By on August 18, 2014

(photo courtesy: www.8thcivic.com)

TTAC regular David Holzman writes:

When my scan gauge says my engine is under 99% load, and I’ve only pushed the gas pedal about halfway down, does that mean, as I suspect, that I can floor it and I’m not going to get more than a drop more power out of it? 

And, in a modern car (’08 Civic, stick), will the computer control prevent me from wasting gas by pushing the gas pedal beyond the point where I’ve reached 99% load?

Sajeev answers:

I’ve wondered this myself, just not enough to research until someone posed the question to TTAC.

Since the dawn of carburetors, vehicles used engine vacuum to measure engine load under the guise of a fuel economy gauge. Earlier EFI machines implemented fuel injector duty cycle to spit out a fuel economy reading. It’s cheaper/easier/simpler to use the fuel injection computer’s powers to calculate an approximate number, but many (all?) newer models use the mass-airflow sensor (MAF) and/or the manifold absolute pressure (MAP) sensor as the basis of these calculations.

As per SAE standard J1979, there are two engine load values: calculated and absolute load value. I suspect absolute load value is used in more customer facing interfaces, as it’s a normalized figure that might be easier to apply across multiple engines, platforms and operation parameters sans re-work. And it probably neuters the data as to not cause end user confusion, warranty claims, lawsuits, etc.

If reading this hamfisted analysis upsets you, methinks you’re a pretty frickin’ brilliant engineer.  Distilling this into an easy to digest blog post isn’t easy, as I was more of a Collegiate SAE wonk. But let’s get it down to one sentence:

Load values are a normalized calculation of engine airflow, which isn’t a 100% accurate measure of the load on your vehicle’s engine at any time.

How’s that for not answering your question and giving me a headache?  I console myself with this Hot Panther Looove:

YouTube Preview Image

Oooooh yeah, muuuuuuch better.

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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34 Comments on “Piston Slap: MAP-ping Engine Load...”


  • avatar
    Sky_Render

    “… many (all?) newer models use the MAP sensor as the basis of these calculations.”

    Many new cars don’t use speed density (MAP sensors); they use mass airflow.

    • 0 avatar

      Correct, amending that right now.

      But some MAF cars still use a MAP: http://www.evoxforums.com/forums/showthread.php?t=126938

      • 0 avatar
        CoreyDL

        Somebody here once explained the big difference between MAP and MAF, and now I can’t remember what they said. However due to that conversation I do recall RAM trucks used a MAP, at least until recently.

        What’s the reasoning behind using MAP over MAF? I believe every car I’ve ever owned has had a MAF.

        • 0 avatar
          Sky_Render

          They’re different tuning strategies. Speed Density utilizes a MAP (Manifold Absolute Pressure) sensor to measure the pressure in the intake manifold.

          Mass airflow (MAF) uses a sensor to measure the amount of air coming into the engine.

          http://www.motoiq.com/MagazineArticles/ID/2254/Basic-control-systems.aspx

        • 0 avatar
          MBella

          All electronically fuel injected cars will have a MAP (manifold absolute pressure) sensor. It will be the sensor most used for calculating engine load. Other sensors are also part of it, like throttle position and MAF (mass airflow). Just because a car is equipped with a MAF doesn’t mean it’s not equipped with a MAP. The MAF’s main purpose is to get an accurate measurement of the airflow to allow for a more precise air/fuel mixture. On the older speed density systems. The air mass is calculated instead of measured. The calculation was based on the pressure of the intake manifold (from the MAP) and the engine RPM. This does not mean MAP sensors went away from with the addition of MAF sensors, they just were no longer used in the main air mass calculation.

          • 0 avatar
            Scoutdude

            No very few vehicles today have a MAP, the MAF is the norm and just like they used to use the MAP to calculate the amount of air entering the engine they now use the MAF to calculate the load. The MAP is making a comeback with the increase of turbo engines. For a while the only company still using a MAP was Honda and they used it to calculate the EGR flow and nothing more.

          • 0 avatar
            Sky_Render

            @MBella:

            Completely incorrect. Most naturally-aspirated cars do not have a MAP sensor at all if they utilize mass airflow (and more and more of them are).

        • 0 avatar
          Scoutdude

          The MAF is much more accurate providing better fuel control, lower emissions and allows the engine to respond to modifications and better compensate for wear over time. The MAP is cheaper. So there was a trend to using a MAF as emissions standards started tightening up. Ford the pioneer in mass market MAF vehicles did it because it allowed easy hot rodding which is what put the 5.0 on top. No need for a chip change like was required on MAP equipped GMs. Now with the proliferation of turbos the lowly MAP is making a comeback.

      • 0 avatar
        Sky_Render

        You’ll find that a lot of forced induction cars still have MAPs to measure boost levels. Both the Ford GT500 (supercharged) and the turbocharged Evo X you linked to are good examples.

    • 0 avatar
      raph

      Ford’s EcoBoost engines don’t use mass air as the resolution suffers due to all the associated piping for the turbo and air to air heat exchanger.

      Oddly my car which is supercharged uses a MAP sensor but calculates but calculates boost for the gauge by an inferred reading from the mass air meter.

      Anyways, speaking of fuel economy one of the best ways to achieve maximum economy is to lug the engine (and by lug I mean either start off in a gear that allows the engine to rev slowly or short shift into a higher gear to achieve the same effect) and hold the throttle wide open since it reduces or eliminates pumping losses in much the same way a diesel does albeit while indicating a low level of vacuum.

      I’m not sure if this can successfully done on an older car using a carburetor and stand alone ignition system but on a fuel injected car using a computer to control the ignition it can be used to good effect.

      • 0 avatar
        Sky_Render

        Yep, I just purchased a Fusion 2.0T and it is using speed density as opposed to mass airflow. That means I can put a stupid-loud blow-off valve on it, because I’m a ricer!

    • 0 avatar
      niky

      The Mirage uses a MAP here and a MAF there… I suppose the extra accuracy counts with the stricter emissions.

      Lots of modern turbodiesels have both.

  • avatar
    wumpus

    A quick question to ask would be, why didn’t the original emailer just press the pedal down a bit more? Car manufacturers have known since at least the 1990s that most car buyers never press the pedal down more than halfway (remember, camrys are the best selling cars. Now figure how often camrys get floored), a condition that seems to have affected our emailer. There is a good chance that the car is already WOT at “halfway down”.

    • 0 avatar
      Eiriksmal

      Er, except that you can push the pedal down halfway and see the throttle valve open to… halfway. Or, you know, drive a car with some power and compare acceleration at 50% vs 100% pedal travel. Huge difference.

      Edit: Yes, this even works on electronic throttles. Remove the air cleaner and MAF tubes, turn the key to “on,” pump the accelerator. Voila, valve movement.

      • 0 avatar

        >>>drive a car with some power and compare acceleration at 50% vs 100% pedal travel. Huge difference.

        It depends. In a low gear, almost always huge difference. In 5th, on the highway, or in fourth, around 30mph for example, there’s often no detectible difference.

        I don’t press the pedal beyond the point where there seems to be no difference (except when I’m pondering this issue), a point which I intuit easily.

      • 0 avatar
        05lgt

        I seem to remember from a previous life that these type valves only supply a 20% restriction (80% flow) at 50% open, and achieve something like 50% flow at 10% open. It gets more complicated with a long string of flow resistance components, at the lower pump speeds the one throttle valve is less % wise of the total resistance etc.

  • avatar
    bumpy ii

    What’s the throttle % look like when the engine load is 99%? ’09 Civic is drive-by-wire, so all the pedal does is roll a potentiometer that tells the ECU to open the throttle plate a set amount.

    • 0 avatar
      sportyaccordy

      It really depends. In neutral you can have 100% throttle and no load. I would bet in lower gears you can have 100% throttle and not have 100% load. On the flip side, in higher gears at lower engine speeds, I don’t think it takes much throttle to fully load up the engine. I.e. you press the throttle a little bit, and you have full airflow and get no added torque for added throttle travel. It’s not linear at all.

      • 0 avatar

        What you describe is certainly what it feels like, and the scan gauge seems to second that.

        @mnm4ever

        140 hp@6300 (19.6 lbs/hp)
        128 torque@4300

      • 0 avatar
        Scoutdude

        Load as reported by a PID is not the load on the engine at the crank it is a relative percentage of the air currently being ingested by the engine vs the maximum air that the engine could ingest at that rpm.

        So to answer the original question if the calculated load is at 99% then the existing throttle opening is providing almost all the air the engine could ingest at that rpm so opening the throttle won’t increase the power at that given instant. However since an engine that is at 99% load is likely accelerating that increased throttle opening will mean more power shortly. The only case that wouldn’t be true is if the engine was in too high of a gear.

      • 0 avatar
        Eiriksmal

        From Sajeev’s link:
        LOAD_ABS is the normalised value of air mass per intake stroke displayed as a percent.

        LOAD_ABS = [air mass (g / intake stroke)] / [1.184 (g / intake stroke) * cylinder displacement in litres]

        Calculated LOAD Value (PID 04)

        The OBD regulations previously defined CLV as:

        (current airflow / peak airflow @sea level) * (BARO @ sea level / BARO) * 100%

        Various manufacturers have implemented this calculation in a variety of ways. The following definition, although a little more restrictive, will standardise and improve the accuracy the calculation.

        LOAD_PCT = [current airflow] / [(peak airflow at WOT@STP as a function of rpm) * (BARO/29.92) * SQRT(298/(AAT+273))]

        So absolute load is air sucked in over the product of stroke length and cylinder displacement. Calculated load is the percentage of air being sucked in compared to air being sucked in at WOT at that same RPM.

  • avatar
    mnm4ever

    It’s an 08 Civic with what, 113hp… does it really matter??

  • avatar
    motormouth

    Not knowing the criteria used across engine load monitoring systems makes them about as useless as an upshift light that’s based on rpm and no other vehicle performance or handling information.

  • avatar
    DenverMike

    It’s trying to tell you to downshift/floor it, if you want faster acceleration.

  • avatar
    cdotson

    “And, in a modern car (’08 Civic, stick), will the computer control prevent me from wasting gas by pushing the gas pedal beyond the point where I’ve reached 99% load?”

    Strictly addressing this statement there are a couple misconceptions evident. Modern cars cannot prevent you from pushing the accelerator pedal further than where you hit full engine load. Pushing the pedal further down actually won’t consume any more fuel anyway. Also in computer-controlled/EFI automobiles the “wasting gas” part starts happening once you surpass 80-85% load as that is typically the threshold at which the ECU starts referencing safe/rich fuel maps and ignoring the O2 sensors. Your ScanGauge should have a setting that will display to you whether you are in open/closed-loop operation. If you monitor that particular gauge you can train yourself to drive within closed-loop control to maximize fuel economy if you so desire. You will notice that at low engine speeds and in higher gears it will switch to open-loop operation with smaller throttle inputs than at higher speeds or in lower gears.

    Given that vehicles now are all either MPFI or DI there is every incentive to massively oversize the throttle body. Larger throttles produce less flow restriction and the need for turbulent flow at the butterfly valve for fuel atomization no longer exists. Larger throttles “leak” a little more and prevent some emissions problems that arrive at high engine speeds when you side-step the throttle.

    • 0 avatar

      >>>“And, in a modern car (’08 Civic, stick), will the computer control prevent me from wasting gas by pushing the gas pedal beyond the point where I’ve reached 99% load?”

      This was a slightly ambiguous sentence. What I meant was, will it prevent me from wasting gas if I push the pedal beyond the point where the engine has reached 99% load. It’s quite obvious that I can push the pedal beyond that point.

      What is open loop vs closed loop operation? What loop? What what are MPFI and DI?

      Thanks.

      • 0 avatar
        bumpy ii

        Open loop= running solely off the preprogrammed fuel maps.
        Closed loop= using the MAP/MAF and O2 sensor(s) to optimize the base fuel maps.

        MPFI- multi-port fuel injection (in the intake header).
        DI- direct injection into the cylinder.

        • 0 avatar
          cdotson

          Thanks bumpy ii, that’s correct.

          Open vs closed loop refers to any control system. A control system is open-loop if it lacks feedback. A closed-loop control system references a feedback mechanism to adjust its input to achieve a desired output. In this case, the O2 sensors close the loop. The engine control unit (ECU) always references MAP/MAF and/or throttle position (TPS) to determine where on the pre-programmed fuel map to start, and references the O2 sensor (or not) to adjust the “fuel trim” to run stoichiometric. When at very high load the assumption is made that you wish for maximum power which is achieved at a richer than stoichiometric mixture, typically 12.5:1. Standard O2 sensors switch high/low at 14.7:1 to help achieve stoichiometric but do not have a readable variance in output when you get to 12.5:1 mixture so the ECU ignores the feedback and runs off the program. Higher-dollar lambda sensors will provide a readable variance in output over a wider range of mixtures and closed-loop control can be maintained with intentionally rich or lean mixtures but this is usually confined to exotics, race cars, or extra-cost efficiency seekers.

          • 0 avatar
            Sky_Render

            Modern engines only run in open loop during initial startup. As an example, the Ford Coyote 5.0 runs in closed loop even during full throttle conditions. The use of a scan gauge allows the reading of both wideband lambda sensors (one for each bank), and shows a solid ~14.0 under nearly all conditions. (Deceleration pegs the air/fuel ratio, due to fuel cutoff.)

            Furthermore, drive-by-wire systems no longer directly map pedal input to throttlebody angle. Rather, the computer produces a “torque target” based upon pedal input and looks that value up in a multi-dimensional array. Throttle position, camshaft advance, spark advance, fuel injector duty cycle, fuel pressure, and myriad other parameters are modified to produce the desired “torque target.” Wideband O2 sensor feedback allows correction of the fuel trims.

          • 0 avatar
            Scoutdude

            Emission controlled engines never run at stoichiometric except by chance when in open loop mode. To control the fuel the computer continuously adjusts the mixture until it gets rich or lean, once it sees that it is running lean it starts adjusting the other way until it sees a rich single then goes back to looking for it to be lean. Not only does this allow the computer to find the right fueling it is done to keep the catalytic converter working properly. The modern catalytic converter has an oxidation and reduction section. The oxidation section needs a lean mixture to provide the extra O2 for oxidation to occur. The reduction portion needs some unburnt fuel to keep the reaction hot and going.

            Cars with wide band O2 sensors still do this because the cat needs it.

      • 0 avatar
        kscott

        Yes, it will prevent wasting fuel. If no more air can go in, then it won’t put more fuel in either when you push the pedal farther. The amount of fuel, for the most part, just comes from the MAF/MAP reading along with the RPM. It’s really an air pedal, not a gas pedal.

    • 0 avatar
      Blue-S

      It should be said that cars/trucks which use a UHEGO (wideband) as the upstream O2 sensor do not need to go open loop at WOT/full load. Since the UHEGO is able to accurately measure the air/fuel ratio across a wide range, the ECM targets a richer than stoichiometric a/f ratio at high load values. It is still in a closed loop mode using the UHEGO input to drive Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) for feedback. The late-model Jaguar and Land Rover stuff with which I am familiar works like this. I do not know what Honda does.


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