Cycle News - Archive Issues - 1980's

~ 00 0') ~ C(") l-i V ~ o .... U o Yamaha's FZ760 has the look of a racabike, from styling to engineering details. The six-speed transmission has close ratios. All the FZ760's valves are operated by cam lobes acting directly on valve buckets; intake tracts are neer-vertical and straight. A conventional, four-valve combustion chamber is typiceHy shaped like this. FZ750 tech: Why five valves per cylinder? By John Ulrich Yamaha's new FZ750 carries the designation "Pure Sports" on its sleek tailpiece, and it's no wonder. The water-cooled 749cc four stroke with DOHC and five valves per cylinder has a bore and stroke of 68 x 51.6mm and a compression ratio of 11.2: 1. The four inline cylinders are canted forward 45° with 12 the bank of four downdraft !l4mm CV carburetors positioned above the cylinder head. That makes the straight intake tracts nearly vertical; the vertical eliminates fuel puddling and the straight shot into the combustion chamber prevents fuel separation, in which the inertia of gasoline droplets .causes them to separate out of the air flow at curves in a conventional intake tract. The advantage is more efficient combustion for increased torque and bener fuel economy. The crankshaft runs in plain bearings and primary drive to the clutch hub is via straight-cut gears; the crank gear is machined directly into one crankshaft flywheel. A chain o££ the center of the crankshaft drives the dual camshafts, and cam lobes act on bucket-style tappets to open the valves. Like the FJllOO, the FZ750 has an alternator located behind the cy linders to minimize crankcase width and the ignition system is on the left side of the crankshaft. The conventional multi-plate wet clutch is hydraulically actuated. The water pump is positioned on the left side ofand below the transmission, and forces coolant through waterjackets in the cylinders and cylinder head. then through an aluminum radiator mounted above the cylinder head to the frame downtubes. A small electric fan mounted behind the radialOr is thermostatically triggered; vents in the bike's framemounted half fairing draw hot air from behind the radiator. The frame itseJ[ is made of square cross-section steel tubing with widelyspaced backbone tubes and with removable downtubes lo facilitate engine installation and removal. There's a single rear shock acting on an extruded rectangular cross-section aluminum alloy swingarm and conventional telescopic forks - without anti-dive - at the front. The 16-inch cast aluminum alloy front wheel carries dual ventilated stainless steel discs; calipers each have two opposed hydraulic pistons. The rear wheel is an 18-inch with a single disc. The front tire is a 120/80-16, the rear a 1~0/80-18. Taking the known diameter of the front wheel and using it to scale the rest of the press photo released at the Cologne, West Germany motorcycle show last week, the FZ750 has a 57inch wheelbase and a rake and trail of 26° and 4.7 inches. Seat height is ~0.8 inches; a rear seat cover is removable to reveal a passenger seating area. In developing the FZ750, Yamaha engineers reasoned that any significant advance of existing four-stroke technology, especially if it were to achieve both increased horsepower and fuel efficiency, would have lo concentrate on cylinder head design. It would have to maximize the volume of fuel/air intake, speed its flow inlO the engine, boost combustion e££iciencY and disperse the spent gases as quickly as possible. These were the basic reasons why high-performance four-strolceengines had switched from two to four valves per cylinder. The FZ750 takes the next step forward. It would have been possible lo achieve the aims outlined above by enlarging the valve sizes in a conventional four-valve combustion chamber. Within the dimensions of the FZ750 cylinder head. however. this simple expedient would have brought problems in its wake. Enlarging valve size necessarily flattens the curve of the combustion chamber, making it more di££icult to achieve the hemispherical cross-section that is most desirable for the highest power outputs. So, very early in its research and development program, Yamaha engineers determined that the normal avenues of four-stroke engine development had basically been covered. It would be necessat:y to cross new design frontiers if the company wanted to make a quantum leap in four-stroke performance and e££iciency. They realized that some fundamental changes in cylinder head design, with The new Yamaha five-valve's combunon chamber is hemispherical in shape. new multi-valve layouts, would be necessary. Using more valves would result in larger intake area. gaining additional induction flow without resorting lo large degrees of valve lift that might promote unreliability. More valves would also allow the use of narrower valve angles. This would permit a hemispherical combustion chamber shape, which would have a smaller volume than more usual cylinder head configurations. The result would be an increase in compression ratio while maintaining the best shape for £lame travel and combustion efficiency. And finally, the weight of each valve would be proportionately reduced. This would allow higher safe valve speeds. permitting higher rpm. It would also permit the use of a single.lighter valve spring without valve float at high revs caused by the weight of a valve being too much for the spring lo handle. Because o£the lighter springs there would be less power loss in valve train operation. With these parameters as the basis for experimentation, Yamaha's development sta££ worked on five, six and even seven-val ve configurations. Of all these, the five-valve layout was found to provide the best all-around answers for the FZ750. The six-valve system (three intake and three exhaust) did not permit a satisfactory combustion chamber shape and required too much reduction in the diameter of each valve. E~periments also showed that the central exhaust valve was exposed to excessively high temperatures and. in any case, tests had proved that two exhaust valves could easily handle the gas £low. Similar problems also a££1icted the seven-valve prototype and the complex desiRn would have

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