Power band not constant 5th gen

Discussion in 'Mechanics Garage' started by Sliding242, May 2, 2021.

  1. Sliding242

    Sliding242 New Member

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    When I keep it pinned throughout any gear, the bike seems to have this weird kind of surge around 7k anywhere before or after that is normal but as I accelerate through that 7-8k range it just has this lag but immediately picks back up through the rest of the gear. Maybe I could post a video. Operation seems normal if I cruise through the gear, not mash them.

    I don't think it is a clutch slipping issue since I don't notice it slipping when I put a load on it (going up a hill in a high gear and other known ways to prove slippage) albeit I do have one clutch problem, the bike will try to creep forward for a second when I start the bike up. Then it wont do it anymore for the remainder of my ride but I notice it when I start it cold. Also have heard about the slave cylinder getting gunked up and causing weird issues.

    The bike has 40k and the previous owner put in a power commander? and I don't think he new how it works since he said he just started pressing buttons till he liked it, I clearly don't know anything about it either.
     
  2. GreginDenver

    GreginDenver New Member

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    This is interesting.

    Could it be that you're feeling the intake system making the shift from "long track" to "short track"?

    When it moves, air acts like a fluid, and at high speeds this fluid nature becomes exaggerated. The mass of the column of air moving through the intake runner and airbox at 7,000 to 12,000 rpm is substantial. When an airbox hits "resonance" the air in the intake column is absolutely jackhammering itself into the backsides of the intake valves.

    As designed the 5th Gen VFR airbox volume is variable between two states. The "long track" (the combination of the airbox volume plus the volume of the long track intake runner) is tuned to reach resonance in the range of 7,000 rpm (depending on the elevation you're riding at). Then the "short track" opens up. This combined volume (airbox + short track intake runner) reaches resonance as you approach 10,000 rpm.

    I'm inclined to believe you're feeling the airbox shifting from "long" to "short".

    All to say that what you're feeling is quite likely perfectly normal.

    The only thing that could make this situation feel truly abnormal would be IF the bike is running a power commander and IF it was dyno-tuned to run towards the rich end of the business (to make "best horsepower" rather than to give any consideration to fuel economy) then the changeover from "long" to "short" track would be much more noticeable than with stock Honda fuel tuning.
     
    Last edited: May 2, 2021
  3. Sliding242

    Sliding242 New Member

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    Now that is interesting about the long and short stage intake. To describe it hopefully better, its like it lags for a 1/4 sec around 7k and then picks up more than ever before? During that blip of a lag, it sounds like rpms climb faster than normal and acceleration hesitates for the moment (made me think slipping clutch) but then yeah picks back up like it never happened and even then some (makes me think not clutch slippage)

    In videos I hear online the bikes sound pretty consistent throughout being pinned.

    I do know the bike was tuned and in top notch shape at one point maybe by original owner (40k now and at least three owners later), it has a handful of mods some of which I dont know, previous owner says he got it up above 160 many times although I do know the speedo is off a little regardless it has been romped on.

    It has the power commander III (one mod I know of haha) so I should do some due diligence on it.
     
  4. GreginDenver

    GreginDenver New Member

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    [​IMG]

    Chapter 21 of the 5th Gen VFR800, entitled "Technical Features", is some good reading.

    When Honda was rolling out the 5th Gen with all of its changes to the outgoing VFR750 the Honda Corporate leadership was concerned about whether or not it would be accepted and purchased by the targeted market segments. So Honda decided to "pull back the curtain" on everything they had done to improve the bike during the generation change from RC45 to RC46.

    Speaking to your situation, I can imagine it would be pretty hard to dyno-tune the air-fuel ratios to exactly, perfectly compensate for the sudden loss of resonant efficiency that occurs when the VFR800 airbox opens up the "short track" intake (to reset the airbox's volume characteristics for the upper end of the rev-range).

    I had always suspected that this shift would cause a "hitch" in the VFR800's acceleration, but I could never feel it on either of my 5th Gen bikes (a '99 and an '01), but that's probably because my two VFRs are running on a stock tune.

    It doesn't surprise me to hear that a VFR800 with a truly top-notch dyno fuel tune could make that airbox shift obvious enough for the rider to feel. The moment when the short track opens is going to be an instant reduction in airbox efficiency, which means that for the exact same injector pulse-width the air/fuel ratio will suddenly become more rich, and if the bike was already running at a really good "max horsepower" air/fuel ratio (something like 13.5 to 1 or 13 to 1, or even something in the high 12s to 1) then the increase in richness might just take the engine slightly beyond the "max horsepower" air/fuel range which will drop the horsepower being produced just a bit.
     
    Last edited: May 3, 2021

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