The Yamaha RZ 350 LC was the Holy Grail of street legal, two-stroke motorcycles and even in a motorcycling city like Barcelona they are few and far between. Too many people going too fast into something that's too heavy and the bikes get turned into a cube of aluminum and fluids. This bike was outside a cafe whose restroom I just clogged so I was looking to make a quick get-a-way from the German man entering behind me but nonetheless I had to stop and shoot a couple of pictures of this perfect bike.
The RZ 350 tipped the scales at 315 pounds, came with 59 horsepower, a six-speed transmission, three disc brakes and a single mono-shock rear, making this a superbike for those who ride on the superslab. One of the more interesting aspects of the bike is the Yamaha Power Valve System (YPVS).
The YPVS helped a motor spool up faster by varying the size of the exhaust port to ensure high exhaust gas velocity. The system worked by placing an elliptical lobe where the cylinder meets the exhaust muffler. The lobe was rotated by cables attached to a servo motor that relayed commands from the CDI. At lower velocities the lobe would partially obstruct the passage from the exhaust port to the muffler to keep exhaust velocities high. At higher RPMs the lobe would rotate, increasing the size of the passage to allow more exhaust to pass into the muffler.
Think of a two-stroke as a wind instrument. A two-stroke has reed valves to allow air/fuel mixture into the combustion chamber and the engine's power output is determined in large part to the acoustic resonance of the intake and exhaust manifolds. Since two-strokes are fairly simple machines with few moving parts and flame propagation pretty well understood, the YPVS broadened a two-stroke engine's torque curve to produce a wider powerband.
The system was easy to package and added very little maintenance, which is why the the YPVS soon found its way onto all of Yamaha's liquid cooled engines. So much success that all of the other Japanese brands began the sincerest form of flattery: Honda followed up with their Automatic Torque Amplification Chamber (ATAC), Suzuki with their Automatic Timing Exhaust Control (ATEC) and Kawasaki with their Kawasaki Integrated Powervalve System (KIPS).