Since its release in 1996, the Rotax 494 engine has seen many uses. Originally
found in the 1996 Formula SLS, the 499cc liquid-cooled twin has also made appearances under the hood of the 1997 Formula 500, Formula 500 Deluxe,
Summit 500 and the Grand Touring 500. After only two years on the market, Bombardier-owned Rotax pulled the spinner in for an overhaul. What came out of the shop is a drastically improved RAVE-equipped power twin. It is featured this year again in those same models, a re-outfitted Skandic 500, plus the brand new MX Z 500.
There have been advances in two-stroke engines through the years. echnology
nuggets like expansion chamber pipes, reed valves, fuel injection and Nicasil
plating have all made their way into the designs from each sled maker. But
from its debut in the 1985 Formula Mach 1 until last season, only Rotax- powered Ski-Doos had a varying-height exhaust valve in the design. We know it as RAVE, which stands for Rotax Adjustable Variable Exhaust, and here's how
it works.
A guillotine slide at the exhaust port is attached to a spring, which keeps it in the low position.
On the other end of the spring is a pressure-sensitive diaphragm (this is what's under the caps you see outside the engine). The exhaust pressure in the cylinder rises as the engine speed rises. When this happens, the guillotine slide retracts and the exhaust port timing is increased to achieve high end power. The springs are preload adjustable to allow different riders to adjust for their specific needs.
The last of the liquid-cooled Rotax motors to not have the RAVE system was the 499cc Model 494. We are pleased to report that this has changed for the 1998 model year. For background, the RAVE-less Rotax 494 of the 1996 Formula SLS dynoed at 89.0 horses last year (November, 1996, pg. 16). With a little modification, we achieved almost 91 ponies. The addition of the RAVE system makes a noticeable difference, as we'll soon see.
When the 494 was released originally, Ski-Doo officials were confident that the motor would provide sufficient power to keep ahead of the competition. "We didn't feel that adding the RAVE system was necessary," commented Ski-Doo's Communications Manager, Dave Thompson. "We felt we had a better engine than anybody else had at that time, and opted not to add RAVE in order to save costs for the consumer."
Our resourceful dyno guys managed to find an all new MX Z 500 just as they were arriving to dealers across the country. With a little sweet talking, and the promise of a shameless mention, Mike Ingles, of Ingles Performance, agreed to let us use one of his sleds.
"The use of the RAVE system makes a significant difference in the engine's performance," said veteran dyno guy Rich Daly. Our archive of old dyno data backs up his statement with hard numbers. The same motor that ran at or near 90 hp with normal exhaust porting now pulls 94.5 horses with everything bone stock. "There's not much you can do to make easy horsepower," said Rich. "We tried different jetting, but the best power we got was at the stock specs adjusted for the conditions."
Considering the new motor hit 94.5 hp, a 5.5 horse gain on the RAVE-free version, we thought maybe more significant modifications were made to the
new and improved Rotax. But Ski-Doo officials assured us that nothing else
had been done internally.
"This is the same motor setup consumers will find in the Summit, Formula 500, Grand Touring 500 and Skandic," explained Thompson. "We haven't detuned it for different models. The main difference will be in suspension setups. Our new MX Z 500 is for the high performance lightweight twin market, but the sport riding or mountain riding consumers also want to find a 94 horsepower engine in their sleds." This year they will.
Again, a special thanks to Mike Ingles at Ingles Performance for letting us use one of his MX Z 500s right out of the crate. If you have need of a Ski-Doo dealer in the Syracuse, New York area, consider giving him a call 315/598-7422.
American Snowmobiler is pleased to work with DYNOPORT in passing along
the dyno tests of key snowmobile models. DYNOPORT is an independent performance and dyno facility located in Union Springs, New York. American Snowmobiler and DYNOPORT make every effort to assure the accuracy of articles and technical information; DYNOPORT assumes no responsibilities for damages due to errors, omissions or applications of any information. DYNOPORT, 1896 Townline Road, Union Springs, NY 13160 · 315/258-9371.
1998 Ski-Doo MX Z 500
Air Density: 94.5
| RPM | CBT | CBHP | BSFC | FUEL | H2O |
| 6700 | 59.0 | 75.3 | .600 | 43.7 | 91.5 |
| 6800 | 59.6 | 77.2 | .488 | 36.5 | 92.0 |
| 6900 | 60.6 | 79.6 | .539 | 41.6 | 92.5 |
| 7000 | 62.2 | 82.9 | .537 | 43.2 | 92.0 |
| 7100 | 64.1 | 86.6 | .509 | 42.8 | 92.5 |
| 7200 | 65.2 | 89.4 | .517 | 44.9 | 92.5 |
| 7300 | 65.9 | 91.6 | .527 | 46.8 | 92.5 |
| 7400 | 66.3 | 93.3 | .523 | 47.4 | 93.0 |
| 7500 | 65.7 | 93.8 | .570 | 52.1 | 93.5 |
| 7600 | 65.3 | 94.5 | .618 | 56.7 | 93.0 |
| 7700 | 63.9 | 93.7 | .557 | 50.7 | 93.5 |
| 7800 | 62.4 | 92.6 | .540 | 48.5 | 94.0 |
| 7900 | 59.3 | 89.1 | .551 | 47.7 | 94.0 |
| 7970 | 55.7 | 84.6 | .570 | 49.7 | 94.0 |
RPM: Engine crankshaft speed.
CBT: Corrected Brake Torque.
CBHP: Corrected Brake Horsepower.
BSFC: Brake Specific Fuel Consumption.
FUEL: Actual fuel flow pounds per hour.
H2O: Water temperature in degrees F.