Now turn the key. Gone is the thrum of that V-8 engine. Instead, you'll be surprised to hear the turbine-like whine of Yamaha's MR-1, 140-horsepower, high-rpm jet-drive engine. Oh, turn the second key, too. You'll have more fun running both of these powerful, fuel-efficient monsters as they accelerate to 10,000 rpm and nearly 50 miles per hour. The sound? More like an Indy car than a recreational runabout.