Piston Slap: Hat Trick!

Sajeev Mehta
by Sajeev Mehta
piston slap hat trick

Jesse writes:

Hello, Sajeev.

My 2013 Outback 2.5i is fine and I don’t have any questions about it. Instead I wonder:

1. Why do car reviews measure acceleration in time but deceleration in distance?

2. Why do high performance electric cars need conventional brakes? I think there was a Mini concept a few years back that had 4 in-wheel electric motors that did all of the accel/decel.

3. Why don’t cars with CVTs have a ‘downshift’ button? Is it too hard on the transmission? Should I stop using the paddles to do so?


Sajeev answers:

Perhaps simple queries deserve simple answers. So let’s do this thang…

1. Relevance: people want to know how many seconds it takes to get to a certain speed, but not how many seconds it takes to stop. A certain stopping distance is far safer for them. Even if reaction times vary and press cars come delivered with varying levels of brake degradation (from previous journos cooking the pads and glazing the rotors), apparently stopping distance and acceleration times still means something to readers.

2. After driving Code Brown, I can guarantee you that traditional brakes are still important in the world of regenerative braking. Nothing stops like a disc brake, both in terms of speed retardation on the highway to the ability to modulate from a high rate (for safety on the highway) to a low rate (for comfort at a red light). Even when you put Code Brown’s regen brakes to full power, they can’t stop in a panic situation. I suspect the G-forces created via panic stop would spike hard enough to damage some component of the propulsion system. No surprise, Wikipedia has a great article on its limitations.

3. Why? Because CVTs don’t need one. You downshift (so to speak) via throttle inputs. You want maximum acceleration? Just go right ahead and bury the throttle in the carpet. Otherwise let the CVT work its fuel economizing magic. Downshifting (and upshifting) algorithms in CVT gearboxes exist to acclimate drivers to CVTs: a stepping stone (get it?) to the future. I mentioned the parallels to motorized seatbelts, I still believe this is true.

Off to you, Best and Brightest!

[Image: Shutterstock user Sensvector]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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2 of 28 comments
  • RHD RHD on May 06, 2015

    Curiously, that Lincoln had the stove-pipe hat Photoshopped on to its nose. Old Abe never wore his hat that way! For proper effect, it should have been near the front edge of the roof.

  • DrGastro997 DrGastro997 on May 06, 2015

    I thought the "B" was for engine braking with cars with CVT. That's what I had in a Corolla rental- it worked fine but did give you a bit of artificial feel of downshifting of gears but was simply engine braking.

  • Inside Looking Out You should care. With GM will die America. All signs are there. How about the Arsenal of Democracy? Toyota?
  • DenverMike What else did anyone think, when GM was losing tens of billions a year, year after year?
  • Bill Wade GM says they're killing Android Auto and Apple Carplay. Any company that makes decisions like that is doomed to die.
  • Jeff S I don't believe gm will die but that it will continue to shrink in product and market share and it will probably be acquired by a foreign manufacturer. I doubt gm lacks funds as it did in 2008 and that they have more than enough cash at hand but gm will not expand as it did in the past and the emphasis is more on profitability and cutting costs to the bone. Making gm a more attractive takeover target and cut costs at the expense of more desirable and reliable products. At the time of Farago's article I was in favor of the Government bailout more to save jobs and suppliers but today I would not be in favor of the bailout. My opinions on gm have changed since 2008 and 2009 and now I really don't care if gm survives or not.
  • Kwik_Shift I was a GM fan boy until it ended in 2013 when I traded in my Avalanche to go over to Nissan.