Cycle News - Archive Issues - 1990's

and about to enter limited production in a car being launched later this year. Or the six-speed EXUP engine powering the Rocket street-legal cyclecar designed by. Gordon Murray of Mclaren Fl fame. Or the fuel-injected XTZ660 five-valve single which powered the first works MuZ Skorpion racer back in '93, and its twincam four-valve successor, the RS660, which debuted last season. Or - well, you get the picture: Russell Sa vory has an inquiring mind, which is why when someone (he won't say who) came to him in 1993 with the idea of developing a supercharged single, it was a project that immediately appealed to him. Two years later, the fuel-injected Yamaha XTZ66D-based RS Tramontana (hot wind - geddit?) is the living proof that supercharging works. If you ever thought this was an old-fashioned concept, better suited to those prewar days of yesteryear when the Gilera Rondine four or BMW Kompressor twin vied for GP supremacy with the blown AJS' V4 or the stillborn Velocette Roarer, think again. Supercharging may have been banned after World War II in keeping with the new mood of postwar austerity, but its time has come again - and the first time you twist the wrist on Savory's supercharged street rod, you'll be convinced. The figures say it all: The stock Yamaha XTZ660 delivered 37 bhp at the gearbox, fresh out of the crate, on the RS dyno, whereas the very same engine in the blown Tramontana now churns out 72 bhp on the same dyno - all without any conventional tuning work like gas flowing or porting the head. Camshaft, valves, etc., are all strictly stock, and compression has actually been reduced to 9:1 by using a forged Omega piston. That piston is mated to a Carrillo rod and welded-up crank, just for peace of mind with all that extra power. It's supercharging that makes all the difference. (Left) Super sleeper: The clean packaging of the supercharger plumbing gives few clues as to the power that lurks within. (Above) Though derived from what was essentially a trail bike, the Tramontana has reworked suspension and brakes, as well as Marvlc wheels and Pirelli Dragon tires, to bring it up to street-rod snuff. (Right) Driven off the counterbalancer with a toothed !?e't, the diminutive supercharger - along with fuel Injection and electronic engine m~nagement • has almost doubled the stock bike's 37 bhp without any loss of ridability. The result is a bike that has to be the quickest point-to-point machine yet conceived for city streets or country roads as well as a wheelie hound par excellence. Gas the Trammie up at the lights, hear the whine of the supercharger a bove the fruity-sounding Gibson exhaust, drop the clutch, and watch the boost gauge in the left fairing flank sweep around to the right as you make everything else on the road eat your dust. Better still, cruise around off the boost, looking for prey - like a Porsche turbo or Honda CBR600. Line 'em up, twist that wrist, and ride the waves of power from 4000 to 6000 rpm - and beyond. Max torque is at 4900 rpm - but, says Savory, the only thing standing in the way of more power is the rev-limiter. "The power curve is almost diagonal," he says matter-of-factly. "And though we've got the limHer et at 8000 revs, compared to 7200 rpm with the stock XTZ, thanks to the stronger rod and welded-up crank, if we were brave enough to set it higher, we'd have more power. We've never had a single problem with the crankcases or bottom end in all the development cycle, so with some proper tuning and a decent camshaft, there's no doubt we could get 100 bhp from this engine. And remember, this is 'just a single.''' The supercharged YZF750 (punched out to 835cc), the next application of the RS blower technology, will max out the concept powerwise, but while the result will certainly deliver outrageous bhp, it's unlikely to be anywhere near as ridable as the punchy, potent Tramontana single. The stock XTZ chassis package is much tauter and streetworthy than its dual sport derivation leads you to expect, thanks to stiffer springs in the revalved forks, courtesy of Maxton, and a Quadrant rear shock that gives racelevel suspension response with an altered link. NWS modified the frame slightly to make room for the blower, and 17-inch Marvic wheels are fitted at both ends, shod with Pirelli Dragons. This is serious street-rod spec, and the result is a nimble, fast-steering, taut-handling street single with unbelievable urge. And no throttle lag: A turbo the Tramontana isn't - instead, there's instant response from the light-action twistgrip and, if you get hard on the gas above four grand, it's the arm-yanking variety, so hold on tight! Only the brakes need attention. Look closely at the pictures and you'll see two front discs but just a single caliper. The downside of being the first person to ride the bike outside of RS Performance or the company who commissioned the project is that Russell hadn't yet gotten around to fitting the second caliper he'd just sourced. Though the Tramontana actually weighs an incredible 11 pounds less than the stock XTZ660, in spite of the addition of a supercharger (the total blower package weighs only 15 pounds, which is offset by stripping off the trail-bike hardware), it goes so much faster you 19

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