Cycle News - Archive Issues - 1990's

.' R iOINGIMPRESSION By Alan Cathca rt Photos by Kyo/chi Nakamura t was only a matter of time before the modem era of engine-management systems and electronic fue l injection arrived in the 600cc supersport arena, and equally inevitable ' that it would be Bimota tha t ushered it in - just as they did a decade ago in the Superbike class. With their latest four-cylinder 600cc sportbike, the YB9SRI now entering production equipped with their own EFI developed in conjunction with TDD, and with the up-rated YZF600RYamaha engine from th e ThunderCat, Bimota has raised the stakes not ju st in the showroom but also in su persport racing and the grand prix world's spinoff Thunderbike class. Yet in doing so, Bimota has also produced a road-legal homologation special that is actually more user-friendly on the street th an the carbureted bike it replaces, thanks to the increased midrange torque, as well as the more responsive pickup of the EFI and extra power it delivers all the way through the rev range. Based on a day' s ride through the Marche hill country behind Bimota' s Rimini base on the first YB9SRI off the production line, I'd say the 600cc street class has a new benchmark sport- I Bimota YB9SRI bike - albeit at the premium price commanded by the Bimota name. A handbuilt, limited-edition, hard-nosed race replica with the extra sophistication of EFI, the YB9SRI sells in its Italian home market for 25,700,000 lire (approximately $16,000), compared to its carbureted YB9 sister at 22,600,000 lire ($14,000) - or a Yamaha ThunderCat at n ot much more than half that. Superbike prices for a tricked-ou t supersport. Still, a large portion of the first batch of 250 fuel-injected Bimota 600s currently being built will certainly end up on the race tracks of the world , especially to challenge the dominance of the Ducati 748SP at European Champi onship level, where the desmo V-twins mad e full use of the combined capaci ty and weight advantages accorded th em und er the rules in effect last season . But in 1996, with the 374-pound minimum weight for all bikes, the Ducati s won 't hav e it so easy - and of course Thunderbike racing , only permits 600cc fours, anyway. That being the case, Bimota's newl y signed Australian w orks rider Fr ed Bayens - fourth in the '95 Thunderbike series and second in th e '94 European Supersport Championship - mu st have a great chance of earning Bimota the GP circus four-stroke crown this se ason. And after defeating th e works Ducat i team to win the ' 95 Italian 600cc title against the odds on the factory Bimota YB9 carbie, Michele Mala testa and team mate Moo Gallina (son of former 500cc team manager Roberto) are bound to be_ serious contenders for the European , Supersport titl e with their new fuelinjected YB9SRI Bimotas. Junior Superbike racing? Sounds like it. Concocting the YB9SRI has entailed much more than simply plugging' in a ThunderCat engine to replace the old FZR600 mo tor and wa ving a magic wand over it to add fuel injection . However, while retaining the same basic format as th e previous 62 x 49.6mm FZR , de sign (chassis pickups are the same as befo re, fo r example, unlike on the YZF1000 ThunderAce, and engine castings remain unchanged), the Cat engine uses forged pistons instead of cast ones for improved reliability at high revs, and for the same reason has an all-new crankshaft made of improved material to cure the high-rpm crank failures su ffered by some Yamaha supersport. The new crank also has light er, smaller webs for improved engine acceleration, which is helped too by a smaller ignition rotor. The 'result delivers 100.5 bhp at the crank, at 11,500 rpm on the Yamah a ThunderCat, fitted wi th 36mm carbs - whereas without any mechanical changes (same camshafts, same 12:1 compression ratio, etc.) the same engine delivers 106 bhp at the crank in Bimota guise, running a grand higher at 12,500 rp m, and translating to 78 bh p at the rear wheel. The difference comes from the TDD / Bimota fuel-injection package, with four 44mm throttle bodies, a single , injector per cylinder, and a high-pressure Bosch fuel pump running at 3 bar . To control all this hardware is an ECU mounted on the rear subframe beneath the seat, with a total of five sensors, .rnonitoring coolant temperature, ambi- . ent air temperature, air pressure, thro ttle position and rpm. Six percent more power at the top end is a hefty improvement that alone would justify fitting EFL But the ben efits are there all the way through the rev range. There's 51 rear-wheel bhp from the inj ected motor at just 8000 rpm, compared to 47 bhp in same-day dyno testing from a carbureted engine, with corresponding improvement in torque from 41.5 lb-ft. to 44.3 lb-ft. at the same revs. This tr a n sl a tes to noticeably improved midrange pickup when you rid e the bike - the kind that mad e me want to compare dyno sheets when I got back to base . Add in the better response coming off a closed or partially open throttle, and the benefits of the EFI start

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