Similar Yet Different: The Nuts And Bolts Of Hinckley's New-Generation Bonneville
As an eight-valve, parallel-twin with double overhead camshafts, the new Bonneville's powerplant is thoroughly modern, though it does share with its pushrod, two-valve-per-cylinder T120 ancestor a 360-degree crankshaft throw, with both pistons rising and falling together in the nickel/silicon-plated cylinder bores. That's a prime recipe for intrusive vibration, which may once have been considered desirable but nowadays is a customer no-no. Triumph's engine team therefore decided to incorporate twin gear-driven balance shafts located at 2 o'clock and 10 o'clock relative to the plain-bearing crankshaft.
The new Bonnie engine also needed to share the T120's power characteristics. The 790cc motor was designed to be "lazy" rather than "potent," which the reasonably long-stroke bore and stroke numbers of 86 x 68mm help ensure. Ditto the T120 engine's look, something designer John Mockett had a role in alongside the engine team. Although the new Bonnie's drive chain was designed to run on the right side of the bike (the T120 runs its chain on the left), several details were designed to keep the basic look somewhat intact, including the finned cylinder and dummy pushrod tube in front of the engine (originally intended to be an oil feed but now acting as a cylinder head oil breather). Although a wet-sump design with no separate oil tank (unlike the original dry-sump Bonnie), the new Bonneville engine employs internal oil lines wherever possible to retain a clean appearance, while twin oil pumps and a sizable oil radiator are fitted for cooling as well as keeping the plain-bearing engine's internals together.
Another ploy in helping endow the new motor with period looks is the Bonneville's TL1000-like cam drive system, with the centrally mounted chain driven directly off the crank to a set of idler gears in order to narrow the camshaft spread. Combined with dropping the cams in the heads to get the pushrod look, this reduces the engine height significantly.
Certain period Triumph traits were deemed surplus to requirements, however, hence the new engine's horizontally split crankcases designed to eliminate the oil stains in the owner's driveway invariably provided by its pushrod ancestor's vertically split engine. So, no comeback for those tarmac restorers of dubious chemical provenance so popular in mid-'60s California.
OK, but why go to all the trouble of having to dress up a DOHC 8-valve design to look right, rather than concoct a more humble, and presumably less-costly, single-cam two-valver?
"Two reasons," says Triumph's Ross Clifford. "One was that we're used to building engines with four valves per cylinder. Another was performance; 55-60 hp was our design target, and we wouldn't have got that power with two-valve heads. We did take a serious look at a six-valve engine with three valves per cylinder (two inlet, one exhaust). The cost effectiveness of going to three valves was marginal, so we opted for four." Running 9.2:1 compression, the design target was achieved early on, and in production guise the Bonneville delivers 62 hp at 7400 rpm, with maximum torque of 42 foot-pounds as low as 3500 rpm.
Since it was always intended that the Triumph twin-cylinder family for which the company intends a long production life would comprise only two distinct models-the Bonneville, and another new bike that will appear in due course-there were fewer design constraints on the new engine, with modularity not such a key issue as on other Hinckley Triumphs. But while it was always intended that the company's new twins should be stand-alone products, there was interest in parts commonality with the company's existing three-cylinder products. Thus, some components from both the older T300 and newer T500 models were used, such as the much-improved T500 six-speed gearbox-well proven on more powerful bikes such as the 955i, and here used on the Bonneville in five-speed guise-mated to a cable-operated wet clutch.
Unlike other recent Triumphs, the new Bonnie is not fuel-injected, but uses a pair of 36mm Keihin carbs fitted with electric heaters to combat icing, and a throttle-position sensor linked to the digital ignition to optimize throttle response. Triumph chose carbs rather than EFI mainly because of the style of motorcycle the Bonneville represents; it's not a sportbike. Cost was also a factor.
Triumph says it can meet all current emissions requirements with carbs, especially when air-injection is used. A catalyst exhaust has also been developed, but will only be fitted in those markets where it's required. The period-looking peashooter-type twin exhausts are uncharacteristically long in order to gain the requisite silencing capacity without compromising cornering clearance-the kink in the pipes helps pull them out of harm's way.