It would have been interesting to be a fly-on-the-wall in the meeting when Harley-Davidson's chiefs sat around to debate the future of their motive power for their flagship ranges. How do you improve on the Evo without killing the golden goose. Why improve on the Evo at all?

To see the full picture you've got to remember that the Evo was brought into production in 1984 and has been fiddled with since but without changing the fundamental engine. It is also important to consider that you can buy what is ostensibly an Evo engine from a variety of manufacturers with a massive array of different features – some of which are held to be better than the original.

So what to do?

We're all aware of the rumours of Porsche involvement in a V4, and have been since the early eighties, but how about a V4? How about overhead cams? Short-stroke engines? How far could you push the Harley aficionados before they kicked up? Do you really want to change a successful formula?

It's a dilemma that must have played on everyone's sensibilities but then, for the first time, the experience of the Japanese motorcycle industry can be brought to bear because a lot of the options on the table have now been tried elsewhere and so far no other mainstream manufacturer has yet succeeded in building a Harley-Davidson. Okay so there are truckloads of V-Twins from a variety of sources with a recognised engineering background, but while they might be better motorcycles in terms of their technology, they lack an indefinable something. It's not just about the 45-degree Vee, 'cos Honda tried that with their ACE Shadow, it's not purely push rods because Yamaha's Wild Star went that way, Kawasaki's VN15 had acres of torque and still went wide of the mark. All good motorcycles in their own right, but not Harleys despite all visual references, and if Harley-Davidson had made bikes that were so ... soulless, they would have been slated for it.

So, if radical redesign is out of the question, what else is left?

Simple, really. Evolution. The evolution of the Evo, which itself was an evolution of the Shovel, of the Pan, and ultimately of the Knuckle. It continues the trend of successive models to use emerging technologies, but it does so against a background steeped in history – and history is something that the Motor Company knows a lot about.

So what can you do to bring the Evo to new levels of performance without sacrificing reliability and durability? To start with, you can spend four years and more than two million miles in finding out.

One thing that has set Harleys apart from the rest of the field since the untimely demise of the British motorcycle industry has been the long-stroke engine. Short-stroke engines can rev higher with fewer mechanical problems than long-stroke ones, and typically have lighter components within them, giving them better throttle response times. Short-stroke engines can use the advantage of lots of firing strokes to maintain momentum and so reduce flywheel weight, whereas a long-stroke will rely on the momentum of relatively huge flywheels to keep the engine spinning between fewer firing strokes. Long-stroke engines are really a legacy of an older technology – steam, which was more reliant on power than speed – and aren't without their problems, most notably that even though the engine spins more slowly, the piston itself has further to go and so travels faster, and causes greater side loads on the cylinder wall creating more chance of wear: which isn't necessarily to suggest that Evos have such a problem, just that the technical solution to reducing that wear is unnecessary on a shorter-stroke.

It can be said that that short-stroke engines generate higher horsepower at higher revs, long-stroke engines develop more torque at lower revs. Of course, if you're not going for the ultra-sports range, top-end power is less important and the characteristics of a long-stroke engine lends themselves to unhurried, gentle usable power and so the gulf between Harley and the mainstream Japanese products grew for a time, but are now converging again.

With their 1600 WildStar, Yamaha broke with tradition and not only made an air-cooled, push rod V-Twin, but also a long-stroke V-Twin. In their case it was far too long a stroke, which made for an even more laid back cruiser but one which struggles to top the ton and is very much lacking at the top end of the rev range. Meantime, Harley reduced the stroke and increased the bore on the new Twin Cam 88 to produce, if not a short-stroke motor, certainly the closest thing we've seen from them yet. From the Evo's 88x108mm, the Twin Cam went to 95.3x101.6mm drastically changing the characteristics of the engine as they went. Moreover, they have built into the barrels the capability of going to 1550cc, which is a bore of more than 98mm and so closer still to a square engine.

What the hell does that all mean?

Well, basically that your Twin Cam 88 Harley will rev more freely than before, but in doing so, it loses some of its natural torque – which the increase in capacity more than compensates for. Greater ability to rev makes the throttle more responsive and increases the tuning potential. It also reduces vibration to some extent, but what vibration remains is harsher – hence the Softail's continued use of the Evo until the 2000 range and a new 88B engine complete with balance shafts, more of which later.

Such a change in internal dimensions requires a modification in cam profiles and timing, and while they were at it, Harley stuffed a second camshaft in there and made each cam responsible for one cylinder. This served two purposes. It gave the engine a name but importantly it gave greater flexibility in the angles that exist in the cylinder heads, and took some pressure off the push rods.

In short, all a push rod does is to transfer an upward motion from the camshaft to one end of a rocker arm, which converts that into a downward motion to open a valve in the cylinder head. It's all simple stuff except that to get the right shape of combustion chamber relies on getting the valve at the right angle, which in turn determines where the valve end of the rocker arm has got to be located, which specifies where the top of the push rod has got to go. The top of the push rod has got to be in a straight line from the bottom of the push rod, because push rods really should be straight to make life easy, and the bottom of the push rod goes into a tappet that is in contact with the cam, and which really should pass through the cases in a way that is not likely to promote wear and tear on its bearing surface. To have all of the cam lobes on a single shaft, as with the Evo and its forebears, reduces your options, because the starting point is compromised by the need to serve two cylinders. The ideal, in some ways, is to follow Sportster practice and have one camshaft per valve, but that further complicates matters, especially when you're trying to retain some of the aesthetic of the original motor.

The twin camshafts are held within their own subassembly, and are chain driven, to reduce noise, and are supported at both ends by bearings, and access via the cam cover shows the cam position sensor and support plate, which also retains the relocated oil pump and serves to direct oil to the filter, con rods and flywheels.

So the simple push rod motor isn't quite as simple as it sounds, hence the general drift towards OHC, but overhead cams demand taller cylinder heads (unless they're Ducati desmodromic OHCs), and the increased height of such a motor would radically change the look of the whole bike, or demand an even shorter stroke to keep the height down.

Having gone to the lengths of resolving the camshaft compromise, they were then able to change the shape of the combustion chamber and both ports, with their associated valves, to improve gas flow, improving power and reducing emissions into the bargain. Beyond that, they also changed the rocker mounts so that they no longer rely on the rocker box for support, as well as machining the tappet guide into the crankcase itself rather than the timing cover as previously – in-line with Sportster practice – both of which serve to reduce mechanical noise and increase durability, while a cross-drilled hole in the crankcase directs drain oil from the heads to the cam compartment near the tappet guides.

The crankcase itself is lighter and stronger with particular attention paid to high stress areas, and a revised interface between the engine and gearbox keeps those two components together reducing the strain on the inner primary drive case. Inside the crankcase lurks a new crank pin with an increased diameter designed to be better suited to the increased torque of its standard spec and performance motors based upon it. The crank pin is a press-fit into new forged flywheels and supports beefier con rods.

Still sticking with air-cooling, the fin area around the barrels and cylinder heads has been increased by 50% which combines with the lower piston speeds, and improved oiling offered by a new oil pump to reduce piston temperatures by 50-degrees Fahrenheit. The oiling system is still dry-sump but now the oil is filtered on its way to the engine rather than the way back which should ensure cleaner oil in critical areas, and which must benefit from frequent filter changes. One nice addition is the oil-pressure controlled oil jets that spray oil onto the underside of the pistons when things start getting a little warm.

The last major redesign is to the breather system which now dispenses with the timed breather gear completely, replacing it with a new breather filtering system allowing breather gases to escape more efficiently.

To show their confidence in the motor, Harley-Davidson offer emission-friendly performance kits and other performance parts through their Genuine Motor Parts and Accessories division.

Layout / type:	4-stroke, 45-degree Air-cooled V-Twin
Displacement:	1450cc
Compression Ratio:	8.8:1 – 9.0:1
Bore & Stroke:	95.3x101.6mm
Torque:	107Nm @ 3300rpm (Touring) 106Nm @ 2900rpm (Shorty Duals)
Power:	68hp @ 5500rpm
Bottom End:	Straight press-fit pin, in-line knife and fork roads, needle roller road bearing and right side main, matched Timken left side main
Ignition system:	Sequential, non-waste spark, MAP-N control (Carb models) Sequential, non-waste spark, Alpha-N control (EFI models)
Cams:	Twin cams, chain driven with spring-loaded tensioners
Crankcase:	Aluminium, vertically split, high pressure die cast
Cylinders:	Aluminium, high pressure die cast with cast-in Spiney-Lok cast iron liner. O-ring base seal
Cylinder head:	Aluminium, permanent mold with 85cc bathtub combustion chamber. Single 12mm spark plug
Pistons:	3 ring, hypereutectic Aluminium, cooled by oil jets
Oil pump:	Twin gerotor, dual scavenge, crank-mounted and driven internal oil pump. Dry sump

And now the 88B ...

No-one seriously expected Harley-Davidson to produce two motors for long and so in 2000 the Softail range also got the Twin Cam 88 in a revised frame ... but no ordinary Twin Cam 88, this was the 88B, with "B" denoting a little device within its modified cases called a balance shaft - or two of the little buggers, to be precise. This was obviously the lesser of two evils, and the idea of rubber-mounting the 88 in a substantially modified Softail chassis must have been discounted because it would have been by far the cheaper option.

So, rather than damping the vibes from the engine, as per the Dyna, Tourer and Buell ranges, the trick is to reduce the vibrations that get out of the engine in the first place. This has long-since been a trick of the Japanese with everything from their parallel twins of the seventies to the modern day cruisers, but it's the first time it's been used on a Harley, and it works. It works so well that the solid-mount Softail that were loved for their look, but castigated for their vibrations are now taking mainstream prizes for most desirable cruiser.

How does it work?

If you put a weight on the outside of a disk and spin it, it will wobble. If you spin it fast enough that wobble will become a vibration. If you stick another weight on the other wise of the disk it will counterbalance the first weight and it will run smoothly.

Four stroke, four cylinder engines do not have the problem because whenever two pistons are rising (one flushing the its chamber of exhaust gases, the other compressing its air/fuel mix), another two pistons are falling (one drawing the next air/fuel mix in, the other forced down after firing): every stage of the four-stroke cycle is happening somewhere and good primary balance is achieved. A V8 is better still, but then as two four cylinder engines on a common crank it was always going to be. Two-stroke twins are as well-balanced in the same way.

Four-stroke twin cylinder engines can only run two of the four cycles at any given time so a compromise has to be reached, and it doesn't matter whether it is a parallel twin or a V-Twin, they both have the same problems to such an extent that Yamaha modified the crank throw of their sportier 850cc TRX parallel twin to make it fire like the ninety-degree V-Twins of Ducati, which was the target market for the bike. Ducati riders, like Harley riders, choose their bike because of what it represents to them and ignored it in droves.

With a Harley engine both pistons are close to top dead centre (TDC) at roughly the same time - separated by the 45-degrees of the configuration, which is small potato-potatoes in anybody's money. The power stroke is not the critical element here because that, and the primary vibration, does not present the major problem - and I apologise to anyone who read this previously and followed me down into a deep hole of misunderstanding. The vibration that is dealt with is a secondary vibration, caused by the reciprocating mass when the crankpin is at the 3 o'clock and 9 o'clock positions. This means that main force of the explosion is spent, and the balance shafts are balancing out the dead weight of the piston rather than their weight under load - a much easier task by all accounts, and something which allows the increase to 1550cc without a problem as the 1550 pistons weight much the same as the 1450 pistons. There is an additional vibration from the 1550cc motors, but that is more the unchecked primary vibration.

As you can see from the pictures and illustrations, a separate subassembly is bolted into modified cases which accommodate balance shafts at the front and rear of the crank. The shafts themselves are chain-driven passing beneath the crank, driven by a new gear on the timing side main shaft, and held taught by no less than three slipper tensioners. The balance shafts are eccentric weights that need to be precisely positioned to ensure that their vibration is synchronised to that of the crankshaft and Bob's your uncle. Get the balance timing wrong and you'll know about it, though, as it would almost double the problem.

As is the case with all balance shafts, it takes its toll on power, and the 88B is no exception, but as to how that relates to its use on the road only time will tell. Certainly a five horsepower loss at the crank is not the best of news, but the torque figure is largely unaffected and even the lesser figure is up on the previous generation's Evo-powered bikes, so it's a small price to pay for a significantly more rideable bike.