Cycle News - Archive Issues - 1980's

The Yamaha OW8 1 's power valv e m echanism and cy linder head exhaust chambers. (Ab ove) The complex exhaust system on t he Honda N SR 500 V -fou r t hat took Freddie Spe ncer to t he 500cc W orld C hampionship. (Below ) Th e simpler, more com pact Ya ma ha V -four of Eddie law son 's. --- High tech wars in Europe By Peter Clifford It has taken the ] apane se a long time to improve on the genius of the legen dary East German two-strok e eng ineer Walter Kaaden bu t, though they have deve lo pe d many of his ideas for th e pas t 20 years, th ey fina ll y swept th e rotary-valve tw o-s tr ok e as ide b y d o rni n a t. b h h 95 d • 0 m g O l l e _ Occ a n :>0 CC 12 ' road racing classes wi th cra n kcasereed-v a lve-i nd uction engines. It wou ld be dangerous to sta te th at th e Japanese h ad invent ed the cra n kcase induction engine, but cert a in ly th ey have de veloped it furth er tha n an yo ne else. Yama ha won the 500cc World C ha mpions hi p with Edd ie Law son and th e OW76 in 1984 and H onda back ed th at up by dornina tin g bo th th e 500cc a nd 250cc cla sses thi s season . . It is a revo lu tiona ry change that threat ens to ecl ip se 'a ll th e European ma nu fac turers. Rot a x: Pernod .' C a r- elli . Parisien!1e, !\IBA, and .Paton s~em stuck with ~otary-valve indue- 1l0na~donlyCag"l\'~hasrace~are~- valve-induction en gine a fter this year s debut of th eir V-Iour Yam aha co pv. Wh at H onda a nd Yamaha have been able to do is make reed va lves that respond so well to cha nges of press ure in the engi ne's cra n kcase that th e gases are drawn in whe n th e pressure is lo w , i.e., wh en th e pi ston is risin g , bu t a re no t thrown bac k o u t thro ug h the car buret or wh en the p isto n descen ds. T his m ay sound si m p le, but ma king a reed-v a lve sys tem that will respond rapid ly a nd stro ng ly enoug h to keep th e ga ses in a t th e righ t m oment. yet n ot be so strong tha t th ey detra ct from th e inta ke ga s sp eed. is a very co mplex science . T he spri ngs m ust be li g h t enoug h so tha t th ey open instantl y an d stro ng eno ug h to close just as quick ly. Further , th e reed pe ta ls m ust be so desig ned tha t they do no t Flutter: hen ce, i t is necessary to st udy th e harm onics a nd natura l freq ue ncy of the arra ngeme nt. T he amou nt o f wo rk requ ired is well wo rt h while wh en th e adva ntages are weighed up . Ro ta ry va lves sap a good deal of power from th e engi ne; just draggi ng th e rotary-va lve discs around agai ns t the fric tion o f the cases and di sc covers is hard work. T he weight of th e d iscs has to be a cce lerated a n d s lowed w i th th e cra n ksha ft. and th is is a furt her load on the engine. The inclusion of d isc va lves co m p lica tes a nd restricts desig n; it a lmost for ces th e eng ine er to p lace th e d iscs o n th e end of th e cra n ksh afts a nd th e ca rbureto rs in line. This li mi ts cyli nder la you t and adds to en gine wid th. Yama ha tried to get away fro m the restr iction o f p lacing the carbu retors o utboard o f th e cra n ks ha ft ends by bu ild ing th eir first V-four 500 wi th di sc valves in the Vee, betwe en th e cylinde rs , a nd drivi ng th em wi th a sys tem of sh af ts a nd gears. T h ey ga in ed in th at th ey co u ld ma ke a narrow engin e a nd give the ca rbu reto rs fresh, co o l a ir from th e front o f th e fa iring. They lost, tho ugh . in th e added fr ict io n o f extra gea rs a nd bea rings. Th e n ew V-Io u r was s ti ll fa st enough to ena ble Ken n y Ro berts to chase Fredd ie S pe nce r ver y h ard in 1983, th ough Spe nc er wo n th e title o n th e th ree-cy li nder H onda. In 1984, Ya m aha unveil ed a reed-v alv e ver sio n of the sa me engi ne. T ha t rea ll y worked because th ey got rid o f th e drag o f gears , bea rings a nd th e discs themselves. The en g ine hel ped Eddi e La wson win th e ti tl e in 1984 but H onda had also ent ered th e reed -va lve-ind uct io n stakes with th ei r NS R500 a nd rh ev went a step fu rther in th e red uc tion o'f interna l frictio n . H o nda had crea ted a master stroke with th eir NS500 becau se th e th ree cylinder engine 's cra n ks ha ft h as o n ly four bea rin gs ra th er tha n th e normal two per cyli nde r. T he sav ing o f tw o hearin gs a nd th e ver y stiff cra n kshaft prod uced wa s not so met h ing that H onda wa nted to give up wh en they m ade th e fou r, a n d, h en ce , th ey mounted a ll four cylinders on a sing le, five -ro ller- bea ring cra n k. Ma king a V-Iou r wit h a si ngle cra n k did mak e the p la cing of th e carb uretors a li ll ie mor e d ifficu lt as th ere was liu le room in the cyli nd er Vee. Th e trip le has its ca rbs in th e Vee. but th ey breat he into th e cylind ers a nd the cran kca ses a re more deeply recessed i nt o the Vee. Pl acin g th e car bs h ere mi ght have dema nded a lo n ger-th an -desira ble in let tract. H o nda co u ld have sprea d th e cyl inder angle beyond 45°, but that m ig ht h a ve ge nerated bal an ce probl em s a nd wou ld ha ve ma de th e engin e larger. T hey deci ded instead to pl ace th e ca r bs behind th e u p per cylinders, and th e ca rbs feed into th e back of th e cra nkcases. Usi ng a si ngle crankshaft restricts th e a mount of space for th e reed valve box es, as a ll fo ur must be in a line across th e back of th e cases. T h e Yamaha has two cranks and more room but to fit th e four box es in a line on the H onda th rea ten ed to restrict th e width of th e reed petal banks, T o a llo w wider box es to be fill ed, th e box es were turned 90° so tha t the split be twe en th e reed banks ran vertical. It is likelv that thi s also mad e th e flow of gases into th e cases a nd between the Ilvwheels smooth er. Bo th th e H onda and the Ya ma h a are very co m pact and this has necessita ted the prod uc tion o f special ca rburetors to mak e th e Vent uri close eno ug h togeth er. T he ban k o f four ca r bs are m ad e in two twi n -choke casti ngs. T he Yama ha uses Ilat-slide ca rbs, per haps because they need a sho rter i n le t tract, whereas H onda has cyli ndrica l-slide Kei h in s, Pl aci ng th e ca rbs beh ind the engine did contri bu te to th e problems o f the NS R500 that lost the 1984 Ch am pi o nship. When th e desig ne rs decided to p lace the gas tan k underneath an d th e pi pes o ver th e top o f th e en g ine it left th e ca rbs to brea th e hot air. That co nt ri bu ted to tro ublesome carburetion, and ma y have given th e Yama ha a power edge. The start of 1985 saw th e gas tank back in th e conventiona l posi tion , and the carbs breathed from an airbox supplied th ro ugh a cha n nel leading from a plas tic scoop un der th e lower fo rk triple clamp. A p lasti c co ver kept th e air aw ay from the h o t engi ne a nd the underside of the ta n k was shaped to form th e top of th e cha n nel and air bo x. T hough th e layou t o f th e NSR changed a liu le between 1984 a nd '85, it wa s a new engine th at Sp encer used to w in th e cha m p io ns h ip. with detail ed changes that ma de i t lighter, m ore co mpact a nd mor e powerful. There were m uch larger changes made to the cycle parts. and H onda threw o u t th e o ld fa bri cated-a l um i n u m fra me in favo r o f a much -sim p ler tw in -spa r design . At th e sam e ti me th ey mo ved th e fu el tank back wh ere it seem s to helo ng . and mounted th e engi ne furth er forward. T h e co nsi de ra ble cha nges these cha nges mad e to th e weight distribution were th e res u lt o f tests made by

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