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Kickdown

Discussion in 'E85 Z4 (2002-2008)' started by vinaysp, Mar 8, 2011.

    vinaysp guest

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    What is kick down and is this operation available on my '07 Z4 3.0i?

    My owners manual says: "Kick-down enables you to achieve maximum performance. Press the accelerator pedal beyond the full- throttle resistance point."

    How do I perform this operation? The manual isn't helpful.
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    Zeichen311

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    :confused: Have you ever driven a car with an automatic transmission?

    The kick-down switch signals the transmission to downshift or "kick down" to the lowest usable gear (based on road speed) when you mash the throttle pedal to the floor. The transmission logic might decide to downshift one or more gears with less throttle input, based on speed and load, but the kick-down switch provides an unequivocal signal that "I want to go as fast as I can and RIGHT NOW."

    You can feel the kick-down point as a "hard spot" in the motion of the throttle pedal. With the engine off, press the pedal as gently as possible until it seems to be at bottom, then press hard. You will feel a sort of dull snap as the pedal overcomes the resistance at this false limit and travels another few millimeters to the true wide-open position. Pressing the pedal beyond that resistance point activates the kick-down function.

    Cars with manual transmissions have a different kick-down mechanism, known as "the driver."

    vinaysp guest

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    I have driven cars with automatic transmission and know that if you floor the accelerator then the vehicle kicks down a gear (though I didn't know it was called 'kickdown' until now). However, I am specifically interested in how this works on my Z4? How long after flooring the pedel will the vehicle kickdown and obtain additional power?
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    MGarrison

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    I would think it should happen pretty darn quick. If you floor your gas pedal as hard as possible, and your transmission doesn't almost immediately downshift to the lowest useable gear and accelerate as much as the car can (you should be thrown back into your seat, a good bit), make sure your floor mat isn't bunched-up behind the gas pedal (or otherwise limited from it's full range of pedal travel). If all is clear and you can definitely fully floor the pedal and the car ain't movin', there's some kinda problem. If you have different driving modes available (sport, etc), try them as well.
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    Zeichen311

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    Ah. No differently than on any other car. There is no "additional power" on tap, it just shifts to the lowest usable gear to make the most of what's available. The downshift should take a fraction of a second.

    vinaysp guest

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    Thanks you to those who have responded so far. I had one more question: Is there a sweet spot in terms of how fast you should be going where you would feel the surge the most when you floored the pedal and activated the kickdown? for example, is there a difference if I did this at 45 mph vs 60 mph? I tried this a couple of times this evening on surface streets and didn't feel much. Maybe I didn't wait long enough before taking my foot off the pedal?

    vinaysp guest

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    Hmmm - so if I used the paddle to downshift, then I would be able to achieve the same results?
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    MGarrison

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    Shouldn't you be gently breaking in a new cylinder head right about now?? ;)

    As far as feeling acceleration by your "butt-dyno", I'd guess the faster you're going, the less forward-thrust you'll feel from a kick-down. A speed where flooring it would downshift you to an rpm point that's slightly before the engine's torque-peak, with the downshift going into 1st or 2nd gear, I think might give the greatest sensation of acceleration.

    vinaysp guest

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    I don't know :( SHOULD I be breaking in that new cylinder head gently? How do I break it in, and for how long is the break-in period?
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    MGarrison

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    I'd say double-check w/ the dealer, they should have a word for ya - otherwise check your manual if it says anything for how to drive your car when new and follow that - barring any info from those sources, perhaps something like no redlining or hard acceleration for awhile. I can't remember BMW's suggested break-in procedures from old-school engines going way back, but they were something like for the first x-many miles (say 500, or maybe 1200) no full-throttle or redlining, and an rpm limit of something like 4500 rpms) and vary engine speed (ie, don't cruise at a steady speed for lengthy periods of time), and then maybe a rpm limit of 5500 for the next 500 or so miles, and somewhere by the time you got to 2500 or 3k miles, you could consider the engine broken in (more or less, as best I recall).

    None of that may apply to modern BMW engines (anyone who knows or remembers may well stand to correct me!), but it might not be a bad precaution to take it easy for a few tankfuls of gas or couple weeks or few hundred miles, even if BMW says 'ah hell, go for broke and have fun!'.

    I don't know that any of this applies to your specific situation, type of engine, or cyl. head, but here's what one engine rebuilder appears to be suggesting -

    http://www.bavengine.com/install.html

    M3Driver guest

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    <chuckle> My late father used to call that the "passing gear".... "Kick it into passing gear and get around him..."
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    Zeichen311

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    "Passing gear!" That was my granddad's term too. I'd forgotten it. :)

    @vinaysp: MGarrison has it right, you are in a break-in period for that new head (shame on me for not picking up on that!). The owner's manual covers break-in procedures but it's pretty much as stated: Do not exceed 4500rpm nor 100mph for 1200mi, avoid wide-open throttle and increase your self-imposed redline gradually after that (I've used a "+500rpm every 100mi" rule of thumb, but there is no hard & fast rule).

    This procedure should be repeated any time major driveline components are replaced. The manual also states this (albeit not prominently) and lists a few example cases.

    In practice, you won't do any serious, immediate harm if you forget to follow these guidelines or outright ignore them. There will be excess wear but it is unlikely to cause problems for 75,000-100,000 miles or more. If you don't keep the car that long, then to be brutally honest anything you screw up now is the next guy's problem. :(
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    Zeichen311

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    Yes. In fact this can feel faster, because you select the correct gear before rolling on the power. When you rely on the kick-down switch there can be a delay (however brief) while the car figures out the best move--then you take off. This lag between input & response is a major reason why us three-pedal drivers don't enjoy automatics much. ;)

    The good news is, BMW transmissions use adaptive logic to tailor the shift response to your driving style. If you use kick-down a lot, generally are pretty aggressive with the throttle and often put the car into situations that demand downshifts, it will learn to downshift more quickly and more often than if you accelerate gently all the time.

    But! Note that if you respond to downshifts by almost immediately lifting off the gas--say, because you're running out of room--the transmission software reads that as, in effect, "hmm, too much, I scared him." Do it often enough and it will be less likely to downshift in the first place. This makes predicting the transmission's behavior a maddening exercise if your driving habits are erratic and variable.

    In short: BMWs reward smooth, predictable driving by being on their "best behavior." The better you are, the better the car will perform.

    vinaysp guest

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    Thank you for that awesome feedback! I have to ask another newbie question here - what is 'redline'?? < embarrassed look>
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    floydarogers

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    Not true. The computer (no more hydraulic-driven logic) in the transmission is able to decide what to do in fractions of microseconds - far, far faster than human minds. And modern automatics shift far faster than humans can move the clutch and shift levers (on the order of 0.1-0.2 seconds IIRC). (It's also why DCT cars are quicker on racetracks.)

    However, the computer is not able to *anticipate* what to do - that's why using the shifter or paddles is better.

    Also: Sport mode on the auto transmissions is a good way to go - the tranny logic uses a different map for shifts and is more aggressive on deciding to downshift.

    If selecting a gear (using paddles/shifter), select a gear that will put the tachometer in the sweet spot of the torque curve. And if you don't know what that is, look it up.

    vinaysp guest

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    On the 3.0i, the max power output at engine speed is 6.250 rpm. The max torque at engine speed is 2750 rpm.

    Is this information supposed to tell me what the sweet spot on the torque curve is?
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    floydarogers

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    2750 - max torque is a good sweet spot. Sometimes you may want a higher rpm - for instance maintaining speed on a steep hill if the hill is so steep that the torque available in a lower gear is not able to hold speed.

    vinaysp guest

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    I just spoke to the service manager at my dealership and he informed me that there was no break in period for the new cylinder head. Sooo...what do I do guys?
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    Zeichen311

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    While I agree with you that this is generally correct, it's an observable fact that you can, in effect, "confuse" the shift logic. Whether it's from adapted parameters, mechanical safeties or something else, on occasion you can stand on the throttle and be stuck waiting agonizing tenths of seconds before anything happens. This happens very infrequently but it does happen.

    As you say, it's primarily because the computer can't anticipate--it can only decide based on sensor inputs, algorithms and accumulated data. On occasion, the decision logic concludes "don't shift"--but detects a continued kick-down/override signal, re-evaluates, and then shifts. The shifts take milliseconds, the decisions mere microseconds but sometimes, it makes the wrong decision--repeatedly and with blinding speed--until all its inputs force the outcome you wanted it to reach in the first place.

    No need to be embarrassed, none of us were born knowing. (Maybe there are a few exceptions hanging around here...you'll recognize 'em eventually. :)) "Redline" is the maximum safe operating speed of the engine, in revolutions per minute (RPM). For practical purposes, it is approximately where the red band on the tachometer face begins--hence the name. The engine will protect itself by cutting off the fuel supply if you try to go more than a couple hundred RPM past that point.

    An automatic transmission will refuse to downshift into a gear you select with the paddles if that gear would force the engine speed to exceed redline at the current road speed. In that case, it will delay the shift until the road speed drops to the point where it's safe, or until you get the message and select a higher gear. This is what we meant by "lowest usable gear" at a given speed--left to its own devices, the transmission will not over-rev the engine.
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    Zeichen311

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    The moving parts in a cylinder head do need to wear in a bit IMO but the techs know whether they changed enough to matter. If they say go for it, I say have fun. :)

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