The high-performance trail class of snowmobiles saw an increase of liquid-cooled sled for the 1976 model year. The added cost and weight, among other factors, previously kept liquid-cooled engines absent from economy or family-based sled.
But liquid-cooling offered a suitable alternative to fan-cooled and free-air engines in high-horsepower snowmobiles where price was not usually a big factor in the buying decision. Like today, high performance buffs wanted the latest, quickest, most modern sled, and many turned to machines with liquid-cooled mills for that fix.
Rupp Industries jumped into the liquid-cooled fray in 1976 with its speedy Nitro 440. The sled was a successful result of mating Rupp’s proven suspension and chassis to a liquid-cooled engine package from Xenoah, powering the trusty Arctic Cat El Tigre torque convertor clutch system.
The new hybrid, carrying the Nitro name, reserved for Rupp’s top–of-the-line trail sled, more than sufficiently replaced the free–air ’75 Nitro, according to Snow Goer editors.
“Available only with liquid-cooling in 440 and 340 engine sizes, the new Nitro boasts even more hot throttle response, even lower noise level and even greater turnability,” editors wrote.
In ’76 Rupp purchased Alouette, a Canadian snowmobile manufacturer. As a result, the same engineering that showed up in seven Rupp sleds for ’76 were available under the Alouette banner north of the border.
The 1976 Nitro engines for the first time were built to Rupp’s specs by the Japanese firm, Xenoah. Xenoah was then the parent company of Fuji Heavy Industries, which had formerly supplied Chaparral engines before it pulled out of the snowmobile business.
In its last season of production, Chaparral introduced a 1974 liquid-cooled, high-performance sled called the SSX in 340 and 440 versions. The roots of the Nitro engine stemmed from that design, but with two additional years of R&D.
The new Xenoah engine carried dual38mm Mukuni carbs with adjustable main jets, calibrated at 1,000 to 2,000 feet. This allowed riders to tune for variances in either altitude or temperature without disassembling the carbs to change the main jet. All it required was a screwdriver to change the carbs to the proper setting.
Despite the 38mm carbs, the Nitro got good gas mileage for its time, according to Snow Goer editors. “In Snow Goer economy runs,” they wrote, “the Nitro managed 13.3 MPG, a step above most 76 sleds delivering comparable performance.“
But Rupp buyers were sledders with performance foremost in mind, and the ’76 Nitro delivered just that.
In Snow Goer speed tests, the sled accomplished a standout 80 mph in the 1/4-mile speed trial, best of all the 26 test sleds in any category – new models, high performance or most popular, according to editors.
Rupp signed an agreement with Arctic Cat to use Cat’s roller ramp drive clutch, plus its driven clutch and the same cross-sled jack shaft, complete with aluminum chaincase equipped with self-adjusting chain tensioners. “This design allows for improved weight distribution, lower center of gravity and better transfer to the clutch,” editors wrote.
Unlike the liquid–cooling systems found on the ’76 Massey Cyclone and John Deere Liquifire, Rupp used an automotive-type radiator for lowering coolant temperatures without depending on snow conditions.
Instead of taking advantage of a separate heat exchanger and the relatively sealed engine room that becomes possible, the Nitro used a radiator core in the console. This required air to pass through the hood and out the cowl below the handlebars, bringing noise level with it. “The Nitro at 80.8 decibels was the second loudest sled tested,” editors wrote.
For 1976, the Nitro’s entire front end was overhauled. The engine was positioned to allow the spindles to be pulled in for easier running through tight trail maneuvers. Also, the ski stance was increased, and the spindle was altered to provide easier steering.
Like engine performance, its handling was impressive, according to editors.
“This sled comes on with incompatible acceleration, yet handles like an absolute dream – very stable and predictable,” editors wrote. “Everyone who rode this steed agreed the sled instills incredible confidence in the rider at the controls, even at top-end, eye-watering speeds.“
Rupp also moved the whole steering shaft forward a few inches to enable the rider to slide up and ride more forward if desired. Combined with new-angle adjustable handlebars, the new arrangement provided a wide range of rider options, regardless of stature. “Whether you have short or long arms and legs,” editors wrote, “the new Nitro will get along with nearly everyone.”
Its comfort stemmed from a rear suspension system that was equipped with aluminum extrusion slides. The unit allows full flexibility in controlling ski/track pressure in the snow,” editors wrote. This was done by either adjusting the rear torsion spring or by adding or subtracting wedges at the front, which control the amount of lift under acceleration.
Aluminum slides ran over a Goodyear track. To extend the track‘s life, Rupp added a set of idler wheels to the front of the slides.
The Nitro’s sleek styling was also a hit with owners, and its side running light an exclusive Rupp offering, set the sled apart from others after dark.
In The End
But despite its speedy reputation, the Nitro was not intended for the race track. For the racer, Rupp offered the Magnum, a liquid–cooled sled with a shorter tunnel, a new swing–link slide rail design for improved weight transfer and smaller race skis, among other components.
Snow Goer editors concluded the ’76 Nitro was not a “high-performance” sled, rather an “ultra–performance sled.”
“When you shut down this sled,” they wrote, “you have no doubt that the new Nitro is what a performance sled should be: nimble, quick, well-mannered, comfortable, reliable and extremely slick-looking.”
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