Story and Photos | Hal Armstrong


Stock Racing takes Center Stage

There were two types of racing in the early 70’s. Cross Country and Oval. Winning a major race or a points championship was in the cross hairs of the 100-plus sled builders during this time period. The factory race teams travelled across North America competing at the main events but the grass roots race circuits provided more exposure for the manufacturers. These events were primarily oval races that featured modified and stock racing classes. A typical weekend could see well over 500 entries competing in numerous classes. 

  Stock oval racing allowed the consumer to bring their trail sled to a local race and compete without having to worry about factory pros on specially built sleds. As stockracing popularity grew, winning meant bragging rights for the manufacturer. The focus shifted to building purpose built stock racing sleds built for the oval. Arctic Cat was the first to bring a “purpose built” stock racer to the racetrack in 1972 with the EXT model. This all new chassis evolved into the 1973 Eltigre consumer sports sled that battled for the consumer dollar against the Ski-Doo T’nT, Polaris TX, Rupp Nitro and Yamaha GPX to name a few.  
 
  The hammer dropped in the winter of 1974 when 
Mercury Snowmobile would build the first Sno-Twister for the Stock D (400cc) class. It met all the rules to be considered a stock snowmobile (minimum build quantity, full lighting package, sound compliance, safety standards, etc) but was purpose built for the oval tracks. The result was complete domination by Mercury that changed the image of the brand in the minds of the consumer. Arctic Cat took notice that winter and started preparation of the El-Tigre Z model built specifically for oval racing. Arctic built the Z to compete in the 250, 340 and 440 classes in the winter of 1975. The rest of the competition built sleds for either one or two classes only. Arctic set out to win all three and also garner maximum exposure for the brand.

The Parts Chassis: Something New- Something Borrowed

 The Z was built on the same chassis as the 1975 consumer El Tigre. That meant aluminum tunnel, fiberglass belly pan and hood. It came with some spin offs from the ‘74 SnoPro racers. Most noticeable were the new leaf spring skis and seat design. Purpose built for racing on ice, the ski was more aggressive and lighter in weight. The seat was right off the ‘74 SnoPro. It was designed to place the rider in the optimum position on the sled farther forward over the skis for better turning. The gas tank was reduced to three gallons to reduce weight and dual fuel pickups improved fuel delivery. Handlebar hooks for the left side allowed the rider to hang further off the side of the sled and had just started to become popular in oval racing. When you sit on one of these machines it feels like you sit in the sled and not on the sled. It was the design of the day to prevent the rider from sliding off the back and to provide some body protection. In reality it was extra weight that was not needed. They do look cool though. Dimensionally a wider 29” ski stance and a reduction in the overall height of the sled by 2” from the ‘74 model gave these sleds a low profile. Compared to today’s sleds at just 88” in length and 35.5” in width they were small.

Twin Sparkplug T7 Engine: The last Kawasaki Free Air Engine Powered Cat

 1975 would be the last year that Arctic would use Kawasaki motors. The ‘75 Z would use the race proven T7 version, which first appeared on the 1973 EXT race sled. It was easily noticeable with its twin spark plug cylinder head. The new T7 series engines were designed with increased transfer port area to maximize fuel delivery into the combustion chamber. The problem with the extra fuel was figuring out a way to ignite it before it was forced out the exhaust ports. Ron Roche former Arctic Cat Engineer understood the problem well. “The early CDI ignitions could build voltage very quickly, but the short spark duration meant that if the fuel delivery in the combustion chamber did not present an ignitable mixture to the spark plug at the moment it fired, mis-fire would be likely. Single cone expansion chambers of the day were not as efficient at forcing a fuel charge back into the combustion chamber as those found today, which could further induce mis-fire. Consequently ignition timing was even more critical. The T7 motor featured twin spark plugs per cylinder to double the opportunity for “lighting” the fuel charge. This increased the potential for more complete combustion and also allowed the ignition timing to be run more retarded than was normal at that time which generated less engine heat allowing the engine to run cooler and improve durability. 

  Three versions of the T7 were available on the ‘75 Z to compete in the three stock classes of the day. A 245 cc, 339 cc and 436 cc. All featured twin Mikuni (VM) slide valve carbs, double ring pistons and high compression heads. A new cold air intake box to route cool air from outside the engine compartment increased performance as well as providing a constant volume of air to the engine. Tuning the intake tract of a two stroke was new territory for the manufactures at the time, but the emergence of intake boxes not only reduced intake noise but increased engine performance. Twin expansion chamber exhaust was a first for a production sled from Arctic. To meet stock snowmobile noise regulations the expansion chambers dumped exhaust into a resonator. This design was used in later production models built for trail use.

Track Suspension: The 1st Generation Adjustable Track Suspension

 Legendary Arctic Cat engineer Dennis Zulawski had worked on the ‘74 SnoPro racer and had begun to learn the nuances of building a track suspension. The Z featured an aluminum slide rail that was designed for transferring weight to the rear of the machine for better acceleration off the starting line and improved weight transfer when cornering to shift weight onto the skis. This was the 2nd generation slide rail suspension from Arctic that was 30% lighter than the traditional Arctic Slide suspension that was standard on all Arctic Cat models. 

  The two biggest concerns for flat track racing are to accelerate as fast as possible and to be able to run through the turns tight with the power on. The weight shift of the driver greatly effects the way a suspension system reacts when coming off the line and cornering. This system on the new El Tigre Z was the secret weapon to give Team Arctic racers a huge advantage. The increased adjustability meant educating the driver and mechanics on all of the nuances of the adjustments, which would affect the sled handling. 

 
  The Z suspension featured two extra long torsion springs to control preload. A new rear arm assembly was adjustable on the rail. With the rider shifting their weight to the rear of the machine the suspension would collapse slightly allowing more weight transfer to the rear of the track allowing the skis to lift off the snow. When the rider shifted his weight forward, the rear of the suspension would unload transferring more weight onto the skis. The adjustability of the rear arm allowed the location to be set for a particular rider weight. 

  The front arm was also adjustable. Limiter straps were not used like today. Instead each front torque arm could be positioned in numerous locations on the rails. Moving the arm forward increased track pressure, reducing ski pressure. Moving it towards the rear reduces track pressure and increased ski pressure. 

  A unique front arm stop made of nylon had a slot through it that the front torque arm shaft ran through. Increasing the length of the front arm stop increased weight transfer front to rear. Shortening it had the opposite effect. Noticeable was the lack of any shock absorbers. All movement was controlled by the extra long torsion springs. This was one of the first track suspensions that entered the world of front and rear adjustability. The Z suspension was not designed to run on a trail. It would have bottomed out continuously. On a smooth racetrack this was a simple lightweight first iteration of a fully adjustable track suspension. The balance between weight transfer for acceleration and cornering had just begun. 

  The track on the ‘75 Z was a full cleat track as opposed to the 2/3 cleated track design used on the consumer Cats. The track was an involute internal drive, a departure from the external cleat drive used in the past. The track length was 112” in length.

Clutching: The Hex Drive

 Power to the track was through the Arctic Hex primary and reverse cam secondary. Arctic Engineer Keni Prasad designed the Hex clutch. The Hex shaft clutch separated the weights from the ramps. Similar to the Ski-Doo TRA clutch the Hex clutch used roller arms to ride against ramps built into the cover of the moveable sheave. The ramp profile controls the shift pattern (up shift and down shift) separate from the roller/ flyweight arm combination, which controls the engine rpm throughout the shift pattern. Torque was transferred through the hex bushings. The Hex Clutch has incredible versatility and adjustability. There are just so many combos of ramps, weights, springs and rollers, which provide unlimited combinations. This easily tunable clutch was used by Mercury on the Sno-Twister as well as Rupp and Sno-Jet. The Hex clutch would be used for 20 years by Arctic.

Summary: Did it Win?

 Looking back, the ‘75 Arctic Cat Z was the last free air Kawasaki powered sled built by Arctic. In 1976 the new free air and liquid cooled Suzuki engines would appear. The Z would also feature the first generation adjustable slide rail suspension that allowed adjustment to the front arm and rear arm to adjust weight transfer front and rear to improve handling and acceleration. 

  Arctic built just less than 4,000 of the sleds. The race results according to the 1975 Snotrack Racing Yearbook revealed that the Z was most competitive in the 250 cc and 440 cc class. While Arctic had a good season, Roger Skime at Arctic Cat was not satisfied. The sled was down on power and even with the new Z track suspension, the narrow ski stance and higher center of gravity prevented these sleds from railing around the corners fast without significant inside ski lift. The ‘76 Z would bring liquid cooling and a new wider lower chassis but the class of the field was the ‘76 Mercury Sno-Twister. Arctic’s turn to dominate stock racing would have to wait till the 1977 season. The ‘75 Arctic Cat Z is the original “Race Special sled“ built for winning on the racetrack. The legacy continues today with the ZR race sleds for Snocross and Cross Country Racing.

   
  Have you ridden the vintage or new model of the El Tigre? Tell us your story on Facebook or Twitter!

 

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