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Thread: Variable Valve Timing

  1. #1
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    Question Variable Valve Timing

    I just read a short article that says That Yamaha may be fitting VVT to the forthcoming V-Max (more advanced than VTEC apparently).
    Excuse my ignorance, but has VVT been around on bikes for a while? I was under the impression that the BMW 'brick' had it. Have there been any others? If not, could this be the next big thing with bikes?
    It has been a good system on my car. Good power and economy.
    Marty

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    Honda pioneered (I think) their vtec system on one of their bikes in the early 80's. Not that I've had any personal experience with their late model bikes with vtec it gives better economy but no more power (bikes already having reasonably aggressive cam profiles so the option to honda was to give a milder cam profile at lower revs for better economy). Now variable inlet tract length is someting worth looking into.

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    Why would you want variable inlet tracts on a FI bike ?
    The contents of this post are my opinion and may not be subjected to any form of reality
    It means I'm not an authority or a teacher, and may not have any experience so take things with a pinch of salt (a.k.a bullshit) rather than fact

  4. #4
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    Um same reason BMW etc put them on their fuel-injected cars. Better response etc.

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    Cylinder filling is optimised at different RPMS by different length runners. I guess bikes having such vast rev range means they'd benefit even more than cars do?

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    Stands to reason.

  7. #7
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    But FI uses an injector pulse which has a variable length and timing.

    I found this as a useful article. Its a few pages long, but gives a good idea how things all hang together these days.
    The contents of this post are my opinion and may not be subjected to any form of reality
    It means I'm not an authority or a teacher, and may not have any experience so take things with a pinch of salt (a.k.a bullshit) rather than fact

  8. #8
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    The cylinder filling is the amount of mixture getting in, not the metering of the fuel. The injector pulse just controls how much fuel is going in, not how much air (mixture) is getting into the cylinder.

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    Quote Originally Posted by TwoSeven
    But FI uses an injector pulse
    FI? F1 surely to goodness?
    "Standing on your mother's corpse you told me that you'd wait forever." [Bryan Adams: Summer of 69]

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    Hitcher, are you taking the piss mate? FI = Fuel Injection.

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    Quote Originally Posted by TwoSeven
    But FI uses an injector pulse which has a variable length and timing.

    I found this as a useful article. Its a few pages long, but gives a good idea how things all hang together these days.
    So pray tell why major auto companies (BMW, Toyota, etc) are employing this on their newest vehicles. If what you are trying to say is correct then obviously they are wasting millions of dollars developing these 'improvements'. Different length/diameter inlet tracts change the speed that the air fuel mixture travels at etc. Different then fuel metering.

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    Quote Originally Posted by onearmedbandit
    So pray tell why major auto companies (BMW, Toyota, etc) are employing this on their newest vehicles. If what you are trying to say is correct then obviously they are wasting millions of dollars developing these 'improvements'. Different length/diameter inlet tracts change the speed that the air fuel mixture travels at etc. Different then fuel metering.
    yes your completely right OAB....I have no idea where twoseven is coming from... differing lenght inlet tracts [and diameter] change the flow of air at different RPM's by manipulating air speed to optimise burn and cyclinder fulling....even to the point that you can develop a supercharging effect at high RPM...

    and this was about variable valve timing this thread I think...and honda's system on the vfr800 isn't real vtec...its a 4 valve per cyclinder head that only uses 2 valves up until a certain RPM and then employs the other two for high RPM running... but the varible valve timing yamaha are playing with alters valve lift and duration...so as to alter timing completely...so it can run high performance cam profile at high RPM for increased power over a middle of the road camshaft profile to suit drivability at all rpm like most bikes...while retaining low rpm and medium rpm performance aswell.... Expect one hell of a flat torque curve!!!!! and one very flexable motor....
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    I made my original post because to elicit out the reasons behind the original statement. Asking questions to get people to explain themselves when they make generic statements is a good way of finding out what they really mean and I can learn stuff.

    My original question was why would you want a variable inlet tract on a modern FI bike, when they often have two injectors set at specific heights (to clarify the question a bit more).

    I understood the reason why two injectors were used on a motorcycle engine was that the first injector works at low speed rpm being situated close to the engine and the second injector works at high rpm. A butterfly valve is used to vary the air speed is it not - I would have thought this was similar to a variable inlet tract - unless I have it wrong which is highly likely

    Anyhow, i-vtec I think is what the original question was about. The latest version alters lift,duration and timing so as to get optimum volumetric efficiency at different rpm. It would be sensible to do this I suspect if you have two injectors set at different heights - As I understand how it works
    The contents of this post are my opinion and may not be subjected to any form of reality
    It means I'm not an authority or a teacher, and may not have any experience so take things with a pinch of salt (a.k.a bullshit) rather than fact

  14. #14
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    Getting back to the original question, forgetting about VTEC......do then, any of todays bikes employ VVT and if not why not? Make it in the real world, not motogp etc. Many car manufacturers seem to be employing it.
    Marty

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    One of the main reasons car manufactures are using such devices is to meet US emission and noise requirements. These are (I believe) not so "tough" for bikes,so bike manufacturers have not yet had to resort to such clever trickery.
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