top and the other either at the bottom (a 180 degree twin), or with a 270 degree crank, sort of halfway. Yamaha’s been a big proponent of the 270 degree setup, featuring on its current MT-07 engine, because it gives a more characterful ‘big-bang’ firing order.
Parallel twins need some help with balancing, so always have a balance shaft, but can make similar power to a V-twin, in a more compact, cheaper package.
PROS: good power, compact unit, only one head and block to machine so cheaper than a V.
CONS: balance needs working on.
GREAT EXAMPLES! Yamaha’s MT-07 is a great little engine, as is the long-running Kawasaki EX650 twin.
SHITTERS! Honda’s NC700 (infamously derived from the Jazz car engine) is extremely dull, though cheap to run.
At first, you might think a Boxer is just a V-twin, with a very wide (180 degree) V – but the name gives a clue to the difference. A 180 degree V would have the pistons moving left and right in unison, since they’d be on the same crankpin. But a Boxer gives each cylinder its own crankpin, so the pistons travel in and out in a mirrored fashion, like a boxer punching their fists together in front of their chest.
Boxers are balanced by the pistons moving in and out against each other, but the offset in the crank pins causes vibration, which needs balanced out.
PROS: transverse installs have the cylinders and heads sticking out each side of the bike, making cooling easy.
CONS: sticky-out cylinders affects ground clearance.
GREAT EXAMPLES! BMW’s latest 1250 Boxer engine is a cracker, with loads of power and grunt.
SHITTERS! unless you have a masochistic streak, avoid the ancient Russian BMW ripoffs…
Unusual layout, with one cylinder in front of the other. Restricted to disc-valve two-strokes really which have the intakes on the side.
PROS: narrow layout
CONS: problems with cooling the rear cylinder, four-stroke would have exhaust and inlet packaging woes.
GREAT EXAMPLES! Kawasaki’s KR250.
SHITTERS! er, Kawasaki’s KR250!
Love the low-down grunt of a big twin, but also want the screaming top end of a four? Then a triple strikes a great balance. It’s halfway between the two in terms of space for valves and high-rpm potential, so can make a great compromise.
Making a 120 degree crank is more fiddly than a 180 degree four or a V-twin, but modern manufacturing has reduced the impact of this.
It can be arranged in a transverse fashion like on the Yamaha MT-09 or ‘fore-and-aft’ like the Triumph Rocket III.
PROS: good balance of top-end and low-down power, narrower than an inline-four.
CONS: complex to make
GREAT EXAMPLES! Almost all of Triumph’s triples are very good engines, especially the mad Rocket 2.3 and the 675 Daytona lumps.
SHITTERS! The ancient Laverda 180 degree triples were super-powerful but horrendously vibey.
A very unusual layout, with only a few road bikes made using it, the V-3 has a ‘V’ with two cylinders on one leg and one on the other. Restricted to two-strokes, with Honda using it for both GP racing and road bikes.
PROS: halfway between a twin and a four
CONS: lots of effort for minimal return.
GREAT EXAMPLES! Honda’s NS400R, MVX250
SHITTERS! er, Honda’s NS400R…