Sled summit. (Ski Doo's 1998 Summit 670 snowmobile)(Evaluation)(Brief Article)
Author/s: Rick Shandley
Winter is a time when many off-road enthusiasts roll their 4x4s into the barn to build, modify or repair them in preparation for spring. It's also a time when folks living in the snow country pull out their off-road machine: their snowmobile. As this winter gets underway, we thought you would enjoy a peek at the 1998 Ski-Doo Summit 670.
The Summit 670 is a high-performance mountain sled developed with new, lightweight materials and motivated by a liquid-cooled, 699 cc Rotax two-stroke engine. The Summit 670's Rotax rotary valve engine is pretty much an exclusive in the snowmobile market. Rotary valve engines use a gear-driven, rotating disc to control the flow of air/fuel mixture into the crankcase. This system boosts the three-cylinder powerplant's performance at high rpm by directing a greater velocity of air/fuel mixture into the crankcase. The air/fuel induction comes from a triple set of Mikuni VM-40 carburetors.
When riding this machine, you don't have to be concerned with when or how the power will it roll on, because it pulls hard throughout the broad powerband. The powerband is not limited to an "ideal" altitude, either, because the '98 Summit 670 features a High Altitude Compensator (HAC), which mechanically adjusts the air/fuel mixture as the altitude changes. Bear in mind that the engine (any engine) loses three percent of its power for every 1,000 feet of altitude ascended.
This snowmobile is a sled for deep powder pushers--a true mountain machine. The Rotax mill ponies up 116 horsepower at 7,800 rpm, enough muscle to carry the front left ski for 20 yards from a full-throttle, standing-start lunch. Mountain riders can point this electric-blue snow rocket straight up a mountainside in what snowmobile enthusiasts call "high-marking." Deep, virgin powder can slow down a lot of machines, but the on-tap throttle response of the Summit 670 allows you to blast your own trails, with the confidence that you'll stay on top of the snow rather than slugging your way through it.
Transmitting power from the engine to the fully automatic transmission is a Total Range Adjustable (TRA) clutch assembly. A centrifugal-force clutch transfers engine power to the transmission in addition to keeping the engine from over-revving at high-speeds. The TRA clutch system allows riders to adjust their top engine-operating speed in 200-rpm increments, without any disassembly.
Stopping power comes from a single Bombardier hydraulic disc brake connected to the driveline. Brake fluid is pumped into dual pistons on each side of the disc to ensure smooth, progressive braking. Routing the brake fluid plumbing outside the brake housing reduces high metal-to-metal temperatures.
New features on the '98 Summit 670 include the Super V track with 2-inch lugs. These lugs are designed to keep snow underneath the track for better flotation, and the taller lug design acts to "shovel" snow out the back of the machine. The new, abrasive running boards feature traction holes that give the rider a solid foothold on the machine, even when wearing heavy snow boots. The new front suspension is narrow (a 37-inch ski stance) for sidehill bite and whisking through tight places. New, wide all-plastic skis have a slight rocker at the keel to reduce pushing through turns and provide better bite. Because of the use of lighter materials, the '98 Summit weighs approximately 550 pounds.
We found the Summit 670 to be highly charged with performance, comfort and
deep powder ability. On the snow flats, this machine accelerated hard and
straight. Weaving through the trees at slow speeds, with deep snow blanketing
the forest floor, was the only awkward experience we encountered. However,
there wasn't a tight spot we couldn't negotiate. This is one mechanical horse
that we'd be happy to call "other" off-road machine.