Piston Slap: MAP-ping Engine Load

Sajeev Mehta
by Sajeev Mehta
piston slap map ping engine load

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:

Oooooh yeah, muuuuuuch better.

[Image: Shutterstock user Joyseulay]

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.

Join the conversation
9 of 34 comments
  • DenverMike DenverMike on Aug 18, 2014

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

  • Cdotson Cdotson on Aug 18, 2014

    "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.

    • See 6 previous
    • Scoutdude Scoutdude on Aug 19, 2014

      @cdotson 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.

  • Chris P Bacon I had a chance to drive 2 Accords back to back as rentals. The first was a base ICE LX. I was underwhelmed. The next was a Sport Hybrid. Like night and day. So much so that I ventured on to the grounds of my local dealer. Was looking for a Sport or Sport-L. Autotrader showed nothing within 250 miles. Dealer confirmed. Told me I'd have to "get on the list" for a delivery, and there was a non-negotiable $3k "market adjustment". I guess I'll have to hope to see one on the Emerald Aisle again.
  • DungBeetle62 I just this past weekend rented one of these for 5 days in SoCal and with $5.29 the best I could find for gas, this ride's wonderful combination of comfort and thrift was welcome indeed. My biggest real beef is with the entire Accord product line - with that angle of backlight, not having this as a 5-door hatch seems a real waste of space.
  • RICHARD I bought my wife the exact car in the picture 3 weeks ago. Acceleration is average for the class. Smoothness of the powertrain, competent ride dynamics, quietness, and comfort are definitely pluses. The styling is restrained for sure, but we weren't looking for a shouty car that doesn't deliver on the design statement. She drives about 8,000 miles per year, mostly around town. At the current rate, we expect to buy about 16 gallons of gas per month. This really is a car that appears to do everything well rather than excelling at a few things to the detriment of others.
  • Ajla "2010-2019 Borrego"The Borrego only had model years 2009 - 2011 in the United States. The Borrego/Mohave did exist in international markets beyond them but the NHTSA of the United States would not be handling a recall on those. It's annoying that apparently the manufacturer, the federal regulator, and automotive press didn't notice this.
  • SilverCoupe The last Accord I test drove was in 1978, but I ended up buying a VW Scirocco instead. The Accords have put on quite a bit of weight since, then, but then again, so have I!