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I have a car with automatic gear (Mitsubishi 1600cc, if it matters) and sometimes when I want to gain speed quickly (e.g. when passing a slower car in a two-way road) I do a kick-down.

I know it causes bigger fuel consumption, but I would like to know if it's also harmful to the engine itself, and if so, how?

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    How is "unintentional" kick-down any different? – Kromster says support Monica Mar 9 '16 at 13:55
  • @Krom "unintentional" kick-down happens when I don't press the accelerator enough, e.g. when driving up a sloppy road. What I mean in the question is when I press the accelerator hard and quick, giving a "fuel boost". Maybe I am not using the correct terms? – Shadow The Princess Wizard Mar 9 '16 at 14:01
  • "Kick-down" is not about a "fuel boost". You do a kick-down to let the transmission know that you want/need acceleration, and the transmission usually reacts by switching to a lower gear to allow better acceleration. And that's it. The engine is pretty much unaffected by this, except for the fact that it will have to produce more power because you put down the accelerator; that's what causes increased fuel consumption too. Accelerator down -> more fuel burned -> more power produced -> more acceleration. That's always the case, no matter if you fully kick down or just slightly press the pedal. – JimmyB Mar 9 '16 at 14:23
  • Your "'unintentional' kick-down" is also just the transmission switching to a lower gear and is perfectly normal operation without any extra stress on any parts. – JimmyB Mar 9 '16 at 14:24
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    In fact, when driving with manual transmission you will manually change to a lower gear when you need more acceleration; and you do so when driving up a slope to keep moving forward too. That's exactly what the automatic transmission does when you kick down. The difference is only in the way you "communicate" with the transmission, and kick down is the means to let the transmission know you want it to shift down. To make it automatic and intuitive to use, the please shift down 'switch' is built into the acceleator pedal. (That'd actually be a topic for UX :)). – JimmyB Mar 10 '16 at 10:25
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No, unless the engine speed increases excessively (in an emergency you will accept this for braking). Just about any vehicle will deny kickdown if your road speed is too high "at that time". It will put stress on your drivetrain, but this should be well within design parameters.

For a long steep descent in a heavy or overloaded vehicle, there may be issues with richer mixtures but this will depend on the delivery method and the age/condition of the components concerned. Keeping it on or above the redline will shorten the life of the engine, timing chains may stretch, valves may "float". If you have worn valve stem seals you will suck in some oil which can foul plugs and clog exhaust components (mufflers, ceramics, catalytic converters).

But in your case, the intent is for the shift down to occur so that the engine is not "lugged" and that more power is available. It is normal, many drivers would even preempt the shift to avoid the delay and perceived "shock".

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