Cycle News - Archive Issues - 1990's

that such a mach in e might carry. More to th e point, it would make Ho nda 's own RC30 successor look pretty sick , a further rationale for Honda 's at first curious decision to make the N R750 roadster a super-refined, elegant, sports bike rather than a highperformance race replica boasting the h ig hest power output on the street. Which is not of course to say that someone, somewhere, won't try to tum it into that ... I Bu t why is the oval piston design so superior on paper (and in practice, too, according to Honda's developmental experience) compared to a ' conventional engine? NR750 engine designer Suguro Kanazawa explains: "When Mr. l ri majiri and his staff were given the responsibility of desi gning a ' New Engine', wh ich basically meant From th e rich, red paint to th e carbon fiber facing of the p lastic bodywork, the N~750 oo zes quality. No expense spared, no detail overlooked. iT"~r-=-~~-:d'" - -- - ----:. . - piston design the preferred choice in any case." According to Kanazawa-san, these adva ntages include a 10% reduction in reciprocating weight for a single oval piston compared to two round pistons of combined equivalent swept volume; a 9.8% reduction in cylinder width for equivalent valve area; a shorter engine stroke for the same cu bic capacity; reduced internal friction; improved combustion; and greatly increased inlet efficiency. "The ideal configuration is to have the length of the inlet tract the same as its diameter," says Kanazawa, " and with more than four valves on a round cylinder, this is impossible, and only really feasible on four-valve cylinders, while the oval piston design makes it very easy to achieve an ideal design which maxim- izes flow. These adva ntages are so many, that it made it worthwhile to try to overcome the many difficulties involved in prod ucing an ova l piston , the greatest of which was the piston ring design and manufacture." H o n da 's success in solving this prob lem was the key not only to making the N R500 and its 750cc successor produce power and run reliably, but it was also a vital factor in productionizing the result in the RC40. However, Honda had made a major design change on the pistons employed in the NR750, compared to their original oval piston design as employed since the start of the NR500 project. This employed straight-sided pistons with semi-circular ends, rather li ke a running track, whereas the street NR750 has elliptical pistons more like a rugby foot ba ll but with round rather than pointed ends. The reason for this, according to Kanazawa-sari, is cost allied with difficulty of manufacture; making eight sets of the flat-sided type for racing is feasibl e, but 500 times that number for even limited production is presently impossible. Only a cynic would say that, since the engine designer also admits: that the most difficult trick of all is to seal the compression at the point where the straight side meets the round end; Honda has decided to keep the secret of how they 've achieved this to themselves, for the time being, at least . . . I The wa ter-cooled dohc N R 75 0 engine is therefore a 9O-degree (no t 85degree, as was the NR750 racer) V-4 with 360-degree crankshaft throws (two-up, like the RC30), with gear camshaft drive on the right side of the engine. The 32-valve engine has a fla t Here it is: The water-cooled, noRC, 9O-degree V-four with 360-degree cra nkshaft. The wind tunnel-developed bodywork is designed to produce maximum do wnload at high speeds as well as look good. It 's both innovative and stri kin g. no t a two -stro ke, for H onda 's return to GP raci ng in the 1970s, they realized th ey must increase valve size as much as possible to improve breathing and achieve more power th ro ugh higher rpm. Bu t th is was very difficult; using a single-cylind er XL250 slave engine for experiments, we tried five, six and seven valves per cyli n der, but the resu lts were not satisfactory - ba sically, we discovered that you must have eit her four, or eight valves per cyli nder. With four valves, we could not achieve the necessary flow, but with eight valves, the problem was how to arrange them on a ro und cylinder, and to o pera te them in a suitable way." "Then one day, Mr. Iri majiri was driving home from his office, when he . stopped at a tra ffic light. In J a pan, th ese are suspended in the ai r, with filter lights for protected di rection ch anges underneath th e three usual lights. He looked up, and saw two rows, each of four lights, hanging in th e a ir side by side. H e su dden ly realized how it would be possible to make an eight-valve cylinder usi ng an ova l piston; when he came to work the next day, we began development of the NR ('New Ra cing') design J" In more than a decade of interest in and support for Honda's bra ve decision to tread the ova l-piston fourstroke route, I have never heard th is explanation as to how it came about; coming as it does from the ma n responsible for th e creation of the N R750 engine, you have to believe it. Strange, but true! But Kanazawa-san has more facinating information to im part: "After we made the decision to use ova l p isto n technology, other advantages became apparent which, ho nes tly speaking, we did not think of at first. O f course, everyone compares one eight-valve oval piston to two four-valve round pistons, and beca use it was forb idden to make a V8 engine for 500cc GP raci ng , people th ought we made the ova l-p iston V4 to avoid this rule. But this is not so: the adva ntages are even gr eater than a round-p iston V-8, making the ova l

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