By on August 20, 2014

 

(photo courtesy: surftherenow.com)

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.

 

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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51 Comments on “Piston Slap: Blogging about Engine Bogging...”


  • avatar
    sportyaccordy

    I’m wondering if bogging = low fuel economy. Fuel consumption is proportional to air flow. When the engine bogs, I’m guessing it’s doing it because it can’t get the air necessary to comfortably handle the load. Plus in high gears, you can open the throttle 20%, 50%, 100% and on most cars you won’t get any more torque. So I’m not quite sure it’s a fuel economy thing.

    It definitely will destroy mounts and eventually internals. Unless your car has crap gearing just downshift to a higher gear.

    • 0 avatar
      HerrKaLeun

      From a Thermodynamic point low rpm at high load would be good. And all commercial shipping diesels agree with me. BUT there is a but. At full load airflow can’t be increased as it depends on rpm. So the car OEM enrich the fuel to be below stochiometric ratio to get more power. So you waste unburned fuel an kill emissions.

      Maybe those Indian cars don’t enrich fuel?

  • avatar
    mike89

    Nice Fiat 682 in that picture. I know that these (and other italian trucks, including even some prewar models) are still very popular in Africa.

    http://img.izismile.com/img/img6/20130225/1000/carthemed_jokes_09.jpg

    http://www.eritreaeritrea.com/images/camion%20italiani%20d%27eritrea/P1040518%282%29.JPG

    http://www.eritreaeritrea.com/images/camion%20italiani%20d%27eritrea/P1040528%282%29.JPG

    • 0 avatar

      I have no idea but that pic could have been taken in Brazil. Those old Fiat trucks were built and sold here and would look like that by now. It looks like the cargo is sugarcane and there is a mule. If may not be, but this is a daily scene in parts of Brazil. On the road, I have seen many trucks loaded like that.

      • 0 avatar
        mike89

        The one in the lead pic is RHD, however.

        BTW, do you still see trucks this old on the road ? And what about the even older Alfa Romeo-based FNMs ?

        • 0 avatar

          Ooops didn’t notice that! Definitely rules out Brazil.

          It’s been a while since I saw one. The last time I saw an old Fiat truck was on a trip to Rio about 2 years ago, while the Alfa-FNM must be at least 10 years (or more) I don’t see them anymore. The Italian trucks really stood out as their design was completely different (and nicer) from the more common Merceds, Scania and Volvo trucks.

          And yes, you still see them out on the road though trucks as old as the one pictured are definitely getting rare. The government has picked up on inspection and many have been retired. I do see a lot of them in the cities and for short hauls between close cities. Out in the country there are probably many more. For instate long hauls I guess the risk of having the truck impounded is too great. 20 year old trucks though are quite normal.

  • avatar
    petezeiss

    If I were a yak I’d find that photo pretty sexy.

  • avatar
    FormerFF

    Here’s an easy clutch wear test: If the clutch is slipping, it is wearing. If it is not slipping, no wear occurs.

    In theory, an engine has its lowest specific fuel consumption at low rpm and a high throttle opening. In automotive practice, the savings are negligible. For many years, Indycar drivers used to short shift when they were in fuel conservation mode, but found it didn’t make all that much of a difference. Now they have different mixture maps they can use.

    There’s no real benefit to lugging the engine. There’s no need to spin it too fast either.

  • avatar
    thalter

    I’d always heard that engines are most efficient at WOT (something to do with pumping losses).

    In fact, I recall several manufacturers working on engines with throttleless intake systems.

    • 0 avatar
      danio3834

      “In fact, I recall several manufacturers working on engines with throttleless intake systems.”

      Diesels and HCCI gas engines.

    • 0 avatar
      Quentin

      I think that saying depends on your definition of efficient. Are you talking about being the most efficient at turning a given amount of fuel’s energy into work or talking about using the least amount of fuel? The saying you are referring to relates to turning fuel’s energy into work. If you engine could be sized to perfectly match the required work output for all conditions, then yes, WOT would be most efficient. Since our engines make a lot more power than they need for most driving, WOT is not the most efficient way to drive. Minimizing pumping losses does help fuel efficiency at the detriment to power which is why you see Atkinson cycle on many hybrids.

      Anyone have a brake specific fuel consumption chart on a B2300?

    • 0 avatar
      Chris FOM

      They’ve gone beyond working on it. The new variable valve lift systems (think Fiat/Chrysler’s MultiAir or BMW’s Valvetronic) basically eliminate the throttle, instead regulating airflow by how much the valve opens instead. They do still have a throttle, but it’s a backup used only in case the VVL system fails. Under normal engine conditions they rut at WOT all the time.

  • avatar
    danio3834

    I wouldn’t be so worried about the fuel used getting up the hill as much as how much you can save coming back down. Keep it in 5th, and coast as much as you can without touching the brakes.

    • 0 avatar
      sirwired

      Shouldn’t that be “pick an appropriate gear and coast as much as you can”.

      Coasting in, say, 3rd on a steep grade uses the exact same amount of fuel as coasting in 5th on a shallower grade: zero. (At least with a modern fuel-injected car.)

      • 0 avatar
        danio3834

        If you’re trying to regulate your speed I guess. If you want the most fuel savings possible, the highest gear will let the vehicle roll out further due to less engine drag.

      • 0 avatar
        lellololes

        If you’re looking to save fuel, running a lower gear downhill won’t directly cost you more fuel, but the increased engine braking from having more friction will cause you to slow down faster, or gain speed slower – and retaining momentum improves fuel economy. As long as speed isn’t excessive, you want to minimize the amount of resistance the car presents to its own momentum, therefore, use the highest usable gear.

        • 0 avatar
          Scoutdude

          Actually downshifting while going down the hill will save gas on this truck, as long as it makes the engine rpm above about 1750-2000 rpm. It has DFCO or Deceleration Fuel Cut Off which means that it will burn zero fuel if you create the conditions for the computer to enter DCFO strategy.

          • 0 avatar
            danio3834

            Is that unique to this model? The DFCO strategies I’ve seen don’t have a minimum RPM threshold that high.

          • 0 avatar
            danio3834

            Either way, at 70mph+ the engine will be spinning faster than 2000rpm in 5th gear in that truck which likely has a 3.73:1 or greater axle ratio.

          • 0 avatar
            Scoutdude

            The DFCO minimum rpm does vary depending on the model specifics. Many vehicles now use what is called ADFCO or Aggressive DFCO which will keep the fuel off until a lower RPM but may still have a higher required RPM for it to enter the strategy.

  • avatar
    Vulpine

    Keeping in mind that I’m driving one of the first-generation JKU Jeeps (an ’08 with the Caravan engine) I’ve found that its best economy comes in at around 1800-2100 rpm no matter what gear I’m in. But the key point here is that you want to be in the best gear for the type of driving where a dirt trail/unimproved road may require 3rd or 4th gear, suburban driving 5th gear and highway driving in top gear. I’ve had 5th gear reach 28mpg in suburban driving while top gear is lucky to give me 25mpg _at best_ on the open highway.

    The engine certainly isn’t lugging at those revs, but I’m at the bottom end of my torque curve, too. any significant grade will noticeably lower my speed and rpms while forcing the engine to work harder to produce the torque trying to maintain speed. By downshifting one gear, the load is notably reduced but on a long grade such as Pat describes, dropping another gear may be recommended even if it does reduce the overall speed by a bit. If your foot has to be on the floor to maintain speed, then you’re feeding it more gas than it needs for the task. Just be sure to upshift again once you crest the hill to bring overall revs back down.

    Short-shifting? Well, as mentioned before it really doesn’t make a whole lot of difference when you’re operating at near max-performance in the first place. On the other hand, for us “everyday drivers”, we certainly don’t need to wind the engine up tight just to roll from light to light, do we? If you want economy, stay out of the throttle as much as you can but don’t let the revs fall below what ‘sounds’ comfortable for the engine. For most engines this would be around 1200-1800 rpm with V8s easily handling lower revs than I4s. Again, with my Jeep I typically shift when revs reach about 2300 rpm, which has the next gear starting around 1800 rpm. People tend to be surprised when my average in-town mileage reads between 2-3mpg better than their own and 20% better than EPA or higher.

  • avatar
    geee

    I usually like to pick up a some extra velocity prior to the hill, and then gradually let off on the accelerator as I go up the hill, letting the car decelerate to just under the speed limit by the top. Seems to be less stressful on the engine (at least as far as how it sounds) and I can watch the fuel economy on the computer to dial in how best to do it. But I definitely dont let the car go down to bog levels – that sound and feeling always makes me think something is about to break.

  • avatar
    chuckrs

    Engine components can be damaged by poor oil pressure at low RPMs too. In the manual for my 1991 Carrera 4, it specifically warns against lugging. To them, luggingt meant below 2000 rpm. Can’t believe this doesn’t hold true to some extent for other makes.

    • 0 avatar
      danio3834

      It would. In general, oil pressure is increases with RPM because the pump is mechanically driven. As RPM increases, so does power and the load on the rotating assembly, so healthy oil pressure is important under those conditions. With a high load at very low RPM, the oil film may not be sufficient to avoid bearing wear.

    • 0 avatar
      Eiriksmal

      Ahhhhh… I never thought about what low RPM + high load would do to the oil’s circulatory system. Very interesting nugget from your owner’s manual.

  • avatar
    madman2k

    According to what I’ve read on hypermiling websites, there is a sweet spot for RPM vs engine load, as far as fuel economy goes. IIRC it’s about 80-90% load. On flat ground it’s usually recommended to upshift as soon as possible, but hills are probably different.

    Personally, when I’m driving a manual up a hill I always use the highest gear that will let me maintain speed, if I start slowing down and the end is nowhere in sight I’ll downshift. Then on the way down I’ll use whatever gear holds me at the speed limit without using brakes.

    I think the only way to find out for sure which way is the most efficient is to use a scan guage or another OBD2 device, and drive the same route trying both methods.

  • avatar
    NeilM

    With sustained open throttle, high load and low rpm (‘lugging’ or ‘bogging’ the engine) you’re maximizing rod bearing loads while minimizing oil pressure to them due to the pump’s low rpm. Altogether a bad combo in a traditional engine: don’t do it.

    That raises a question with regard to new generation gas turbo engines, which are designed to come on boost at low rpm to provide early torque onset. I hadn’t thought about it before, but they must have oil pressure systems designed to provide similar high pressure at low rpm. Or at least I hope they do…

    • 0 avatar
      HerrKaLeun

      The Prius has electric oil pump. Pressure independent of ICE rpm.
      Toyota really figured that car out beyond the famous hybrid. All aspects are truly modern.

      • 0 avatar
        madman2k

        +1
        It’s a car people love to hate, but I enjoy mine.
        Great car for using cruise control up and down mountains on the interstate – the CVT gradually adjusts and holds the speed without any of the several-gears-at-a-time downshifts a modern 4 cylinder car with a regular transmission will bombard you with. Plus, you get better MPG than almost anything else even if you run the cheapest ethanol-blend gas.

        I don’t feel like a car can’t be enjoyed just because it’s an “appliance.”

        Besides, it’s has the same 0-60 time as a Delorean.

  • avatar
    Toad

    There is a simple way to determine what engine RPM is best at different vehicle speeds and loads: ride the same route on a bicycle.

    If you try to ride up an incline in a high gear, what happens to your legs and heart rate? Your legs/joints/muscles start to wear out quickly. The rods connecting the pistons to your engine crankshaft are no different. Drop down a gear on your bike and you notice you can cover the same ground with a lot less wear and tear on your body; the engine in your car is no different.

    Whenever you drive your car it is easy to imagine how your driving style would wear you out if you were bicycling on the same route. Treat your car as well as you treat your body and it will last a long time.

  • avatar
    albert

    It´s relatively simple. When you have full throttle and your vehicle decelerates on a slope, you are overloading your engine. Is this bad? Well, normally when you are on the point of the torque curve where the torque increases when the revs are slowing down, you will come to the point where there is equilibrium between available torque and the torque needed. Or: at that point you will hold your speed. If you do not reach this point, you would have been far better of by downshifting in time, because you will be too slow. The rest of the traffic will curse you.

    Will too low an engine speed do damage to your engine?
    Lubrication is not an issue. Modern engines have sufficient lubrication from idling speed up, even under full throttle. Clutch wear is a real problem. Not the friction materiel, but the damping springs. Really an issue with two mass fly wheels (think of modern diesels in Europe!). The cost of replacing your DMF is way more than the savings in fuel, even with the European fuel pricing.
    The lesson of this: keep you engine in the correct power band. Shift up and down when necessary and don´t wait too long.

  • avatar

    Higher gear more fuel economy. Shifting under 3k rpm too. The trick is too do it without having the engines shuddering. A Brazilian website did several tests on this about a year back and showed this time and again. It has to do with how modern cars manage fuel. So going up a hill in high gear, pedal to the medal doesn’t do damage and saves fuel, but shift lower before the engine shudders, that is when damage can be done.

  • avatar
    DenverMike

    I just can’t stand the sound and feel of a lugging, lurching engine that actually wants to stall. If it could talk, it’d ask you to please learn to drive. And why not just start in overdrive???

  • avatar
    heavy handle

    Here’s the theory: your engine is most efficient at full load and wide-open throttle. So that’s a vote for holding 5th all the way up the hill.

    In practice, it depends. High load and low RPM can mean detonation (knock), which would be a bad thing. Your ECU may also go rich at full throttle, which will reduce power and efficiency.

    The best way to know what your specific engine is doing would be to record OBD2 data feeds and compare. Does your timing go wacky by the time you get to the top of the hill, does the engine go into open-loop, does the A/F ratio go stupid rich?

  • avatar
    PrincipalDan

    I downshift in manual transmission cars on steep hills mostly because I hate to slow down. Good to know that it was the right thing to do all along.

  • avatar
    TheyBeRollin

    In my totally non-scientific test of fuel economy, now that I live in a place with a lot of smaller hills and lower speeds, I found that higher RPMs help. I averaged about 28-30mpg and peaked around 32-33mpg on mostly-highway trips when I lived in SoCal. My mileage after moving up here was abysmal, peaking around 27mpg, so I did an experiment.

    Keeping the car in a range where it doesn’t lug, but pulls along effortlessly seems sensible, roughly 2000-2700 rpm, so that’s what I’ve always done. My test was to run it the entire time at higher rpm so it never decelerated going up hills. Mysteriously, if I aim in the 2500-3500 range at all times (run in lower gears longer, shift when it hits the top of the range, then down when it drops below the low end while under power) it uses less fuel and I will see 32-33mpg. It feels weird to drive like that, but it saves gas, so I know it is more efficient.

    As far as I can tell, fuel-injected engines get the best fuel economy somewhere just under the peak of their torque curve. I haven’t figured out if this applies to high gear on highways/freeways because aside from some long downhill stretches I don’t have anywhere nearby where I can use a gear higher than 4th.

    Another strange thing I’ve found is that my GF’s automatic car gets dramatically worse fuel economy here than it did down there. We’re talking a 10+ mpg worse. I believe this is somehow related.

    • 0 avatar

      “Mysteriously, if I aim in the 2500-3500 range at all times (run in lower gears longer, shift when it hits the top of the range, then down when it drops below the low end while under power) it uses less fuel and I will see 32-33mpg.”

      Methinks you’re running closer to your torque peak to get that benefit.

      • 0 avatar
        TheyBeRollin

        I’m sure that’s the reason, too.

        It looks like the best results are seen around 70-90% of peak torque. The peak is 4250 for the engine in my car, so I could probably still do better if I was even more rev-happy.

        It was surprising because I believed that (short of lugging the car) less throttle input would always result in better fuel economy.

  • avatar
    dal20402

    All of this discussion makes some sense for naturally aspirated cars.

    But with turbo cars you have an additional factor: whether you are in boost or not. I get the best fuel economy out of my turbo Forester (which still isn’t very good economy) by keeping the revs ridiculously low, as much as the Jurassic 4-speed auto will allow. That way I stay out of boost, and far less fuel goes into the engine even at the same level of acceleration. If I drive as if the redline is 2000 rpm, I can get 22+ mpg in mixed driving. That goes down to 19 or so if I let the engine spin to 3000-3500 rpm, which is what the car wants to do if you don’t carefully manage it. To drive an auto, turbo car with such low revs requires a lot of finesse on the throttle.

    • 0 avatar
      TheyBeRollin

      Newer cars with turbos have peak torque really low in the RPM range, though.

      This is clearly more difficult with an automatic.

      • 0 avatar
        krhodes1

        And this is much of the reason I call BS on most manufacturer claims that the auto gets better fuel mileage than the manual. Only on the test it will, not in the real world with me driving. Exception being silliness like the ridiculously short geared Fit.

  • avatar
    Scoutdude

    As long as you don’t need more than about 75% throttle keeping it in 5th will give you better economy. 2000 rpm is not bogging the engine. The farther open the throttle is the lower the pumping losses. However you reach a point where the vehicle drops out of closed loop and enters enrichment mode the best fuel economy can be had when you are just under the point where it will enter enrichment mode.

  • avatar
    RogerB34

    B2300 L4 engine specs show 112 hp at 4800 rpm.
    Best torque is 135 ft lbs at 2400 rpm.
    The answer is to maintain 2400 rpm under load by upshift or downshift.

  • avatar
    Searcher

    With big trucks best grade-climbing economy is by not going full-fuel, allowing speed to drop off gradually, and downshifting as necessary until a gear is reached where you can accelerate and then holding that gear at about 1500 rpm.

    eta:it should be noted that on those big diesels 1500 rpm will be about midway between the torque and horse peaks

  • avatar
    moore101

    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.

  • avatar
    Baldpeak

    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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