Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/126808
~ 00 - 0') The stripped-down GSXR (below) shows the air intake above the top frame tubes. The GSXR comes without a centerstand, to save weight. By John Ulrich There's more than one way to make a motorcycle faster and quicker: Make more power or reduce weight. Suzuki engineers used both approaches in building the GSXR750, a machine built to claim th e title o f World 's Fastest 750cc Motorcycle. The co m pany took a co nve n- 10 su m p, a nd six-speed tra ns m iss io n. T he chassis so u nds eq ua lly co nve n tional , with telesco p ic from fo rk s, a si ng le-shoc k sw i nga r m a nd dual cra d le la you t. Th e o n ly unusu al fea ture o f the frame is the ma terial it 's m ad e of, bei ng a luminum castings a nd ex tr us ions. T he tri ck facing the me n a t Suzuki was to ge t m o re power a nd redu ce weig h t a t th e same tim e, without los ing engi ne reliabilit y or chassis dura bilit y. And in tw o days o f ra cetrack tiona l co ncept - th e inline, tr an sverse. DOH C fo ur- cy li nde r - a nd deve loped i t to meet th eir perfor mance goa ls. T he engi ne's basic speci ficati ons in clude bore and stroke of 70 x 48.7mm, d isplacemen t o f 749cc, a p lai n-bearing cran kshaft . four valves pe r cylinder opened by for ked rocker f1.nJl,s.'Vvb.s,~l;,\ . t:m~~#IAi v~t:~s.) 'I:J • • • ~C;; Lip& Pl .rJ~.ll;tL1Li~ .Qr. j QYWi~ ~ I from a ro u n d th e wor ld , none of the GS X R 750s in the tes t flee t s u ffered a n y fa il ures, d esp ite co nstant revvi ng to 11,000 rpm a nd to -the-sto p s, fu ll t hrott le accelera tion off co rners a nd down th e ultra-long Ryuyo test co urse straigh ta way. T he fact that none o f the bikes blew u p even when su bjected to so me w ha t ham -fisted testin g a t th e track ma kes i t easier 10 bel ieve Suz u ki's clai m th at a p rotot ype e ng in e wa s s u bjected to 30 hours o f wide-ope n d yn o tes tin g a t a co ns ta nt 12,300 rpm wit ho u t failure. Yet the G SXR750 engi ne h as a clai me d 104.5 hhp a t 10,500 rpm , a n 11,000 rpm redl ine, a nd weighs 161 pou nds with air c1eaner/air bo x, ca rburetors, ignition a nd oil. Com pare that 10 th e we ig ht o f the 86-b hp-al -9500-rp m GS750 engi ne introduced in 1983. aga in wit h ai r clea ner/airbox, ca r b uretors , ign iti on a nd oi l: 176 pounds. T he key to red uci ng engine weigh t, Suzu ki e ngi neers sa y, was th e developme nt o f th e Suzu ki Advanced Cooling Sys te m (SACS) , a fa ncy n ame for o il coolin g. T he horsepower devel o p m en t goa l th e co mpany had set for the eng ine - 100 PS (98.6 bhp) -was 100 high fo r stree t bike rel iabil it y wi th air cooling; addi ng wat er coolin g wou ld a llow hig h h o rsepower o utpu t with good street reli ability, but would ad d weight. T he so lu tio n was 10 use engi ne oi l to nOI o n ly lu br icate, bUI a lso cool th e e n g i n e. T he oil does n' t r un th ro ugh o i l j ack et s, a nd th e en gi ne is heavil y fin ned: th e system m igh t m ore acc u ra tely be described as o il-assisted air cooli ng. II works by pumping large q ua nti ties o f oi l throu gh the cy linde r head . but i t' s not as sim p le as that. To m ake th e syste m work, th e en g ineers found th ey had to in cr ease th e size of the cy lind er head a nd make the floo r of th e ca ms ha ft/va lve- trai n cavity wide and fla t. T he oi l is blas ted in to th e ca m shaft/ valve-tra in cavity a t th e ra te o f 2 1 quarts per minute, fro m th e 5.3-qua rt wet sump. T ha t m eans the ent ire con lent o f t he su m p is s ho t throug h the cyl inder head about four times each mi n ute. The huge vo lume of cooli ng oil , which is sprayed down agai nst th e cy lin d er h ead casting fro m eight feed lu bes b u ilt into the cam co ver, swi rls along the floor of th e ca vi ty - separated fro m th e co mb us tion cham be rs bv the min imum amou nt o f m et al need ed - a nd pi ck s up heat , th en runs th rou gh a pa ir of tunnel s d ro p pi ng straigh t from th e front o f the cvlinde r head back int o the cra n kca ses. . If th e large flow o f coo ling oi l ca me in co ntact with th e sp in n ing cams a nd ca m cha in, it co u ld in crease Irict ional losses a nd would foam a nd fro th . T o prevent th at , a set o f baffles bol ted 10 th e floor o f th e ca m sha ft/ va lve-tra in ca vity keeps th e o il from co ntactin g th e m ovin g part s - th e oi l sw ir ls a nd r u ns fo rw ard underneath the baffles. Wh en th e oi l re tu rns 10 the cra n kcase s. it 's picked u p by the sma llervo l u me side of the dua l oil p ump, sent thro ugh a spi n- on filt er a nd a huge. a lu m in u m oil coo ler mou nted in l D nt ;o flLhe l,ame tkm:Dtubes, a nd o I travels th ro ugh a co nven tio na l lubri cati ng system. T ha t system su p p lies o il un d er p ressu re to the cra n kshaft, rocker arms, cams haft beari ngs a nd transm ission. T he m ost exo tic feature o f th e lubricat in g syste m is th e use o f o il j ets set int o th e cra n kcases to sp ray th e undersides o f th e pi sIOns as a coo li ng m ea sure. T he oi l cooler, closer 10 a radiat o r in size th an 10 a n y oi l cooler we've see n before, is ca pa ble o f transferring 8000 kil ocal ori es o f heat p er hour, co m pa red 10 th e GS 750's coo ler ca p a bi li ty o f 1950 kil ocal ori es per hour. T he efficiency o f th e syste m ca n be see n in th e m aximum temper ature m easured in th e spark plug a rea o f th e cy linder head cas ting: 500° F co m pared to a typical a ir- coo led m achi ne's 554° F. T ha t's hi gher th an a typical wa ter-cooled engi ne's 392°, but Suzuk i engineers say that 500° F is lo w eno ugh 10 a llo w good, reli able power o u tp u t, a nd tha t i t's no t worth th e ex tra bulk a nd we ig h I o f a wat ercooli ng syste m to ge t a n eve n lower temperatu re. Severa l recent ai r-coo led Suzu kis routi nel y h it oi l temperat u res o f 320° F wh en ru n ha rd , a nd Suzu ki en g ineers say they ' ve see n oi l tem peratures o f 33 1° F in stoc k GS 750 engines. T he sa me type of test in g with th e GSX R750 engi ne has yie lded a m aximum o il temperatu re o f 257° F, a nd d estructi on -testing of a proto ty pe engi ne a t Iull-throule o n th e d yn o never sa w o il temper atures exceed 275° F. G SXR750 oi l tem peratures in norm al rid in g tests have ranged between 194° F a nd 212° F, co m pa red to a n ai r-coo led bike's 302° F in th e sa me type o f tests, accord ing to Suz u ki engi neers. Whil e th e p rimary goa l o f th e coolin g system development was to a llow hi gh horsep ow er ou tpu t with stree ta ble re lia bility, a n im port ant side be n efi t is th at th e sys tem a llo wed th e weigh ts o f individua l compone nts to be reduced. T he cyl inder h ead . for exa m p le, does n' t have to ha ve as much m ass to transfer hea t from th e co m bustio n chamber a rea to the cooling fins - com pared 10 th e GS750's head - because the oi l cooli ng system carries a way m uc h of the heat. So th e GS X R 750 's cylinder head weighs 4.85 pou nds less tha n the G S750's cyli nder head . T he sa me applies to th e G SXR's cy linder - 2.2 pounds lighter than th e GS750 's cy linde r. The GSXR 's val ve cover is cast m agnesium a n d is 1.76 pou nds light er than th e GS750 's cas t a lu mi n u m cover, a nd th at d iffere nce ha s n o th in g to d o with th e coolin g syste m . But ther e' s an ent ire li st o f o th er co m pone n t part wei ght reductions m ade possible by th e GSXR's lower peak operating temperatures -lower temper atures m ean parts buil t of less m et al will be eq ua lly re lia ble. th e engi neers sa y. The rundown o f parts a n d th ei r rel ative wei ght sa vi n gs (G S750 vs. GSXR750) is impressive: C ra n kcases, 3.3 pou nds; cra n ks ha ft . 2.2 pou nds thanks 10 lig h ter flyw hee ls, a thi n ner p rimary gear and sma lle r main beari ng (fro m 36m m to 3'l m m) an c.! rod .(Irpm 36m m .to 32mm ) jour;