Piston Slap: The Auto-Erratic Transmission?

Sajeev Mehta
by Sajeev Mehta
piston slap the auto erratic transmission

Longtime TTAC commentator Mikey writes:


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?


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.

Join the conversation
3 of 69 comments
  • Mikey Mikey on Aug 06, 2014

    Thank you so much, Sajeev... And all the B&B. I've certainly had all my questions answered. As I said, the Impala is perfect for me. For now. After reading the comments, I do have some concerns with the long term prospects of the 6 speed? As a life long GM buyer I've dealt with a few issues. Front disc brakes, and rotors, steering racks, trim falling off, motor mounts,S15 ball joints, and "A" car rusting backing plates, soft cams, and "in the gas tank" fuel pump problems. Oh! did I mention head gaskets? Never, in 45 years of driving GM, have I ever replaced a transmission. I ran an 81 Monte Carlo with a metric 200 over 100,000 miles. For sure, it did some weird shifting. I do think it needed a tranny when I dumped it. That Monte was a great car, I watched it come down the line. I got my wife a plant pass, and she drove it off the Roll Test. Thanks again guys

    • Sajeev Mehta Sajeev Mehta on Aug 07, 2014

      Nice job keeping a 200R4 running that long. Even if it was going sour, swapping to a TH-350 was pretty painless, if I recall correctly. Edited the article to reflect that you, our longtime reader, wrote this.

  • Mikey Mikey on Aug 19, 2014

    Sajeev.....A little late here....Thanks. BTW....I still read TTAC every day. We survived after Farago moved on, and we will survive J.B's departure. Keep up the good work!

  • Jeff S The question is how long will Ford offer the Mustang as a pony car? Dodge is sun setting the Challenger at the end of this year and it is doubtful if the Challenger will come back as an EV. Rumors are the Camaro name will be used on an EV and that will mostly likely be a crossover. There is not enough market for a Detroit muscle or pony car. It is sad to see not only the last of the cars like the Camaro and Challenger go but to see most cars go. Soon this site will have to change its name to The Truth About Trucks (TTAT).
  • Oberkanone Does GM build anything to compete with this? Does GM build any competent hybrids?
  • Dukeisduke So, it'll be invisible, just like all other Gen 6 Camaros?
  • Alterboy21 The gov't has already mandated control of your vehicle. 10 years ago they required cars to have ABS and traction control.I am not sure I agree that automatic breaking is ready for primetime, but taking control of a cars driving behavior is not new ground for the NHTSA. 
  • Parkave231 Collector's Edition hood ornament or GTFO.