engine design includes backpressure
to help keep the bang in there..
unless full throttle or forced induction
open exhausts take away potential
at lower revs in a basic engines..
[thus db restrictor potentials]
Well, technically 'backpressure' is always bad; what you want is maximum exhaust scavenging which is accomplished through uniquely designed resonance waves in the exhaust system itself. These vary depending on engine characteristics, power bands, header design, and a million other things. So typically when you change this as in changing to a slip-on, you're changing the resonance patterns in the pipe and shifting the powerband.
Usually this is a shift up in higher RPMs where rapid flow-through is more beneficial, at the expense of low-end power. What's always confounded me about our little CBRs is that any
exhaust combination besides the factory system provides more power and torque across the entire rev range
. This leads me to believe that either the factory exhaust is way too restrictive to meet noise requirements that it was impossible to effectively design the pipe for efficient scavenging, or Honda simply underdeveloped the pipe for any performance reasons at all, and it was strictly designed simply for noise and emissions.
This seemed to change slightly with the 300r's exhaust design, which is lighter and a bit throatier than the 250's. Some people have done a direct swap/conversion on their 250, but I don't believe I've seen any dynos to see if there were any gains from this. That would be interesting.