2002 yamaha sxviper

Yamaha's SXViper is the most anticipated consumer sled we've seen in many years. The Yama-faithful are talking it up to be the best model in the world. The rest of the market is at least acknowledging it as one hot unit. At the heart of the conversation lies the new engine. While it's not something completely new, it does have a character all its own.

Yamaha took the basis of its SXViper triple from the proven SRX engine. "We started the Viper's engine with the SRX's crankcase," explained Yamaha's Greg Marier. "The bore and stroke is the same as on the SRX, too, but there are a lot of internal differences that let us achieve the performance level we have."

The SXViper engine uses Yamaha's new Forced Air Induction system to boost horsepower at speed. "What happens is that air is packed into the airbox through the scoop vents near the headlight," Marier explained. "We found it was worth about three horsepower, although you won't see that on normal dyno tests where the sled is sitting stationary on a flow bench. We measured the pressure in the airbox, then pressurized our dyno room to put a number to the power gain."

The Viper's 33mm rack-style Mikuni carbs are vented to the airbox. This evens out the air pressure from the airbox to the float bowls. "What we learned by tying the carb vents to the airbox was that the engine runs cleaner at low rpm because the airbox is actually in a vacuum mode down low," Marier explained. "By lowering the float bowl pressure to match what's happening in the airbox, we don't see a lot of fuel going into the engine, causing a bog. As the sled accelerates, the pressure in the airbox increases, and that transfers to the carbs as well. With higher pressure in the carbs, more fuel is flowed, producing more power."
The whole Viper engine block is lighter than the SRX. The cylinders on the Viper have been recast, reducing material on the exhaust side and redesigning the ports. Although it shares the same general crankshaft design, including heavy duty roller bearings and welding for strength and durability, Yamaha engineers cut as much material as they could to shave a pound and a half of rotating mass.

Our production 2002 Viper test sled showed good top power at 122 horsepower at 8500 rpm. Remember there's a bonus three hp when you get up to speed from the air induction. But the better story is that it maintains its peak torque of 76 foot pounds from 7100 rpm all the way up through 8200. "This is a very wide band," noted dyno guy Rich Daly from Dynoport. "It's wider than any other engine on the market today. With that kind of torque curve, you're always able to backshift the clutches and it'll just keep pulling. You can't hardly miss!"