Flapper Mod

pic courtesy of Sir James & Andy B (www.hondahornet.co.uk)

Very simple to do:

1. Remove the seat & left side panel
2. Remove the 3 screws that hold the airbox lid on
3. Remove the hose from the vaccum switch & plug the hose with suitable sized screw or bolt. You can then tuck the hose behind the intake air temperature sensor that is attached to the airbox directly behind the vaccum switch.
This is a flapper door actuated at certain rpms that cuts off the the majority of the air flow to the airbox! On the RC51 disabling and/or removing this mechanism in the ram air tunnel yielded more linear power delivery (we believe that on the RC51 it was put there for noise abatement issues so that it would pass EPA testing & not performance related as Honda claims because it runs better without it & it is very likely that it was put on the 919 for the same reasons as the induction noise is the loudest part of the 919 & the EPA has gotten real strict lately on noise emissions)

The theory is sound (no pun intended) in that by closing off part of the induction area at lower rpms & throttle positions intake velocity is increased, but on this type of application the results are negligible at best. If this type of flapper valve really worked as they claim it does then every single Factory racebike in any professional racing paddock would all have flapper valves installed in them, but they don't!

I believe this was done by Honda for no other reason than to keep the noise down for EPA testing! The induction roar is howlin' mad now especially at freeway speeds & the bike no longer has a surge of power at 5500 rpm, but it is not because you are losing horsepower it's because it is making the power curve more linear & producing better power below 5500 rpm so when the power picks up at 5500 rpm you no longer feel it as an increase since the power flows much more smoothly now.

Since this mod is so simple I strongly urge any of you to disconnect & plug the vacuum line & try it for yourself. If you don't like it then put it back on...


Where this type of testing gets tricky is in the fact that dyno testing will only give you full throttle acceleration results & mods like this may hinder a full throttle run, but be beneficial to a partial throttle roll-on etc... That is when you have to weigh out which you need more. Most people don't ride their streetbikes around pinning the throttle to the stop & if they have to give up some WOT acceleration in return for better lowend or midrange roll-on acceleration or throttle response they usually will.