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(Left) Russell savory is the brain. behind the project. 20 need more stopping power. The way you can get the rear brake howling in protest when you work it hard with the present setup only confirms this. Twice as much power using a stock cylinder head, with no weight penalty or throttle lag - all this didn't come about overnight, and the Tramontana is the result of two years of continual development, some of it frustratingly abortive. RS began by sourcing the smallest commercially available supercharger, made by OPCON in Sweden - a twinrotor design of which 10 prototypes were built: three for the RS Yamaha project, one for Harley, and the rest for Honda for use on a 660cc microcar project. ow tha t the unit has gone in to series production, Savory is planning to build a small series of Tramontana replicas for around 10,000 pounds each, based on a new XTZ donor bike. Or, of course, the same five-valve engine also powers the MuZ Skorpion and Yamaha's own Italian-built SZR660 roadster... Having sourced the blower, the next step was to decide on a drive system for it, and after four or five failures, RS hit on the system now employed, which sees the supercharger driven by toothed belt off the engine's gear-driven counterbalancer. This has the added advant~ge of employing a cush drive to smooth ou t the gear shock from the pulsing effect of a four-stroke single that fires only once every 720 degrees of crankshaft rota tion. "This is the reason it's so hard to turbocharge a single," Russell says, "because you only get one exhaust-gas pulse every two engine rota tions, so there's not enough to drive the turbo unit. Supercharging a single is equally difficult, which is probably why nobody ever did it in the prewar days - you have to wait so long between bangs, it causes terrific shock to the drive syste.m. We had lots of failures because of this. It took months of trying different tapers, different keyways, alternative belt designs and moving the drive about the engine, until we eventually got it right. Toothed belts would tear the.mselves to shreds at about 3000 rpm - even the HDT ones used for oil pumps on Formula One car engines. Eventually, Uniroyal came up with their PolyVee belt used on high-torque pumps for power steering, and this gives a bit of slippage, which solved the problem. We called this the Le.mon Engineering project - suck it and see, spit it out if it didn't work, ·then try something else till you get it to work. And eventually, we did." Running the supercharger at 1.7 times engine speed was a conservative step aimed at eliminating this problem, even if such a slow blower speed is at the outer edge of OPCON's performance envelope. But together with the more precise fuel metering of the R5developed Weber-Alpha fuel-injection system, this has resulted not only in reliability but also in amazing fuel economy. Savory says the Tramontana's fuel consumption' is almost identical to a stock, carbureted XTZ - the EFI's increased efficiency compensates for the extra thirst of the supercharged engine, in similar conditions. A vital function is performed by the little black intercooler box on the righ t side of the engine, which acts as a plenum chamber, cooling the incoming air before it's mixed at the very last moment with fuel. The box is internally baffled, and its volume and design are critical factors in delivering performance, as well as acting as an air damper to resolve the problems of the shock from the 720degree firing pulse. Developing the EFI for the supercharged application was also a hit-andmiss affair, admits Savory - though he was helped by the work he'd done on the same engine for his Sound of Singles racers, so he knew where to place the sensors and had an idea for the basic map before compensating for the blower. The syste.m he's ended up with has a throttle body behind the steering head that sucks air down int~ the supercharger, which then compresses it, feeds it to the baffled plenum chamber, where it is cooled and stabilized before passing through the manifold to the ports. A balance pipe between the plenum and manifold maintains constant pressure, while an oil feed delivers engine oil to the top. of the supercharger. After running through it, it drains out the bottom and thence into the bearing housing fitted to the crankcase. There's no need for extra oil capacity, nor even a cooler. Instead, the XTZ's liquid cooling copes fine, and even.on a hot day, the fan hardly came on at all even when I was sitting in traffic. Not a turbo. The main problem was where to stick the twin injectors: Dyno testing a variety of different .locations delivered the chosen option of fitting them both in the cylinder head, one feeding each of the pair of inlet ports of the five-valve head, with the fuel squirting in very late just before the valve. That delivered a 5-bhp increase all through the powerband. But though a bigger 52mm throttle body delivered even more power, it did so at the expense of bottom-end response and ridability - hence the smaller 40mm one fitted now, which delivers magical, meaty response low down: The Tramontana will pull any gear you throw at it. Perhaps that's the biggest surprise about riding it: Here's this point-andsquirt street rod, with the performance of a turbo but the immediate throttle response of a fuel-injected superbike, that's tractable and user-friendly enough to pull hard from low revs or trickle around town off the boost. What a trick - the best. of both worlds. I bet it would be easy to adapt this concept to off-road conditions, where you could take real advantage of all that torque to get performance without sacrificing agility on sand or loose surfaces. How about a supercharged four-stroke single for Open-class MX racing - or the ParisDakar? Don't laugh - after riding the Tramontana, I know this is a wind. that could blow hot in Africa, too. It's also a new window on performance street tuning - let alone a roadracing application where rules permit. Volume bike manufacturers need to take a fresh look at supercharging with an open mind, because Russell Savory's dedieated development - assisted by Tony Hart (brother of Fl engine man Brian), who made all the drawings for the project - has proved that it has real potential for production appliea tion. The Tramontana I rode is the only one out of the three prototypes that RS built that will remain in England, and it's been sold to the same customer who's commissioned the supercharged YZF four. But Savory has another application in mind. "I really want to supercharge one. of Yamaha's new TRX twins," he says. "The engine has all the potential to be a great package for this application - and there are no rules against supercharging in BOTT racing, either! I'm a Yamaha man through and through, and I reckon this would be the most exciting and logical application for this technology. I hope someone comes and asks me to make one. I'm ready!" L~ RS Yamaha 660 Tramontana Specifications Engine .. Uquid-cooled SOHC five-valve single·cylinder four-stroke w/ gear·driven balance shaft ~re " stroke . . . . . . . . . . . . . . . . . . .. Displacement . Output Compression ratio Fuel/ignition sy_m Gearbo" . . . . . . . . . . . .. Clutch C is Su.penslon Front Rear Wheelbas. .. Weight Brakes Front Reer Wheels/ti _. . . 659cc 72 bhp at 7200 rpm (at gearbox> , 9.0:1 Weber Alpha engine-management system with electronic Ignition system and fuel injection with 40mm throttle body and twin injectors. and belt·driven OPCON twin-rotor supercharger ............. . 5-speed . Multiplate wet w/ heavy-duty springs Tubular steel open-cradle AO.mm Yamaha telescopic forks modified by Maxton Rectangular steel swingarm with Quadrant shock and rising-rate linkage ......... .............. . ... 57.9 In. 385 lb. dry ....•..•.......... .Dual 282mm Yamaha discs with four-piston calipers .. ' ... _.Single 220mm disc with two·piston caliper . Front Rear Year of construction . . . . . . . . . . . . . . Owner . .100 x 84mm 120/70-17 Pirelli Dragon on 3.50-ln. Marvic wheel 160/60-17 Pirelli dragon on 5.50-in. Marvic wheel . 1993 RS Performance. Harlow. Essex. England.