1974 Mercury Sno-Twister
When the history of snowmobile racing is written there will be a select few sleds that will be recognized for totally dominating the competition. The 1974 Sno-Twister was just such a sled. It arrived at the time when Mercury Marine had decided to go racing after manufacturing sleds that had not established the company as a leader as they were known for in the outboard motor industry. The Twister racing outboards struck fear from their arch rivals OMC and were known for developing big horsepower from inline 4 and 6 cylinder motors. On the snow, the Mercs were for the most part “boat anchors” with two skis and a track.
The Sno-Twister was the complete opposite that would turn the image of Mercury snowmobiles up side down and literally overwhelm the competition for the entire season. The Twister story is also as much about the people that were brought together to bring the project to life as the machine.
It is astounding to think that Mercury in the early 70’s did not have a high performance engine lurking in the Advanced Research Dept. When upper management decided the time was right to go racing in mid 1972 the task ahead was daunting. Not only did they not have a motor, the project needed a new chassis, clutching and a track suspension. We’re talking a complete new snowmobile. That was just half of the task facing Mercury Management. The second part was recruiting the people that could pull it off.
Dr. Ted Morgan, Director of the Advanced Research Group at Mercury knew a lot about outboard engines but not much about racing snowmobiles. His first recruit was Lyle Forsgren. Lyle was working at Rupp Industries when the call came from Mercury. While at Rupp, Lyle was responsible for developing the Magnum race sleds raced by Snowmobile Racing Hall of Fame member, Gene Bloom. Lyle Forsgren’s short stint in the snowmobile industry was predecessed by over ten (10) years of building open wheel race cars. Forsgren was a mechanical engineer who worked at Boeing in the 60’s during the day and built his own open wheel race cars at night. The Forsgrini chassis would become so successful that he left Boeing to build his race cars full time. Forsgren was a rare talent indeed. Not only could he design, he was a machinist and fabricator. A “hands on” engineer, and perfect to lead the Sno-Twister program.
With the project leader now in place, Morgan went looking for an engine development leader. Les Cahoon was hired right after completing his Ph.D. from Queen’s University in Belfast, Northern Ireland. Queens University is recognized as a leader in internal combustion research. Les Cahoon’s Ph.D. study was focused on computer modeling of gas flow dynamics in a two-stroke engine. This work was years ahead of its time. Les brought science to the art of two-stroke engine porting, intake and exhaust design.
The “Brain Trust” was in place to lead a small group of people working in the Advanced Research Group on the Sno-Twister program. The new Sno-Twister was to be built with a 400-cc engine to compete in the most competitive and popular stock racing class, Stock “D”. The program mandate was to build a sled that was a winner on the track but capable of being a solid trail sled, just in case the chequered flags were hard to come by. The deadline for a machine ready to roll down the production line was early September 1973. Lyle Forsgren started work in October 1972
Chassis Development
Forsgren established basic goals that the new sled design would need to meet. They were;
a) Engine placement forward over the skis.
b) Wide ski-stance for stability
c) Small track contact patch
d) Light weight
e) Adjustable slide suspension capable of good weight
transfer for acceleration
f) Not too radical of a design, the sled had to be sold as a trail sled just in case.
To improve cornering stability a 4130 chrome molly tubular front end located the ski spindles 35” apart. The spindle caster angle was set at 30 degrees to improve steering stability at wide-open throttle. To further improve high speed cornering a track contact patch of just 32” on the snow reduced the potential for understeer. The increased weight over the skis by placing the motor just behind the tubular cross member created the near perfect amount of balance between the force generated by the skis turning in a corner and the push of the track. Pictures of the Twister in competition with the SnoPro mod sleds in ’74 show the Twister’s carving the inside line as well as the factory specials.
The drive train layout featured a gun-drilled jackshaft to reduce rotating weight. The use of a jackshaft between the secondary clutch and chaincase was a new concept found on factory race sleds. The chaincase was cast from magnesium, which again was common on factory race specials of the day.
Other features of the chassis were an aluminum tunnel and a stamped aluminum steering post /dash assembly. The fiberglass hood enclosed the free air engine with an offset headlight to maximize the size of the air intake. The belly pan had a scoop molded into the front to direct cold air directly onto the crankcase. Every design feature was built into the sled for a specific purpose. That included a 6-gallon gas tank and a track suspension that provided a comfortable ride for trail use. The chassis was built extra strong, just in case the sleds were not a winner on the track. Sled sales were in a slump in ’74 with the oil crisis and 1000 Sno-Twisters (which was the minimum build quantity to meet stock classification rules) would have to be sold to trail riders just in case the sled missed the mark on the track.
The track suspension also designed and patented by Forsgren was one of the first rising rate designs to hit the snow. The suspension used a double bell crank arrangement that generated a 25% increase in the spring rate as the suspension compressed. The front arm also used a bell crank that operated the front track shock. Again, a rising rate design. The other notable feature was a linking arm that connected the front pivot point to the center of the slide rail. As the suspension compressed the front link arm effectively allowed the front of the suspension to communicate with the rear of the suspension. Dual coil over shocks at the rear of the suspension and single front shock with dual torsion springs smoothed the bumps.
The suspension did have a notable design flaw. The rails were flat with no front curvature. A set of bogie wheels mounted on the front of the rails were prone to falling off, resulting in the rails jamming the track.
The track was also a Forsgren design. Forsgren went with an all-rubber track with an involute drive. The slide rails rode on steel flat bars molded into the track. The steel bars were exposed to the snow to better dissipate heat. The rubber track was another unique feature to the Twister, as the common track design of the day (for oval racing) was a steel cleated track, which was not great on ice. Forsgren developed the all-rubber track in conjunction with Goodyear.
Engine
When Les Cahoon started working at Mercury in January 1973 he had less than 9 months to specify a motor for the Sno-Twister. Coming from Ireland he had no concept about snowmobiling but he knew engines. Mercury at the time was building two strokes for their trail sleds. The most powerful was a 644cc twin producing 50 hp @ 6400 rpm. The Advanced Research Group also had a 6-cylinder inline 440cc motor in development that spun at 13,000 rpm. Les Cahoon quickly dismissed this engine and started looking at the aftermarket to see what was available. At the time the choice was down to two suppliers. Kohler and Sachs. The choice was made to go with the Kohler 340cc crankcase. Les Cahoon would put his engine modeling formulations to work using the big mainframe computer at Mercury. The results would be the cylinder porting dimensions, port timing and combustion chamber design that would be sent to Kohler to build the 65mm chrome bore cylinders. The heads would feature the Kohler starburst cooling fins that maximized cooling surface area.
The engine also would feature twin expansion chambers designed by Cahoon that would dump into a resonator to increase back pressure and still meet the noise standards of the day. This feature was unique to a stock production snowmobile at the time. Only the Yamaha GPX would feature twin pipes in ’74. Again using the computer at Mercury and his ability to program the equations to calculate the pipe dimensions, the lead time to develop was substantially reduced.
Also key to producing the most powerful 400cc motor was the design of an intake box for the 36mm Mikuni Carbs. Again using computer modeling, the intake tract length of the air box was quickly figured out to return every drop of fuel spit out the intake ports back into the engine. The airbox was also designed to ram cold dense air from the front of the machine. The colder the intake air temp, the more horsepower an engine can make (if jetted properly).
The Kohler engine package with the Cahoon intake box and twin pipes would power Sno-Twister riders all across the snowbelt to more chequered flags than all other manufacturers combined. Lyle Forsgren sums up the Twister engine program. “The trick to winning races is to make a leap forward without making it too obvious. The Cahoon calculations were all buried deep inside the engine. When the hood was lifted there was a whole lot of head scratching. What was the secret?”
Summary
The original Sno-Twister dominated Stock D like no other sled before it. All across Canada and the USA, Mercury drivers claimed championships. At the 1st race of the season in Ironwood, Michigan, the new SnoPro circuit was getting underway. The Sno-Twisters were overshadowed by the SnoPro machinery circus. That was until the Stock D final was run. A total of 160 sleds competed in this single class to see who would make the final. On Sunday the ten sleds that lined up for the green flag were all Mercs.
After the Ironwood race, Lyle Forsgren and Les Cahoon loaded up the station wagon and headed back home to begin work on the ’75 Twister. The Sno-Twister would be the new benchmark race sled for the competition and would rejuvenate interest in stock racing for many years to come.
By Hal Armstrong
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