Sunday, March 26, 2017
Flawed octane rating testing by Consumer Reports
Recently Consumer Reports published a piece how premium grade gasoline may not be needed for vehicles due to manufacturer recommendation versus a requirement. The article has several flaws and omits some key factors. One aspect is the testing used two vehicles that had recommendations for premium grade gasoline but not a requirement. The recommendations are usually only for performance, but the testing was lacking in a variety of situations.
It was suggested that if the feel and the sound of the engine indicates knocking, it’s a good idea to use gas with higher octane. Feel and sound are not good indicators of knocking or pinging. It is more serious when it is audible but using the human ear as a guide is deeply flawed. A data logger for the knock sensor and reading how the engine fuel mixture reacts is the only way to determine if the octane is causing knock. Knock is detrimental, audible knock is very bad.
Consumer Reports claimed the power difference using premium versus regular grade gas was measured with a zero to sixty mile per hour timed acceleration test. A 0-60mph time is a very poor comparison of horsepower. It is too launch dependent and only runs through the first two gears typically. A more powerful car may have a more difficult and therefore slower launch due to managing traction. This confounds the data even though it makes up the time once rolling. There are two reliable ways to measure horsepower without removing the engine; a chassis dynamometer and full throttle acceleration from a roll at a given speed and gear selected. The testers cannot feel a few more or less horsepower either. Not many people are able to accurately.
The article never mentioned mid-grade fuel nor the premium octane rating ranges. 91 octane is prevalent in California but 92 and 93 octane is available in other areas. Are factory stock vehicles tuned for 91 or 93? The difference can be significant. Are any production cars tuned for 93 versus 91? In powerful cars, especially with forced induction, just what would the difference be? An estimate is it can be a ten to thirty horsepower difference.
The last sentence hinting at reliability is irresponsible. It is implying a vehicle requiring premium is less reliable. Yet a performance engine is highly engineered with extra robust components. Also a first tier gas supplier may have a superior formulation in its premium grade. For instance Shell's V-Power Nitro+ has seven times the government mandated additive package that is proven to reduce valve deposits, corrosion and cause less wear versus other brands. That would imply superior reliability.
It should be noted that the testing didn't vary atmospheric conditions and temperatures. Nor take into account the summer versus winter grade of gasoline. Engine output varies significantly based on intake air temperature also called altitude density. It also should be disclosed that vehicles with forced induction tend to require premium fuel, and not simply recommended unless specifically noted. Especially when the engine is tuned for performance rather than economy.
Measuring performance differences demand proper instrumented testing and procedures. It also requires disclosure of testing conditions and more than a cherry picked facts and tests.
Link to Consumer Reports article: