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?

Thanks!

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

More by Sajeev Mehta

Comments
Join the conversation
2 of 51 comments
  • 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

  • Dave Holzman A design award for the Prius?!!! Yes, the Prius is a great looking car, but the visibility is terrible from what I've read, notably Consumer Reports. Bad visibility is a dangerous, and very annoying design flaw.
  • Wjtinfwb I've owned multiple Mustang's, none perfect, all an absolute riot. My '85 GT with a big Holley 4 barrel and factory tube header manifolds was a screaming deal in its day and loved to rev. I replaced it with an '88 5.0 Convertible and added a Supercharger. Speed for days, handling... present. Brakes, ummm. But I couldn't kill it and it embarrassed a lot of much more expensive machinery. A '13 Boss 302 in Gotta Have It Green was a subtle as a sledgehammer, open up the exhaust cut outs and every day was Days of Thunder. I miss them all. They've gotten too expensive and too plush, I think, wish they'd go back to a LX version, ditch all the digital crap, cloth interior and just the Handling package as an add on. Keep it under 40k and give todays kids an alternative to a Civic or WRX.
  • Jpolicke In a communist dictatorship, there isn't much export activity that the government isn't aware of. That being the case, if the PRC wanted to, they could cut the flow of fentanyl down to a trickle. Since that isn't happening, I therefore assume Xi Jinping doesn't want it cut. China needs to feel the consequences for knowingly poisoning other countries' citizens.
  • El scotto Oh, ye nattering nabobs of negativism! Think of countries like restaurants. Our neighbors to the north and south are almost as good and the service is fantastic. They're awfully close to being as good as the US. Oh the Europeans are interesting and quaint but you really only go there a few times a year. Gents, the US is simply the hottest restaurant in town. Have to stand in line to get in? Of course. Can you hand out bribes to get in quicker? Of course. Suppliers and employees? Only the best on a constant basis.Did I mention there is a dress code? We strictly enforce it. Don't like it? Suck it.
  • 1995 SC At least you can still get one. There isn't much for Ford folks to be happy about nowadays, but the existence of the Mustang and the fact that the lessons from back in the 90s when Ford tried to kill it and replace it with the then flavor of the day seem to have been learned (the only lessons they seem to remember) are a win not only for Ford folks but for car people in general. One day my Super Coupe will pop its headgaskets (I know it will...I read it on the Internet). I hope I will still be physically up to dropping the supercharged Terminator Cobra motor into it. in all seriousness, The Mustang is a.win for car guys.
Next