This Hitchcock pipe is nice. It's a bit larger diameter than the stock RE pipe. With a Goldie silencer, it makes less power than stock! The bottom end power is gone! Jack had one on his bike. It didn't work well. He went back to the stock bike with the Goldie.
Ron Greene said the D&D pipe was worth 4 hp. I'm assuming he meant over the stock pipes. If true, that's a good hike!
Bare
Bare
That doesn't surprise me at all, about the reduction of low rpm torque after losing the smaller I.D. stock header. The smaller pipe has higher flow speeds and improves scavenging at lower rpms.
However, it loses the advantage at higher rpms because of being small. So, it's a two-edged sword.
We see the same thing with the Iron Barrel engines when people install a 350 exhaust system on the 500 to improve low rpm torque, and it works. But it loses a lot of power at higher rpms because it's too small to work at the higher rpm range.
With a port that is designed to work into a larger pipe, it should be a better overall match. I'm not going to say that the larger I.D. pipe will give better torque at low rpm than the smaller I.D. pipe, but with the right exhaust port it should give a decent low rpm performance while being much better in the higher rpms. It's all a matter of balancing the performance throughout the rev range.
With any fixed diameter and fixed length exhaust system, there is only one area where it is ideal, and the rest is compromise. Making a very useful compromise is the art of it.