The GistAs outsiders looking into the world of snowmobile drag racing, car guys need to seriously readjust their perceptions of speed. Straight from the factory, the average Ski-Doo, Yamaha, and Polaris snowmobile weighs between 500 and 550 pounds yet cranks out roughly 140 hp. That's good for mid-11s in the quarter-mile with high 10s just a few simple mods away. Can you say street bikes for the snow? With such favorable power-to-weight ratios, it was inevitable that guys would start running them at dragstrips, and converting snowmobiles for asphalt use is surprisingly simple. All it takes is swapping in an on-road-style track and installing custom front skis with wheels for steering. Both the NHRA and IHRA sanction snowmobile classes, but for a small group of racers in the Northeast, it just wasn't hard-core enough. They formed the National Speed Association two years ago to promote the fastest snowmobiles in the world in its Pro Stock class. The series competes at tracks throughout the Northeast, with the winner at each event taking home $1,000-not too bad for such grassroots-level racing. To add to the excitement, all races are run in a heads-up format on a pro Tree. Get this: The fastest pass on record in Pro Stock is an 8.42 at 154 mph.
Vehicle SpecsMuch like a tank, a snowmobile relies on tracks instead of wheels, and the assembly is driven by a continuously variable transmission (CVT) integrated into the clutch. A cogged track drive linked to the driveline propels the track, and A-arm and trailing-arm suspension systems absorb bumps. Most NSA Pro Stock racers run three- and four-cylinder 1,000cc engines pushing 250-300 hp. Power adders are prohibited, but otherwise rules are pretty loose. Common tweaks include bumping up the compression ratio and enlarging the ports. Since two-stroke motors don't have camshafts or traditional valves, timing is altered by manipulating the height and location of the ports. By far, a snowmobile's expansion chamber-the automotive equivalent of an exhaust header-has the most profound impact on increasing power. With no exhaust valve, air/fuel escaping out the exhaust port is a common problem with two-stroke engines, and exhaust pipes with just the right amount of taper help revert some of the mixture back into the combustion chamber. Snowmobile and rider combined must weigh at least 650 pounds, and although four-stroke snowmobiles are given weight breaks, they can't compete with the outrageous power output of the two-strokes.
Fun FactsWith the assistance of power adders, snowmobiles have run as fast as 7.90.Thanks to CVT transmissions, snowmobiles reach their 9,400-rpm power peak within half a second of hitting the throttle and hold that rpm the entire course of the run.Unlike a four-stroke engine, a two-stroke has its ports located in the actual cylinder case. The term "porting the heads" doesn't apply.With studded tracks, snowmobiles can run nearly as fast on ice as they can on asphalt.Racers must have current NHRA or IHRA competition licenses to compete.Tracks measure 136 inches long and 10.5 inches wide.
Gimme MoreCheck out the NSA Web site at www.nationalspeedassociation.com to learn how you can get in on the action. -Stephen Kim
Lingo of the MonthLambdaDifferent fuels have different stoichiometric air/fuel ratios. In theory, the ideal mix for gasoline is 14.7:1 for a complete burn, and for alcohol it's roughly 6.5:1. Lambda, which is the actual air/fuel ratio divided by the stoichiometric ratio, makes comparing the leanness or richness of different fuels much easier. For example, a lambda figure of less than 1 indicates a rich mixture, while lambda greater than 1 reflects a lean mixture, regardless of the fuel type an engine consumes. -Stephen Kim
On the Webwww.eng-tips.comGetting solid tech info from most online forums usually requires sifting through pages of hearsay and mythology. That's not the case here, where well-educated engineers discuss esoteric theories and concepts using advanced mathematical formulas and SAE papers to fuel their debates. Some fascinating topics as this is being written include a prototype six-stroke engine that features two power strokes and methods for heating up fuel lines to run biodiesel in cold climates. Although it doesn't offer much practical information for the average hot rodder, it's great entertainment for those who like high-end discussion merely for the sake of high-end discussion. There are also separate forums for aeronautical, computer, and structural engineers. -Stephen Kim
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