Cycle News - Archive Issues - 1980's

Magnesium castings. t itanium fasteners. oval pistons. eight valves per cyl inder - the exotic NR500. C laimed power was 135 bhp in 1982. Each oval piston ran on two connecting rods; the pistons are 41 mm wide and 93.4mm long with straight front and rear faces measuring 52.4mm. The en ginee rs with him d idn 't think th at more th an 100 bhpwa s p ossible, wh ich was 15 bhp m or e th an Mik e Hailwood 's inl in e ai rcoo led four o f 1968. Yet 120 bhp was th e goal. - Th e engine wo u ld have to rev hard and breathe deeply, much more deepl y than a n y four-stroke engine of its size had ever done. More valves and more revs , more th an 20,000 rpm would be necessarv. B u t the n one day as Irimajiri was driving to work he had an idea . Sitting at the traffi c lig ht s he looked up a t the light and wondered why the pis tons in the en gine h ad to be round. If th ey were ova l, more va lves cou ld be fitted into the com bus tion cham ber. T he project was then restarted wi th a V-four engi ne a n d oval pis tons ru n ning on doubl ed-up connecting rods. On th e outside it loo ked lik e a V-eight, and it ran lik e one, but inside each pai r o f cylin ders had been joined together to form an o val with stra ig h t sid es. ) . One journ al ist wa s even du ped into th inking the en g ine wa s a Veig h t and wa s th rown into co n fusion wh en h e real ized th at H onda co u ld never race such a n engine . No sooner had th e proj ect started th e probl em s showed up. Seal in g th e pi stons with o ther-tha n -ro und rings was th e first headach e an d a va rietv o f co n fig u ra tio ns wer e used. H o';da cracked th e problem in the en d but just how is still being kept a secret. It 's revealing that Honda hasn 't exposed the nature of the camshaft s or the crankshaft. For an engine to ru n at 20,000 rpm it needs very ex tended valve timing: th at ma y .ha ve been the reason the engine was hard to start. Remark ab ly, the NR500 is otherwise very straigh tforward in its interna l construction. To keep weigh t do wn to 110 pou nds, the cases are cas t in magn esiu m alloy with ova l stee l lin ers p ressed in. The crankshaft is bu ilt up, j ust as the four and six cyli nder raci ng engi nes o f the I960s were, with oil mis t lubrica tion . The rods, two per pisto n, are alloy steel. Angle between the cylinder banks isn ' t the ideal goo for perfect prima ry bal a nce. Instead 100° was chosen so Cos wo rth 's four-valve cha m ber develo ped for th e. F-l DFV three-liter V-eight engine ha s sp a wned severa l others with Yamaha 's five-va lve a rran gem ent the latest and m ost sophisticated. The N R500 cha m bers, being lo ng a nd with a large surface ar ea for their ' volume, co u ldn ' t offer Ii worse arrangement by comparison, a nd th is is reflected by the low co m p ressio n ratio o f 12.1 to I. If it were any h igher, th e pis ton cro wns, whi ch alr eady have deep p ockets to clear th e eight valves, wou ld affect combustion. With 270 bhp per liter, the NR had a specific power 90 bhp per liter higher than th e Cosworth F-I car engine. . Even so, riders had difficulty controlling that power which came in at 8000, ro se to 70 bhp at 12,000 and th at th ere is more roo m for th e four · tw in 30m m-cho ke carbure tors ' with flat chrome -p la ted slides. From th e top th e two magn esium cyli nde r heads look as if th ey're o ff a V-eight. Eac h has lfi valvesset in line with a fairl y wid e 38° ang le between th e exhaust a nd inl et banks. .Th e four inl et valves for each " cyl( inder " a re made in titanium with a head di ameter o f 19.5mm and stern 3.8mm in di ameter. The ex ha ust va lves ar e a llo y steel with a 17.5m m head di ameter. To ensur e adequate speed of com bustion, two 8mm spark plugs are used . From the design of th e N R500 it looks as if Honda's engi neers took the same four -stroke design pa th a s they had done in the I960s; by exploiting thei r abi lit y to get their engines to rev to astronomic level s. The mech anical rev limits o f a n • engine have usua lly related to th e maximum average piston speed. This gives a ro ugh idea of the loadin gs on th e cra n k, co n neci ing rod s, pi stons ' a nd valv e gear. U p to the end o f the 1970s, 4000 feet per m in ut e was th e limit. But more recentl y im proveme nts in mat er ial s have a llowed th e limit to be push ed far h igher th an thi s. Desp ite usin g an extreme ly short . stroke of 36mm, a t 20,000 rp m th e NR500 'ha s a mean pi ston speed o f 4725 feet per min ute, beyond which th e p ow er begins to drop away drama tica lly because o f th e lim its of th e engine's breathing efficiency and in creasing internal fricti on . T o give a swe p t vo lume o f 499cc. th e pist on s a re 41 mm wid e and 93.4mm long with straight front and back faces 52.4mm long. An equivalent round pi ston would be 66.4mm in di ameter, so th e N R is ex trem ely oversquare by normal standards, _ The diffi cu lty of usin g such a layo ut is that th e efficiency o f the combustion process is compromised. Latest developments in combustion cha m ber design have mov ed tow ard m ore co m pact cha m bers that concentrate the combustion chamber volume around th e spark plug, but without using flat sq u ish ar eas which can quench the mixture a nd lead to in creased detonation. then increased almost lin earl y to th e peak o f 135 bhp. Engine braking proved its undoing too. On closing th e thrott le th e co m bina tio n of min iscule flywh eel in ertia and m assive interna l fricti on resu led in rear-wheel hop whi ch was o n ly slightly overcome by an . antihop clutch that was in corporated in to Honda's road bikes . What killed th e NR was its weigh t a n d difficu lt sta rting. Its on ly chance was a change in reg ulations pu tt ing a fuel lim it on machines, as in F- I car racin g now, which wou ld have placed more im porta n ce o n the bike 's 30% better consu m p tion compared to the two -strokes. H onda 's efforts to get th e FIM to a llow turbos in the 1986 racing seaspn have given the NR concept a new lease of life. • 17

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