VTEC power dip

Discussion in '6th Generation 2002-2013' started by tlarrea, Oct 24, 2007.

  1. tlarrea

    tlarrea New Member

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    I posted in mechanics garage, but I thought I'd ask here and get some opinions from other 6th gen owners, hope that's ok.

    Just got an '02 VFR VTEC, and I've noticed when riding that at the 7000rpm mark where the VTEC kicks in (i can hear the change in engine noise) I have what feels like a bit of a dip in power. It's most noticeable when I'm not pushing the bike too hard, in 1st or 2nd gear, not really accelerating too hard, the VTEC kicks in, and there seems to be a slight drop in power, as though there's some extra drag or something. However when accelerating hard, I don't really notice anything, and the bike quite happily (maybe even eagerly) revs up to 10k+ with no issues. Also, when in 3rd and up, i can feel a bit of a boost in power as the VTEC kicks in if I'm accelerating from say 4k rpm though past 7k, so the problem seems to only manifest in lower gears, and generally only when I'm at say 5-10% throttle, over that I don't notice it, but perhaps that's just because the extra throttle is masking the problem?

    However on the way home yesterday, i was accelerating in 2nd or 3rd and hit the 7k rpm mark and this time, the bike seemed to get a sudden kick of power which i hadn't experienced before, makes me wonder if that's what is supposed to normally happen when the VTEC engages as opposed to what's previously been happening.

    So I'm curious to hear from other VTEC owners about what the VTEC transition feels like, try and work out whether there might be a problem with mine or not.

    thanks,

    Tim.
     
  2. eddievalleytrailer

    eddievalleytrailer Member

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    On my '07, it feels like the 4 barrel on an old Holly or Quadrojet carb on an old muscle car. It kicks pretty good. WAHOOOOO!!!:whoo:
     
  3. John451

    John451 Member

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    Perfectly normal on the Gen 6 VFR, Honda seem to have added the slight dip just prior to the Hyper VTec engagment, on the '02 think it around 6800 and from '06 around 6400....

    See below link from this sites "My25thVFRVision" for a clear pic of the dip in his '07 Dyno run it occurs a few hundred lower than your '02 but otherwise very similar.

    Gen 6 Dyno chart:
    http://vfrworld.com/forums/general-vfr-discussions/10733-07-25th-dyno.html
     
  4. turtlecreek

    turtlecreek New Member

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    remove your 02 sensor and replace with a 330ohm resistor and this will all but go away.
     
  5. tlarrea

    tlarrea New Member

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    Do this in conjunction with the PAIR value bypass or no? I quite like the instant response of the FI, will this affect that throttle reponse by introducing something similar to carbi lag? Or does it just smooth out the power response across the throttle range?

    Sorry for stupid questions, I'm not terribly mechanical, but hopefully a quick learner. :smile:
     
  6. chomper

    chomper New Member

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    Depending on where throttle postion is when vtec kicks in you can experince a surge or a dip then a surge. some have installed power commanders and 02 blocks. I found after changing stock exhaust and doing a pair valve block made a big difference much smoother transition. try a pair block first cheap mode dosen't cost anything. lots of threads on this subject that discribe it in more detail. simple mode lift the tank and block the rubber hose on the left side of the air box all this dose is stop fresh air from entering the exhaust and giving 02 sensor a false reading. pair valve is just some more emission crap to clean up exhaust. worked for me
     
  7. chomper

    chomper New Member

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    If you bypass 02 sensors you dont need to do a pair block and yes it smoothed out throttle response from mid through vtec. do a pair block first if you dont like it its totally reversible
     
  8. tlarrea

    tlarrea New Member

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    Ok, another stupid question about how the VTEC works, just trying to get a handle on the relationship between the various components.

    When the extra valves open up as VTEC comes in, does that suddenly dump extra exhaust which the O2 sense detects as running too rich and compensates with a leaner mixture, producing the slight power dip? Or is this completely unrelated?
     
  9. John451

    John451 Member

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    Have a read of the below Review with Valve diagrams from the Gen 6 launch, it may help with understanding the VFR's "Hyper" VTec.

    2002 VFR Review:
    http://www.mcnews.com.au/Testing/Honda/VFR800/Page_1.htm
     
  10. tlarrea

    tlarrea New Member

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    ok, did a pair valve block (used an old computer mouse ball, cut the rubber coating off it and it's the perfect sized ball bearing), seems to have smoothed things out a little, however the idle seems to be sitting under 1k rpm now. Do i need to adjust the idle after blocking pair valve at all?

    Also, I think that the power dip might be more prominent once the bike has warmed up to 90+ deg C.
     
  11. TheoriginalDuplicate

    TheoriginalDuplicate New Member

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    Does the 330ohm resistor richen or lean out the air fuel ratio in relation to stock . Have you tested resistor that are more and less than 330 ohms? If you replace the O2 with a resistor wouldn't that make your air fuel mixture just get leaner and leaner as the rmps rise?
     
  12. junktionfet

    junktionfet New Member

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    I've noticed this 2 or 3 times as well... but I have to be really really light on the throttle when I cross that 6400 RPM threshold. It feels as though the power drops a bit, then picks up again.

    I think it's just the timing of when the extra lifters kick in, perhaps an interaction with the the fuel/air moving through one half of the intake port suddenly being diverted partially to the other.

    I'm sure the ECU does something with the injector pulse during that transitional period (since the volume of air entering the cylinder increases), yet the ECU has no way of knowing the precise moment when the other valves are fully engaged... so occasionally the mixture either spikes too rich or too lean for one stroke. No big deal really.
     
  13. OKCVFR

    OKCVFR New Member

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    Does neither, when you install the resistor it fakes the ECM out into thinking that the O2 is still connected but not seeing anyting other than stoemeteric conditions. What this does is not allow the ECM to go into closed loop operation and the ECM will not try to lean things out when it sees a slight rich condition. Remember the O2 and ECM, when motor is at operating temp, will try to maintain stoemteric conditions for the sake of emissions. This is why you get slight stumbles and dips in power at certain loads when riding. Don't think of it as leaning or richening but dissabling the emissions programing. Hope this helps. :wink:

     
    Last edited: Jan 15, 2008
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