Saturday, June 16, 2007

MD First Ride: 2007 Yamaha YZF-R1

2007 Yamaha R1

The 2005-2006 R1 was often criticized for having a powerband biased too heavily toward the top-end; the bike was a rocketship when the rider kept the needle in the upper quarter of the tachometer, but in the lower RPM range, it was out-powered by rivals like the suzuki GSX-R1000 and kawasaki ZX-10R. This "600-like" powerband was great on the racetrack, but in everyday street riding, the (relative) lack of low-end grunt and mid-range pull was disappointing.

As I discussed in Part One, the R1 design team was tasked with the seemingly contradictory goals of eliminating or mitigating these shortcomings, while at the same time significantly reducing the new bike's emissions. To accomplish this, they decided to give up yamaha's traditional five-valve cylinder head design in favor of a more conventional four-valve layout, which allowed for a smaller combustion chamber and thus a higher compression ratio (now 12.7:1). The four throttle bodies feeding air to the new cylinder head are topped with an innovative system, called 'YCC-I', which varies the length of the intake runners depending on RPM and throttle position. Below 10,400rpm, the intake runners are 140mm in length, which significantly boosts low-end and mid-range power output. When the R1 spins past 10,400rpm (assuming the rider is applying at least 57.5% throttle), the upper section of the intake runners is lifted away, leaving the air to enter through the 65mm lower sections - a length optimized for high-rpm power output.

The throttle bodies also feature the same 'YCC-T' (yamaha Chip Controlled Throttle) throttle by wire system that first debuted on the 2006 R6. Although the throttle is still connected to two cables which run down to the throttle bodies, these cables do not directly actuate the throttle body butterflies. Instead, the rider's wrist motion is translated by a throttle position sensor, which tells the ECU how hard he wants to accelerate. The ECU then makes the decision as to how far and how quickly to open the butterflies, maintaining maximum air velocity in the intake tract to maintain maximum acceleration (consistent with the demands of the rider, that is). Why not locate the throttle position sensor directly on the twist throttle and eliminate the cables, you ask? Well, besides actuating the throttle position sensor, the return cable actuates a small arm that manually closes the throttle body butterflies. Thus, in the unlikely event of an electronic malfunction that left the butterflies open (the throttle-by-wire equivalent of a stuck throttle), the rider can manually close the butterflies by merely closing the throttle.


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