FALLACY ONE -- A DOHC ENGINE
FALLACY ONE -- A DOHC ENGINE IS MORE FUEL EFFICIENT
Actually the opposite is true. A DOHC engine always consume more fuel than a Pushrod 2-valve engine of a similar displacement and technological content. In fact, a pushrod engine is frequently more fuel efficient than a DOHC engine of smaller displacement! Why? Because a DOHC engine has four times as many camshafts and twice as many valves -- that equals more friction and more fuel wasted overcoming friction. Whatever "breathing" advantage of a DOHC head design is irrelevant for fuel economy purposes because fuel economy is not measured at wide open throttle. At part throttle, your throttle body is choking the engine and the source of ALL aspiration drag. DOHC engines' advantageous lie in its ability to flow enough air at high rpm (7000~10000 rpm). But to support operation at such engine speeds they typically have cam lobe profiles which have high lift, long duration and significant overlap. These type of cam grinds have poor intake velocity and lousy scavenging at cruise speeds. Unless you have a cam lobe switching implementation -- like VTEC or VVTL-i -- such cam grinds REDUCE output and fuel economy.
FALLACY TWO -- A SMALLER DISPLACEMENT ENGINE IS MORE FUEL EFFICIENT
The case for lower displacement equaling lower fuel consumption is shaky at best (just like the case for global warming). It takes burning X amount of fuel with a corresponding amount of air to make power. It takes power to move a car. To burn X amount of uel you need to ingest a given amount of air. Doing it with a larger displacement incurs a bit more friction from bigger pistons. Doing so with smaller displacement force fed by turbos incurs lower compression, whereas doing so by spinning a smaller engine to 8000 rpm incurs low torque numbers which necessitate lower gearing. Statistically the fuel economy impact are not very different! Don't believe me? Let's look at real world statistics.
Example #1: The Cadillac ATS-V for example has a 3.6L Twin Turbo V6 with 464 hp / 445 lb-ft and a 17 mpg (city) / 25 mpg (hwy) EPA rating. The 2017 Camaro SS has a 6.2L pushrod LT1 with 455 hp / 460 lb-ft and a 17 mpg (city) / 28 mpg (hwy) EPA rating. Both cars weigh about the same and use the the GM 8L90 8-speed automatic with a 7.0:1 ratio spread.
Example #2: The Chevy Cruze 1.4T has a (153 hp / 177 lb-ft) 1.4L turbocharged 4-cylinder with 30 mpg (city) / 40 mpg (hwy). The Honda Civic 2.0 has a 2.0L (158 hp / 138 lb-ft) 4-cylinder engine with 31 mpg (city) 40 mpg (hwy). Again, both cars weigh within 50 lbs of 2900 lbs.
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What we just engaged in is a scientifically discussion with real engineering facts. All the propaganda you hear by about "downsizing being the only way forward", and the practice of marginalizing everyone who says otherwise, is FAKE NEWS.