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post #11 of 13 Old 12-12-2017, 09:06 AM
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Quote:
Originally Posted by shisoshin View Post
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.

Tis better to be thought a fool and remain silent, than to open one's mouth and remove all doubt.
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post #12 of 13 Old 12-16-2017, 03:54 AM
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Quote:
Originally Posted by cbrlocal View Post
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.
I did the same, bought a 300 straight through exhaust when I did the 305cc conversion. Bike became night and day when all done.
Picture showing the pipe, bottom fairing would not fit good so left off.
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post #13 of 13 Old 12-16-2017, 05:19 PM
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'backpressure', simplified, includes
the opposite of drawing fuel/air mix
from combustion chamber into exhaust,,
where it is lost to potential 'bang'..

high performance engines allow for this
wastage of unburned fuel into exhaust,
in pursuit of maximum scavenging
for max power at highest rpm..
for economy based road engines [eg ours]
more bang conserved or not wasted,
= better performance/economy..

so its a balancing act..
throw out the dirty bathwater
while keeping the baby..
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