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I have 2008 Subaru Outback (160K miles) with an automatic transmission and have been curious about the Sport Mode that can be shifted into. I read in the car's manual that it increases pickup and is preferable for driving up-hill. I was curious how it accomplishes this, and if it has other consequences as well. (I have googled this, but have found a lot of different and unsatisfactory explanations.)

As a bonus component to this question, I have a bit of a wonky transmission that tends to "catch" or "lurch forward" when I attempt to accelerate too fast directly after breaking. When I drive with Sport Mode activated it seems to reduce the frequency of this quite dramatically. Why would this be the case?

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    "How exactly is a rainbow made? How exactly does a sun set? How exactly does a posi-trac rear-end on a Plymouth work? It just does." Joe Dirts Dad...classic... – Moab May 26 '16 at 23:49
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Sport mode will "increase pickup" by providing a more instantaneous response to throttle inputs. As akadian mentioned, the car will stay in lower gears as you accelerate, allowing the engine to stay in the optimal rev-range for higher torque output. The computer, by activating sport mode, will also not have to 'think' about what gear you should be in for optimal efficiency when you press the throttle, so the electronic throttle and transmission are able to more quickly react to throttle inputs.

As for the wonky transmission: If you haven't changed your transmission fluid/filter in a while, the solenoids that facilitate gears shifts can get 'gummed up,' preventing them from reacting quickly to shift requests from the ECU (engine control unit). This can cause the jerkiness you described, as it is allowing the engine to rev up before the car is ready to accelerate in the desired gear. When the transmission finally shifts, the engine is already accelerating causing it to "catch" and "lurch forward" as you described. The wonkyness is not as pronounced in sport mode because the transmission will have already shifted down during braking to keep the engine in the optimal power band.

Edit: The ECU will also learn what kind of inputs to expect from its driver over time. If someone else had been driving the car for a while, or multiple people drive the same car, the ECU will expect certain patterns and agressiveness, which can lead to a jerky experience if it assumes incorrectly.

  • I will look into my transmission fluids, appreciate the tip! (and, of course, the answer) – DeathByTensors May 26 '16 at 23:14
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Typically, a spirited driving style involves staying in gears until the higher RPMs to make most out of each gear. This generally provide mode torque and more enjoyment.

With this in mind, it's easy to understand that a "Sport Mode" would allow you to use the lower gears for longer as they provide more power, which makes it more "sporty".

On the contrary, when driving in "normal" mode or "eco" mode, you car will attempt to maximize fuel efficiency, which is accomplished by reving the engine as low as possible. Sometimes it can be achieved by skipping gears.

For the bonus question, the car is trying to select the perfect gear for the way you drive. Unlike a manual, the car doesn't have the luxury if getting your direct input. The only way it knows what to do is by reading your throttle. When you attempt to accelerate too fast, since you're not in sport mode, your car might try to down shift to let you accelerate faster hence getting up to speed quicker (Accelerations are not fuel efficient, unlike holding a constant speed)

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