Cycle News - Archive Issues - 1980's

The five-valves-per-cylinder Yamaha FZ750 has impressive horsepower and rock-solid handling. It may be the best sports bike of 1985. Before settling on five valves per cylinder. Yamaha engineers experimented with seven valves - four intakes. three exhausts More valves. eotta have more vahes: DeYeloplne the Yamaha fZ750 ~ In 1977 plans were drawn up for a fourstroke engine at Yamaha's advanced engine technology research section. The development aim was a four-stroke that could surpass two-stroke power. Extensive research was performed to try and get the desired power from a four-valve cylin- -e 70 der head. The search for a higher rev ceiling, better combustion efficiency, more intake volume and less mechanical loss led 10 the development of a seven-valve head. Since more valves seemed 10 offer a good solution, research began on a single-cylinder engine with·a seven- valve head. Theoretically, the sevenvalve head could supply the ideal intake volume in a linear curve up to a 20,000 rpm rev limit. This head featured four intake valves and three exhausts, with twin spark plugs. Pleased with earlv test results, Yamaha built a four-cylinder version. However, results of further tests were unsatisfactory. Fundamentally sound and revving 10 20,000 rpm, the design was unsuited for the comparatively small displacement of motorcycle engines. Whatever potential the design had, it could not be fully explo· ited at the technical level existing in the 1970s. Nevertheless, the knowledge ob· tained about the combustion chamber shapes early pointed the path that future four-strokes would have 10 follow in search of performance. The engineering goal of increasing the mechanical octane value at highcompression ratios began during development of Ihe 5000cc twin-turbo 800-horsepower V-8 'engine for the Toyota 7, a Group-'] racing car that Yamaha participated in developing. The goal was to surpass the pentroof combustion chamber, considered at the time to be the best for' high· preformance engines. Company policy dictated that technical developments be adaptable to r:nass-production machines within a few years. The question became how to improve the performance of each cylinder with a IOtal engine displacement of between 500 to 75Occ? However, the seven-'-'alve head was not suitable for mass productiQn due 10 cost, the overly-high rev limit, the problems of cramming seven-valves into such a small bore area, and th'e interference between the two flame fronts from the dual plugs. A six-valve head was developed next, using three intake valves and three exhausts. Mechanical loss was reduced and intake volume remained almost the same. However, it too had 10 be discarded due 10 hot spots between the exhaust valves and other problems. A five-valve head was the next logical step. With two exhaust valves placed fartber "apart, the hot spots between the exhaust valves were eliminated. The effective rev limit was slightly lower but intake volume remained high, and power output and torque characteri tics were excellent. Most important, the five-valve layout permined an ideal bi-convex lens-shaped combustion chamber. Unlike the pentroof combustion chamber, with Yamaha's five-valve head and lens-shaped combustion chamber there is no need 10 raise the pislOn crown to increase the com· pression ratio, and the distance between the plug electrode and the recessed piston crown is generous. Flame propagation is ideal, reaching every corner ofthe com bustion chamber quickly and uniform ly. T,lJis permits high compression ratios without detonation. As development" of the five-valve head and lens-shaped combustion chamber continued, drastic changes were taking place in the motorcycle industry. Sales of all makes began 10 drop off, and protective import regulations appeared imminent through- out the world. Facing this situation, Yamaha determined that its future machines would have 10 have a longer market life. In 1980 planning began on prototype machines that would eventually culminate in the FZ750. TIe basic engine configuration was now decided. The proven five-valve head and lens-shaped combustion chamber would be used by aJl development teams. At Yamaha, products are usually developed in a step-by-step manner: separate 'groups develop different machines at the same time, and some are dropped from the list afterevaluation. This goes on and on until the final goal is reached - the product. A number of projects were started by different groups. One group's engine, designated 001, was to use the FJ1100 frame - which was under development at the time and reflected Yamaha's latest design concepts and new hydraulic lifters. Another group was told to develop a V-4 engine. Another group was given no conditions at all. The first group started development by mounting a liquid-cooled five-valve head on an existing inline· four cylinder block and crankcases. The cylinders were slanted about 5° as in conventional engines. Directdriven hydraulic lifters were stipulated for the 20 valves in the head. for improved reliability and ease of maintenance. Already more than 100 different combination of valve springs and hydraulic lifters had been examined in a search for the best possible combination, assisted by a CAD (Computer Aided Design) system. In short, group 001 tried every possible approach in this engine configuration. Another group developed dual outboard cam drives with twin separate cylinder blocks in an in line configuration to eliminate lTead distortion of cylinders. Still another group developed a V4 with hydraulic lifters and rockerarms. Again, the three groups shared only two things in common: the fivevalve head and liquid cooling. And no group was aware of what the others were doing. In early 1982, results were evaluated. At this time the machine's capacity was set at 75Occ, as Yamaha planned to enter four-stroke racing in the near future. J;he engine developed" by group 001 was chosen. Although the other groups' engines were technically notable, they were dropped ~cause their practicability, noise, compactness, mass centralization and center of gravity were inferior to those of the group OOl's engine. While much was learned from the projects (the 1985 Maxim-X, for example, features the five-valve heads and 5° slanted cylinders developed by 00 I), all the designs had to be discarded. For example, the direct-driven hydraulic lifters lacked response over 10,000 rpm and were noisy when the engine was hot and the oil pressure low. As design goals were narrowed down, the pace of development picked up. TheearlierOOI was re-designated the Oil and given an all-new chassis. Its engine was to be a base power unit capable of winning TT·I races, Daytona, and other major events. Hydraulic lifters were discarded in favor of inner-pad shim adjusters for valve clearance adjustment. But to ensure instant thronle re ponse, and adherence to Yamaha tradition, the valves were lifted directly off the cams, without rocker arms. Yamaha's factory GP team suggestedeliminating the somewhat wide rear-end of the fuel tank dictated by engine and intake components, so the cylinders were slanted forward markedly. moving the carbs up and forward. A new group. formed to develop the Oil, started their project by listening to racing riders and electrical specialists. They reconsidered every possible approach to the engineering goal at hand - an ideal combination of engine and frame. Their studies revealed that the inline four is the best enRine configu-

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