6th Gen Accord F23 Vtec Operation

Discussion in 'Accord Tech' started by Mark2001Accord, Oct 16, 2019.

  1. Mark2001Accord

    Mark2001Accord Well-Known Member

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    Is the Vtec operation of the 6th Gen Accords similar to Integras and preludes? While going through some documents there was some info i might have misunderstood. I always thought the Vtec switched over at 5250 RPM or something similar at certain percentage of throttle and load. The F23 reds at say 6300. So I could see why the ceiling would be done differently.

    Is it possible the Accords F23 Vtec keeps the 2 valves shut for mileage and emissions? Then opens only for a throttle percentage ? When my accord was stock a hardwired LED from the VAFC harness showed Vtec actuating at low RPMs like 2500. I cannot find a Vtec spec in the manual section, and that is suspicious.

    This would need to be tuned differently then a B or H because the hi cam / low cam RPM range is the same. Am I on the right track here understanding this?

    Currently my Vtec parameter is set wndowed 5000 upper at 15", lower 4000 at 1" vacuum in Hondata's Smanager. It was my understanding it was delayed for an RPM when I tuned it. This might not be true. I might need to revisit this tune.

    It has been years since I built these. I kept poor notes , and records. So I need to catch up on it all again. Then again I might just be paranoid.
     
    Last edited: Oct 16, 2019
  2. trillobite

    trillobite Active Member

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    In terms of determining whether or not VTEC on these Accords keep 2 valves shut, pull the valve cover off, turn the engine and look :)

    The last time I did a valve lash adjustment on my Honda 3 years ago, I believe all valves are always operational, but open farther when VTEC kicks in. However my memory tends to be flawed.

    I might do another valve lash adjustment this weekend, if I do, I will let you know what I find out.
     
  3. Nam1911a1

    Nam1911a1 Well-Known Member

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    Yeah the f23 doesn’t have that huge a difference between the high and low cam lobes and mine would crossover around 2300-2500 rpm. Then a smooth hard pull all the way to 6k.

    But no the engine does do what Wikipedia states and it cannot keep one of the intake valves closed. It’s wikipedia after all. Honestly the only reason I ever wants to change my vtec crossover to above 3000k is that I cruised at 3k rpm otw to work 60 miles round trip and didnt want to be in vtec the whole time.
     
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  4. Mark2001Accord

    Mark2001Accord Well-Known Member

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    Not shut , but close. Using the term 'shut' was a poor choice. 2 valves, meaning the intake side. Vtec is only on the intake side on the F23's system.

    The F23 Vtec crossover point seemed to behave much different than other models. Another forum member chimed in, also suggesting operation down in the 2000RPM range. It is still oil pressure dependent for proper operation, so I would think the system would not kick on too early.

    It only lifts the one valve slightly, unit you give it enough throttle. It does not seem to be RPM dependent. When it rides on the middle rocker then both valves are being lifted.

    That is a major difference, no? Seems to shut down for idle, not open higher to rev.

    Section 6-10 discusses Vtec in the manual, but does not cover the operation.

    intake valves open
    pri-lobe 34.481mm
    secondary 37.725mm
    in Vtec riding on the center rocker
    both valves lift on a 39.725mm lobe

    Figure lobe base circle is around 30mm.

    A difference of 3mm is huge, a cam only lifts like a 1/2 inch total. So, ya..shut.
     
    Last edited: Oct 17, 2019
  5. Nam1911a1

    Nam1911a1 Well-Known Member

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    The difference is enough that I can feel the vtec engage at 2200 rpm and the engine pull just a hair harder as I accelerate. It’s much easier to feel with a manual trans as there is zero slip from the torque converter.

    When I did my valve lash job the low cam on the f23 opens BOTH intake valves still. Not just one.
     
  6. Mark2001Accord

    Mark2001Accord Well-Known Member

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    The way I understand the system the one intake valve only opens not even half way. It is held shut until you push the gas pedal. Not RPM limited.
     
  7. Nam1911a1

    Nam1911a1 Well-Known Member

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    where did you get that information?
     
  8. Mark2001Accord

    Mark2001Accord Well-Known Member

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    The specs are from the manual.

    I am not saying that one valve is held closed. One intake valve opens half way, the other intake valve opens like 80% . Soon as you accelerate , not cruise or idle, the Vtec kicks in and they both lift of the 39.725mm lobe.

    There are numerous threads suggesting operation of Vtec happens down in the 2000 RPM range.

    I tuned my car, at the time i did not account for this.
     
    Last edited: Oct 19, 2019

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