Piston Slap: The Auto-Erratic Transmission?

Mike writes:

Sajeev,

I bought a 2014 Impala LT with a 2.5 four cylinder, and a 6 speed auto. I'm a 60 year old guy, that's driven more cars than I can count. I'm still in awe that the engineers have figured out a way to move a car with the weight and size of the Impala with a 155 cu. in. engine. I love the car, with its comfort, and size, it suits my needs perfectly. I'm getting great gas mileage, with mostly city driving. Were flirting with 5 dollars a gallon up here.  I'm willing to sacrifice power for economy.

I'm rarely on the highway these days.  However I do find that at highway speeds{ 75 mph or so} the slightest touch of the gas pedal, will cause a down shift. The tach will jump from 2200 up to the high 3000′s in an instant. Does the 6 speed down shift sequentially, 6 to 5? Or will it go back 6 to 4th?

A week or so ago, I think it was "Kenmore" that was talking about a 6 speed Honda?  The discussion revolved around the transmission " clunking" as it downshifted at below 10 mph. I find the Impala does that under certain conditions.  Is this normal?

Thanks

Sajeev answers:

Occasional clunking is normal until some third-party disassembles a metric ton of these gearboxes, points to a poorly designed part and goes on the Internet saying, "ZOMG Y U ENGINEERS BE SO CHEEP HERE?"

And by that I mean that we shall never know. Regarding the frequent downshifting, I recently rented a four-cylinder Buick LaCrosse, same problem.  Hell, even a V6 Mustang rental constantly shifted when I breathed on the gas. On a mostly flat stretch of highway!

This frustration is why I referred to these units as auto-erratic in my review of the CVT powered Mitsubishi Mirage. People think CVTs suck, rightly so.  But many of today's self-shifters suffer from computerized analysis paralysis.

It's not entirely the autobox's fault: with only 186 lb-ft of torque peaking at a somewhat high 4400rpm, don't blame the Impala for a 6-5 or 6-4 downshift because you feathered the go-go pedal. That's just the way it is…unless you get a 74hp/74lb-ft Mirage with a Nebraska-flat torque curve.

But is this a problem? Not really: any auto-erratic box attached to a low-end torque free motor shall do this.  It bothers me too, but I'm spoiled by vehicles with a fatter torque curve. I wager you are too, in your 60 years on this earth. That said…

Bonus!  A Piston Slap Nugget of Wisdom:

Let's consider the epic struggle between horsepower and torque. There was a time when most everything made power like a modern turbo diesel. Back when the battle for peak performance numbers and increasing redlines in boring family sedans and pickup trucks with a 4000rpm torque peak were unheard of.  

The good old days?  Not entirely sure.  But it'd be fantastic to see today's technology applied to a fatter torque curve instead of sky-high horsepower battles. There'd be a superior driving experience and better fuel economy (less throttle needed), with a modest penalty in full throttle acceleration. Or so says the Piston Slap Guy…

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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