It was easy in the old days. If you wanted more power, you simply added more inches, cubic inches, that is. But carmakers today are faced with increasingly stringent standards for emissions and fuel economy. One of the great challenges here in the U.S. is the Corporate Average Fuel Economy (CAFE) standards requiring a corporate average of 35.5 miles per gallon by the 2016 model year, and stretching out to over 54 miles per gallon by 2025.
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To the credit of the engineers and designers, so far the goals have been met. Engines are being downsized, but the technology they contain allows them to produce the same amounts of power their big brothers were just a few short years ago. Computer controls allow engineers to alter intake tracts across the operating range, change the valve timing to maximize efficiency at low and high speed, all in addition to the precise control they already had over fuel mixture and ignition timing.
But even with the high tech advances, a gasoline engine is still a gasoline engine. More simply, it’s an air pump and the more air we can pack into it, the more fuel we can burn, and the more power we can make. If a restriction forms at either end of the engine (intake or exhaust), it won’t be able to bring in all the air it’s capable of, and performance will suffer. And don’t let the word “restriction” limit your thinking. Anything that impacts airflow through the engine can be a “restriction”. There’s the obvious, like a clogged air filter, and the not so obvious, like retarded cam timing.
But first things first. How do you know if reduced breathing causes the drivability issue you’re chasing?
Volumetric efficiency is a comparison between the amount of air actually moved through the engine versus what the engine is theoretically capable of moving. In a healthy normally aspirated engine, volumetric efficiency will peak at a little lower than a perfect 100 percent. Expect normal to be around 80 to 90 percent. If there is help packing air into the engine (turbo or supercharger), that normal is going to exceed 100 percent. Look for numbers in the 100 to 120 percent range.