The
combustion process—gasoline and air burned in the cylinders to
produce power—also produces byproducts, carbon being the most
relevant to this discussion. Carbon deposits inside the engine
disturb airflow and air/fuel ratios, adversely affecting performance,
emissions, and fuel economy to the point that it might set off the
check engine light—and trigger the need for potentially expensive
repairs. Decades ago, it was routine procedure to partially dismantle
engines to remove carbon deposits. The Brits called it decoking.
There was also the “Italian tune-up,” running the engine near
redline to burn off carbon deposits.
Modern
engines are even more sensitive to carbon deposits on such components
as intake valves and fuel injectors. These deposits cause a rough
idle, hesitant acceleration, and knocking or pinging, among other
symptoms. In 1996, the federal Environmental Protection Agency
decreed that gasoline refiners must add detergent to every grade of
gasoline—regular, midgrade, and premium—to prevent carbon
buildup. But eight automakers—Audi, BMW, Fiat Chrysler Automobiles,
General Motors, Honda, Mercedes- Benz, Toyota, and Volkswagen—deemed
the federally required amount of detergent additive insufficient.
They formed an alliance called Top Tier to test and certify that
gasoline brands have additional detergent additives to keep their
engines performing as they were originally designed.
Last
year, AAA hired an independent engine testing lab to see if Top Tier
gasoline cleans engines any better than non–Top Tier brands. The
result? Top Tier gasoline averaged a whopping 19 times fewer deposits
than non–Top Tier gasoline after only 4,000 miles of simulated
driving. And for motorists who’ve been using lower-quality
gasoline, the AAA study had good news. There’s no need to tear down
the engine and decoke. As stated in the final report, “Engine
carbon deposits formed when using [non–Top Tier gasoline] can be
largely removed by switching to a gasoline that meets Top Tier
standards”—though it may take a few thousand miles of driving.
More good news: The average price difference between Top Tier and
non–Top Tier brands was just 3 cents a gallon. AAA also found
non–Top Tier gasoline reduced fuel economy by 2 to 4 percent,
meaning that the higher cost of Top Tier brands is mitigated by
improved fuel economy. Seems to me like pretty good reasons to stick
with Top Tier gas. To find brands meeting Top Tier standards, go to
toptiergas.com/licensedbrands.
A
note of caution: Don’t confuse gasoline quality (which
refers to detergent additives) with grade (which
refers to octane rating). Buying a lower octane rating than specified
in your car’s owner’s manual also harms performance and could
cause engine damage. Buying a higher grade than required is a waste
of money.