dialing it in

AmSnow.com is now SnoWest.com

Yamaha engineers are dialing in on the sweet spot. Their new Nytro has great power, competitive weight and excellent handling.

When Yamaha started on its 4-stroke snowmobile program, the big question was: Could a 4-stroke be powerful enough to compete with 150-horsepower 2-stroke rockets?

First, the RX-1, and now, the Apex proved it could compete, even though competitors claimed these sleds were too heavy. Yamaha came next with the Vector for the 120-hp class, an instant hit with consumers looking for a good all-around trail sled. To further prove a 4-stroke could be lighter, Yamaha followed with a new Phazer powered by an 80-horse twin.

The Phazer is a fun machine, but what if you could have more power in a Phazer chassis?

Lighter + More power
Yamaha was already thinking this way, and this season it married a higher horse version of the Vector engine with a lighter Phazer-style chassis, creating the new FX Nytro. It weighs in at 525 lbs. dry, including electric start and reverse.

With 138 hp at the crank, Nytro slots right in with Polaris' 700 Dragon and Ski-Doo's 2007 800 Blizzard. When they are all filled, Nytro may even be a few pounds lighter! We had the chance to dyno a new Nytro engine along with Yamaha's Genesis 120, used in the Vector and Venture, to confirm the Nytro's power increase.

With a freshly calibrated dyno, the two tests were run on the same day, within an hour of each other. The Genesis 120 pulled 122.6 horses at 8,400 rpm with torque peaking at 84.5 ft.-lbs. as low as 6,600 rpm. The Nytro engine produced 138.6 hp at 8,600 rpm with peak torque of 94 ft.-lbs. at 7,100 rpm and still pulled a strong 72 ft.-lbs. all the way down at 3,500 rpm. (See the full Dyno Report) Yamaha engineers claimed a 14-horse increase, but they are usually conservative; our actual readings showed a 16-horse jump, plus an additional 10 ft.-lbs. of torque.

How did Yamaha's engineers gain the horsepower?

First, the engine has a 3 mm larger bore than the 120-class Vector engine. That's a 7.5% increase in displacement and would account for 9 hp. The new displacement was chosen to fit in with the ISR racing rules for the WPSA Snocross Open Class, where Yamaha launched an aggressive factory team program last year with the previous year's Champion, Robbie Malinoski, and Japanese Champion, Yuji Nakazawa. Unfortunately for Yamaha, Malinoski is moving to Ski-Doo this season.

Nytro also gets extra power from new camshafts that add 10 degrees to the intake duration with valve lift increased to 9.2 mm from 8 mm on the Vector. The exhaust timing duration remains the same, but lift is increased from 8 mm to 8.7 mm. Intake and exhaust ports have been improved for increased flow, and larger fuel injector bodies replace the carb set-up.

A nice feature has been added to the injector body to reduce engine braking from the 4-stroke. An extra passage bypasses the throttle butterfly and is controlled by a solenoid valve. When the throttle is shut, the bypass valve opens and allows extra clean air into the motor to reduce engine braking. If you pay attention, you will notice that it coasts with only slightly more engine braking than a 2-stroke.

Lighter crankshaft, camshafts, wrist pin and valves improve throttle response, and cut the Nytro engine's weight by 10.2 lbs. compared to the Genesis 120 used in the Vector. The Yamaha triple mounts the clutch directly on the crankshaft, so the weight of a speed reduction gearbox also is eliminated.

With the engine mounted as far down in the chassis as possible, the center of gravity is low and centralized, allowing for extra ground clearance without a tendency to roll in corners.

Minimalist philosophy
Nytro's chassis also uses the Phazer minimalist philosophy. An old racing adage says "the lightest parts are those that are not there." Exit conventional hood and pan. The chaincase is part of the die-cast bulkhead, and the cooling circuit is only a front tunnel cooler with a top-mounted radiator.

Each chassis component was studied, lightened, combined or eliminated. The OEMs know weight savings are not accomplished in one specific part, but smaller savings in a multitude of components that add up to a larger total.

I spent a lot of time on the FX Nytro in spring testing out West, and the driver-forward ergonomics are excellent. Everything, from the seat contour, to the handlebar position, to the footrests, made me feel comfortable and in control. Usually, after a day's riding, my shoulders are sore from pulling on the handlebars, but I had no problems here. You get no feel of heaviness.

The front A-arm suspension and the rear double-shock skid performed flawlessly with the updated calibration, and I had a blast throwing this sled around on the trail without feeling I was on the limit. While some competitors' shocks fade when pushed, Yamaha's units maintained full dampening through all kinds of conditions.

This is becoming more important with the new sophisticated suspension systems. If engineers build good suspensions, but bean counters in purchasing try to save a few bucks by installing sub-par shocks, the effort is wasted. Consumers will remain unimpressed.

Yamaha has had a strong shock development strategy, which included buying a majority share in Ohlins 20 years ago. Ohlins is one of the world's top racing shock makers; Yamaha has managers and engineers working at the Ohlins factory on rotation before they move back to Japan.

You may not always see expensive Ohlins units on a Yamaha, but the production shock technology has been heavily influenced by engineers who learned their trade apprenticing at Ohlins. Add to this the continued suspension development for snocross sleds, where bump performance is paramount. Hard charging drivers push the ma-chines to the limit; any weakness will quickly show up.

Racing speeds up development, as the deadline to fix a problem is only a week away, rather than next year. Combine the handling of this light sled with the snocross suspension technology, and an engine that pulls like an 1000 on top end but accelerates out of corners like an 800, and Yamaha has a new machine I'd be happy with.

After a day of busting trails, I came back feeling good, with no shoulder or back pain, and that's important to me. I like to ride, but I am not in the shape I once was, and its fun to find a sled that helps you feel like a racer and still feel good at the end of the day.

Olav Aaen is a long-time contributor to AmSnow. As a mechanical engineer and president of Aaen Performance, Olav has been heavily involved with snowmobile performance since 1968. Aaen Performance is best known for pioneering performance pipes and introducing the roller clutch to the snowmobile market.