Piston Slap: Bogging, Tripminding, Economizing

Sajeev Mehta
by Sajeev Mehta
piston slap bogging tripminding economizing

Joel writes:

I have a manual transmission car and am traveling on the highway at a speed that has my RPMs low for mpg. Now say I come to a hill, and my car can hold its speed and rpms, but I have to floor it or nearly floor it to keep my speed. I could downshift, where my engine rpms would go up, and I wouldn’t have to give as much gas through the pedal. My question is, is downshifting using more gas than keeping it in the same gear?

Now, I do realize that this is pretty much an OCD kind of question, but then again, if the answer is rather large, it would be good to know. And, if the answer is negligible, that would be cool too. So, if you know the answer, or is the kind of thing to put on the website, I’d love to know the answer. Thanks in advance.

FYI: the cars in question are a 2001 Honda Civic and a 1985 Volvo 245, and a 2007 Honda Fit, and all are sticks. That said, I even converted the wife to driving a stick, the Fit is hers!

Sajeev Answers:

I answered this question once, well before there was a Truth About Cars. Years ago I installed a somewhat desirable, vacuum fluorescent display’d Ford “Tripminder” computer in lieu of the bean-counted clock in my 1988 Mercury Cougar. Proud of my accomplishment, I religiously monitored my instantaneous fuel economy to ultimately answer your question. And the answer was most inconclusive.

You have three factors in play: gearbox ratios, engine torque, and long term durability. Let’s stick with that final point, because going uphill at low rpms with high throttle effort isn’t a good idea for the engine. The extra load can have a negative impact on any number of bearings, shafts, etc. And while you might not notice any problems today…

Then there’s engine torque: if you have a gutsy V8 or turbo diesel at the helm, the motor is far less likely to “bog” at lower rpms when you hit the throttle. But most importantly, it’s game over when the motor bogs, downshift to save your engine all that unnecessary stress.

Finally there’s gearing: wide ratio or short ratio gearboxes in particular. When I did my fuel economy tests on my (wide ratio) four cog automatic-motivated Cougar, the rpm jump out of overdrive was quite significant. And mileage went down considerably compared to nursing the big V8 up a crest in top gear. After I made the switch to a six-speed stick, the rpm jump from 6-to-5 was far smaller, keeping the rpms low while avoiding the “bog” was no big deal.

Grain Of Salt Note: Houston is a relatively flat city, so my Tripminder’s calculations have little to no relevance in places with real terrain changes. No driving condition is the same, so I’d recommend this: avoid engine bog at all costs, downshifting accordingly with no regard to the trivial change it might have on fuel economy.

(Send your queries to mehta@ttac.com)

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  • Panzerfaust Panzerfaust on Apr 06, 2010

    One of the gaping problems with getting the most out of a tank of gas is the matter of time. Most aircraft pilots do not refer their range with the term 'miles per gallon' rather it is 'pounds of fuel per hour.' Which means that the most effecient setting of an aircraft engine might not yeild the least amount of fuel used, over a given distance because you're in the air longer and therefore are consuming more fuel than if you actually were moving at a higher speed. Those of us old enough to remember experienced this first hand when the Carter administration inflicted the 55 mph speed limit upon us. What we discovered was that on a long trip of say 300 miles we started with the same amount of gas as we did with the 65 mph speed limit, but got to our destination with less gas in the tank. Why? Because the difference in gas mileage between the two speeds was offset by the amount of time spent on the road. So in reality the difference between 65 and 55 was about one or two miles per gallon, but the time difference to travel the same distance was 45 minutes to an hour, and so I could actually use more fuel on a long haul at 55 than at 65.

    • Wmba Wmba on Apr 07, 2010

      Ahem... If you get more miles per gallon at 55 rather than 65, time has nothing to do with it. You have fallen into a classic trap, I'm afraid. Time is irrelevant to an mpg figure. It's miles per gallon, and miles and gallons have no time component in them. By your "argument", at 30 mph, the amount of gas used on the trip would be even higher! I had a friend who thought that driving home, which is due north from here, was obviously uphill. So he always expected to get worse mileage going home for the weekend, rather than when he came back south for the work week. Don't think he grasped the concept of sphere. Meanwhile, I conducted an experiment up a long fairly steep hill today. Speed 60 km/hour. Consumption in fifth, 19.2 litres/100 klicks. Fourth, 22.4, Third, 25.2. Coming back downhill, consumption in the same three gears, 2 l/100km. You guys can do the math. Fifth was most efficient. Subaru Legacy GT. I've noticed this many times on my instant economy readout. Lower gears eat gas, despite torque peaks, etc, etc. There is a reason why car companies make their automatic trannies grab that next highest gear as soon as possible, and it's better fuel economy. Hardly a revelation. As long as the engine and gear combination can provide the necessary torque to maintain speed up the hill, the lower engine speed, and hence higher gear will give better economy, unless the fuel injection cycle has to "overcompensate" for a higher gear. LJK Setright used to chortle at the usual old rubrick about damaging a "lugging" engine. Old Wife's tale. Mr. Locock above, an engineer in the automotive industry in the Antipodes, I believe, points out the real problem. The engine leaps around on its mounts and cannot run smoothly at too low an rpm under load (350 rpm in an old Chev stovebolt six, I'd say). My legacy will glide along at 1100 rpm in fifth lockup at 31 mph (50 klicks) on my secondary road behind Toyotas and Buicks, and accelerate smoothly from there if you don't goose it too much. If you do, it shifts to second and rockets the hell out of there. I really fail to understand the mystery in all this. Pumping losses are lower at wide throttle openings. For the average modern overpowered car, a very high gear will usually provide all the torque required to go up most hills, and the lower engine speed means fewer injection cycles per second. The time to shift down is when, with foot on the floor, the car slows down anyway.

  • Whuffo2 Whuffo2 on Apr 07, 2010

    There's not one totally correct answer here. On older cars with carburetors and distributors then maximum efficiency was when the engine was at full throttle, at it's torque peak RPM, and fully loaded. Those conditions are rarely met and seeking them in the real world is usually futile. It gets more complicated with fuel injected engines. I haven't studied all of them, but most of the common electronic injection systems run the engine lean under most conditions; the computer seeks the air / fuel ratio that is best for the catalytic converter, not fuel economy (for what that tidbit is worth to you). When you floor the throttle, the computer will shift into a "power enrichment" mode and temporarily drop out of closed loop operation - this gets you a nice burst of power but it isn't very efficient. On these cars, running full throttle at the torque peak and fully loaded results in very low efficiency and lots of unburned fuel going out the exhaust. If I had to come up with a "one size fits all" rule - use the gear that provides the lowest RPM as long as there's still sufficient torque available to accelerate the car. If you hit full throttle and can't accelerate it's time to downshift. And try to avoid holding the throttle fully open for any length of time - it's very wasteful.

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