Piston Slap: Blogging About Engine Bogging

Sajeev Mehta
by Sajeev Mehta

Pat writes:

Hi Sajeev,

I have a question about driving style that I’d like to pose to you and the B&B. Part of my highway commute is a steady 2 mile grade. With a running start of 75 mph, my 2007 Mazda B2300 slows to about 62 mph by the top of the hill when I keep it in 5th gear, with the engine turning about 2000 rpm. I can maintain 70+ if I drop into 4th and floor it, but I’m a cheapskate at heart. My question is, is it really more efficient to lug up the hill in top gear, or am I just kidding myself and doing irreparable damage to my engine?

As an aside, I recently traveled to Vietnam and I noticed that all the cab and minibus drivers upshifted extremely quickly. Typically they were in 4th gear by about 15-20 mph, and really lugging the engine (I rode mostly in Toyotas). Besides the obvious lack of quick acceleration, any downsides to this kind of driving style? How much gas could be saved?


Sajeev answers:

How funny: I noticed the same problem in India. Be it Maruti, Toyota, Honda or Hindustan Ambassador, you’d hear a horrible “chug” of engine bogging on a regular basis. It keeps stressed out drivers from “unnecessary” down/up shifting in dense urban conditions, if that was the point.

Unless we’re talkin’ about a friction-challenged road starting from a standstill, never intentionally engage in engine bogging! Each engine/transmission/body combo handles loads differently, there’s no magic boggy-RPM number: the unique sound of engine bog is all you need to know. Depending on the severity of the bog’s shake, this increases clutch wear, damages motor mounts and maybe even stresses the weakest link in your reciprocating parts (crank+rods+pistons).

In your case: bogging up a 2 mile grade kills fuel economy. Being that low on the torque curve combined with massive throttle inputs means you’re burning fuel with little return on investment. Clutch wear? Probably not. But accelerating near your torque peak (3750 revs) gets you up the hill with the most efficiency (least throttle input) so you can spend more time “cruising” on flatter terrain at lower rpms, sooner. More to the point, 3rd or 4th gear is your friend, my friend son!

Bonus! A Piston Slap Nugget of Wisdom:

Perhaps your (presumably stock) Mazda needs the SCT tune (low-octane) and modest intake/exhaust plumbing modifications of my Ranger. Its your sister-ship, ya know. The volume of low-end torque below 3000rpm increased dramatically to the point that 2nd gear with steep parking garage grades was doable, and almost worth the extra throttle input. Almost.

[Image: Shutterstock user N K]

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.

Sajeev Mehta
Sajeev Mehta

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  • Moore101 Moore101 on Aug 20, 2014

    How many miles are on your pickup? You might have the beginning of a plugged cat. I had a 89 Camry that drove great on flat land but was a dog going up hills, replacing the cat solved the issue. For the MPG questions when towing a trailer with my Nissan Titan I get better mileage going up hills in 3rd at higher RPM than 4th or 5th but my truck is an automatic.

  • Baldpeak Baldpeak on Aug 21, 2014

    What you want to know for efficiency is Specific Fuel Consumption or SFC, sometimes called Brake-Specific Fuel Consumption or BSFC. Here's a page that has a bunch of plots. http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_(BSFC)_Maps

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