Dude, you HAVE to further explain how the smaller diameter, more restrictive, stock exhaust system affords more flow capability than the larger diameter Holeshot system simply because it incorporates a crossover pipe.....especially at low-to-moderate rpms?!?!
I think you misread what is being said. If the amount of exhaust is constant, then a larger exhaust pipe will be able to expel the gases more quickly, but the flow rate of the smaller pipe will be greater. At lower rpms, where the smaller pipe is able to keep up with the flow and completely expel the gases, the higher flow rate is desirable since the intake valve overlaps the exhaust valve opening: the high flow rate helps bring in fresh air. So smaller pipes are generally better at lower rpms than larger pipes.
On the flip side with the smaller pipe you lose some high rpm power since the pipe is not able to flow out the exhaust gases fast enough. A crossover pipe can ameliorate some of that problem by giving an alternate path for the gases to follow. So a smaller pipe with a crossover can be a best of both worlds situation on a motor with a wide powerband like the FJR. Having no experience with designing such a system it seems like a black art to me because you get into issues of tuning where the crossover pipe goes by the exhaust pulses.
For this size engine, I consider 1/8 inch to be a significant size increase. Engines with twice the displacement per cylinder will have powerband shifts with a 1/8 increase in diameter. Don't think 1/8 diameter, think of the increase in total area that results.
As for kicking the powerband higher, the FJR doesn't have the other components to take advantage of the increase in pipe diameter. It would need longer duration cams with perhaps more lift, larger intake and exhaust ports, and maybe larger throttle bodies. Then to keep it from being a dog down low because of the longer cam duration, you would need to bump the compression ratio up.
Good explanation, but the primary issue is optimum velocity. Everybody forgets that power is about flow, and exhaust gases slow down in a larger pipe than they do in a smaller one. The key is to achieve optimum velocity for the size engine and desired rpm range. If velocity slows down you lose the scavenging effect pulling stagnant exhaust gases out and allowing for a more complete cylinder fill of fresh air and fuel.
Just like too big a carb, or too big an intake tract or port, power is lost because velocity is decreased. Crossover pipes primarily enhance low end torque and not as much on the top end as one would think. That is why when designing an exhaust the crossover pipes are not the same diameter as the rest of the exhaust. They are usually about 75% of the pipe they attach too. But they are many variables in designing an exhaust. The crossover location, collector length, all affect power.
The FJR is not an R1 or a CBR. As a sport tourer, Yamaha designed it with great low end and midrange. A good exhaust should not result in a loss of low end or mid range power. IMHO larger primary pipe diameter was a step in the wrong direction for this application.
And yes, I have designed a few exhausts in my day, but they were for 8 cylinder cars. Computer software programs (Engine Analyzer PRO) and dyno results have backed them up.
