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?
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 firstname.lastname@example.org. 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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