What Makes a V4 Engine So Special?

Engine configuration is a compromise, and for a motorcycle it’s a vital one considering its interaction with the rest of the machine – both in terms of package and dynamically. However, there’s clearly more than one way to do it since there are a wide variety of possible motorcycle/engine configurations that work well, with all of their creators claiming that they are the panacea. So let’s think about the advantages and disadvantages of some…

Width is probably the most important dimension regarding how a motorcycle feels, especially for the novice rider. Simple, then: let’s build a single or a V-twin. While this is good for width at the top of the engine, the gearbox is probably the widest part – getting the gears and the clutch on a single shaft dictates this. So a two-cylinder bank probably gives the minimum practical width. And a V-configuration is nearly always more expensive to manufacture than an inline one for any given number of cylinders, too.

Of course wel all like performance, and to generate it more cylinders always helps. This is because for the same capacity and bore/stroke ratio smaller cylinders give a shorter stroke, and this, within mechanical limits, gives the ability to rev. The Honda VFR800F engine has the same bore and stroke of the original 800 from 1998, which shared its bore and architecture with the Honda RVF759R (RC45) superbike, so it can clearly make power.

A shorter stroke equals shorter engine height, so one can make a case for a V4 over a parallel twin, if the installed length can be managed, because for a good-handling motorcycle you need a short wheelbase and a long rear swingarm.

This is where it gets challenging for any V-engine. Stacking the gearbox shafts helps, but it can’t benefit a V-engine as much as an inline one because the rear bank usually leans backwards. Honda have previously employed clever packaging here, using side-mounted radiators to move the engine forwards. Simultaneously the lack of engine width allows the radiators to hang under the frame spars in front of the rider so that compactness is not compromised… and then for the newer designs the radiators return to the front, making it slimmer, without any alteration to the frame or geometry.

So, thinking about the V4 engine in comparison to an inline twin, triple or four one could expect that for the same capacity the V4 would make good power and be equal narrowest. One might expect the others to be lighter but that’s not necessarily the case, because the 90 degree V4 doesn’t need balancers and its crankshaft is short. Plus there’s an aspect that the V4 shares with race bikes – it has minimal inertia torque. This is because the cylinders aren’t all at the dead centers simultaneously; with the uneven exhaust note it all helps with the feel-good factor.

Of course, the Honda VFR also has VTEC as part of its carry-over technology, which, contrary to what’s been written in numerous road tests, is not inherently a bad thing. Variable valve timing gives an engine many personalities, and cutting one of the two valves out on the intake side undoubtedly does good things for gas velocities, mixing and combustion at low speed so that torque can be boosted and fuel consumption improved. The key to smoothing the switch-over point will be more computing power; and as the new motorcycles are a lot smoother, with noise the only real indication, it’s something they’ve cracked. Using variable intake length would be a very good thing with VVT – one for the facelift?

However, it’s not all a bed of roses for the V4 engine. Exhaust routing is problematic and it’s almost bound to be expensive due to the extra cylinder head and timing drive. But MotoGP shows it can be one of the dominant layouts and it does have some real packaging and dynamic advantages there that translate to the road. And, perhaps most importantly,V4s have real character and sound brilliant.

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Tuesday, 06 December 2016 @ 03:21 AM ICT
It is not a rule that inline-4 engines must have a flat crank (which means that all pistons come to momentary stop at once). Check out Yamaha's crossplane 4's as used in 2009+ R1 and new MT-10. In the same manner, a V4 could be designed so that all cylinder reach TDC and/or BDC at the same time (just a 90-degree crank phasing away). Each design has its pros and cons, but perpetuating old stereotypes will not allow full analysis of said characteristics