25-04-13, 02:51 PM
Old Brit twins with 2 valves per head revved to 6k rpm or thereabouts, no? 
Yamaha's 5-valve head Genesis-era motors go almost double that and there's a massive difference in gas flow characteristics at such high rpm. Tuning the exhaust to deliver optimum performance at 11,500 rpm - taking into account valve overlap, gas velocity, resonance, scavenging etc - means that there would inevitably be a compromise in header length, collector box/manifold design and so on without something like an EXUP valve.
The EXUP valve isn't blocking the gas flow above 6k rpm but below that the valve rotates into the gas flow to alter the resonance and scavenging characteristics of the system. The 11" header rule was fine for Brit twins because they operated in a relatively narrow rev range. The EXUP in effect provides a variable length header to provide good exhaust performance across a 10k rpm range.
Most who have removed or disabled the EXUP do notice a detrimental effect on low-mid range performance, irrespective of any jetting changes. You're fortunate in that you aren't able to discern any loss.
Yamaha's 5-valve head Genesis-era motors go almost double that and there's a massive difference in gas flow characteristics at such high rpm. Tuning the exhaust to deliver optimum performance at 11,500 rpm - taking into account valve overlap, gas velocity, resonance, scavenging etc - means that there would inevitably be a compromise in header length, collector box/manifold design and so on without something like an EXUP valve.
The EXUP valve isn't blocking the gas flow above 6k rpm but below that the valve rotates into the gas flow to alter the resonance and scavenging characteristics of the system. The 11" header rule was fine for Brit twins because they operated in a relatively narrow rev range. The EXUP in effect provides a variable length header to provide good exhaust performance across a 10k rpm range.
Most who have removed or disabled the EXUP do notice a detrimental effect on low-mid range performance, irrespective of any jetting changes. You're fortunate in that you aren't able to discern any loss.