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Just wanna help
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There's no shortage of opinions on who is to blame for gas-price gouging. One thing that's certain is
drivers tend to economize at the pump during extreme price rises—they buy cheaper, lower-octane gas.

In the old preelectronic days, cars would protest such parsimony by pinging like a pachinko parlor, but most
modern cars don't complain audibly, so maybe they don't mind. Or do they? And conversely, is there any
benefit to be had by springing for the expensive stuff when you're feeling flush?

To find out, we ordered a fleet of test cars—some calibrated to run on regular, others that require
premium—and tested them at the track and on a dynamometer.

But before we go into the results, let's go to combustion school. When a spark plug fires, it
does not cause an instantaneous explosion of the entire cylinder's charge of fuel and air. The spark
actually lights off a small kernel of air-and-fuel mixture near the plug. From there, a flame front
expands in every direction, gradually igniting the rest of the air and fuel. This takes some time, as
much as 60 degrees of crankshaft rotation.

Meanwhile, the air-and-fuel mixture that the flame front has not yet reached is experiencing huge
increases in pressure and temperature. If any part of this air-and-fuel mixture gets heated and
squeezed enough, it will explode spontaneously, even before the flame front ignites. This
self-ignition is called detonation, or the dreaded "knock."

Now for the chemistry lesson: Oil is a hydrocarbon fuel, meaning the individual molecules contain
carbon and hydrogen atoms chained together. Modern gasoline is blended according to various
recipes, the active ingredients for which include about 200 different hydrocarbons, each with a
spine of between 4 and 12 carbon atoms. One of them, isooctane, consists of 8 carbon and 18
hydrogen atoms (C8H18) and is exceptionally resistant to exploding spontaneously when exposed
to the heat and pressure found inside a typical combustion chamber.

Another, n-heptane (C7H16) is highly susceptible to such self-ignition.

These two compounds are therefore used to rate the knock resistance of all gasoline blends. A gasoline
recipe that resists knock the way a mixture of 87-percent isooctane and 13-percent n-heptane would is rated
at 87. Racing fuels with octane ratings over 100 resist self-ignition even better than pure isooctane. The
octane ratings for regular-grade fuel range from 85 to 87, midgrades are rated 88 to 90, and 91 and higher is

Mind you, premium fuel does not necessarily pack more energy content than does regular. Rather, it allows
more aggressive engine designs and calibrations that can extract more power from each gallon of gasoline.

An engine's tendency to knock is influenced most by its compression ratio, although combustion-chamber
design also has a large effect. A higher ratio extracts more power during the expansion stroke, but it also
creates higher cylinder pressures and temperatures, which tend to induce knock. In supercharged engines
boost pressure behaves the same way. That's why the highest-performance engines require higher-octane

If you feed such an engine a fuel with insufficient octane, it will knock. Since it is impossible, for now, to
change an engine's compression ratio, the only solution is to retard the ignition timing (or reduce boost
pressure). Conversely, in some engines designed for regular fuel, you can advance the timing if you burn
premium, but whether this will yield additional power varies from engine to engine.

Knock sensors are used in virtually all new GM, Ford, European, and Japanese cars, and most
DaimlerChrysler vehicles built today. According to Gottfried Schiller, director of powertrain engineering at
Bosch, these block-mounted sensors—one or two of them on most engines and about the size of a
quarter—work like tiny seismometers that measure vibration patterns throughout the block to identify knock
in any cylinder. Relying on these sensors, the engine controller can keep each cylinder's spark timing
advanced right to the hairy edge of knock, providing peak efficiency on any fuel and preventing the damage
that knock can do to an engine. But, noted Schiller, only a few vehicles calibrated for regular fuel can
advance timing beyond their nominal ideal setting when burning premium.

Older or less sophisticated cars with mechanical distributors do not have the same latitude for
timing adjustment as distributorless systems do and therefore may not always be able to correct
for insufficient octane or additional octane.

We should note that even cars designed to run on regular fuel might require higher octane as
they age. Carbon buildup inside the cylinder can create hot spots that can initiate knock. So can
malfunctioning exhaust-gas-recirculation systems that raise cylinder temperatures. Hot
temperatures and exceptionally low humidity can increase an engine's octane requirements as
well. High altitude reduces the demand for octane.

Got all that? Good. Let's meet the test cars and ponder the results. At the lower-tech end of the scale was a
regular-gas-burning 5.9-liter Dodge Ram V-8. This all-iron pushrod engine has a mechanical distributor and
no knock sensors, so the computer has no idea what grade of fuel it's burning. A Honda Accord V-6 with
VTEC variable valve timing represented the mainstream-family-sedan class, and a 4.6-liter V-8 Mustang
stood in as an up-to-date big-torquer. Both of those were designed to run on regular unleaded.
Our premium-grade cars included the hard-charging 333-hp, 3.2-liter BMW M3 straight-six boasting
individual throttle by wire for each cylinder and enough computing power to run Apollos 11 through 13.
A Saab 9-5 gave us a highly pressurized 2.3-liter turbo. For the sake of repeatable track testing, all but
the M3 were equipped with automatic transmissions.

We ran all vehicles on both grades of fuel, at a drag strip near our offices and on a Mustang eddy-current
dynamometer. On arrival, all fuel tanks were drained and filled with 87-octane Mobil
regular fuel and driven for two days before track and dyno testing. The tanks were drained again and filled
with 91-octane Mobil premium and again driven for two days to allow time for the engine controllers to
acclimate to the fuel type and tested again. All dyno and track results were weather-corrected.

Our low-tech Ram managed to eke out a few extra dyno ponies on premium fuel, but at the track its
performance was virtually identical. The Mustang's knock sensors and EEC-V computer found 2 hp more on
the dyno and shaved a more impressive 0.3 second off its quarter-mile time at the track. The Accord took a
tiny step backward in power (minus 2.6 percent) and performance (minus 1.5 percent) on premium fuel, a
phenomenon for which none of the experts we consulted could offer an explanation except to speculate that
the results may fall within normal test-to-test variability. This, of course, may also be the case for the gains
of similar magnitude realized by the Ram and Mustang.

The results were more dramatic with the test cars that require premium fuel. The turbocharged Saab's
sophisticated Trionic engine-control system dialed the power back 9.8 percent on regular gas, and
performance dropped 10.1 percent at the track. Burning regular in our BMW M3 diminished track
performance by 6.6 percent, but neither the BMW nor the Saab suffered any drivability problems while
burning regular unleaded fuel. Unfortunately, the M3's sophisticated electronics made it impossible to test
the car on the dyno (see caption at top).

Our tests confirm that for most cars there is no compelling reason to buy more expensive fuel than the
factory recommends, as any performance gain realized will surely be far less than the percentage hike in

Cheapskates burning regular in cars designed to run on premium fuel can expect to trim performance by
about the same percent they save at the pump. If the car is sufficiently new and sophisticated, it may not
suffer any ill effects, but all such skinflints should be ready to switch back to premium at the first sign of
knock or other drivability woes. And finally, if a car calibrated for regular fuel begins to knock on anything
less than premium or midgrade, owners should invest in a tuneup, emissions-control-system repair, or
detergent additives to solve, rather than bandage, the root problem. Class dismissed.


  • Like
Reactions: Pat Riot

Premium Member
2,049 Posts
a bit more refined than my expose' on premium vs regular and compression....but the same results....only, I wish they would have used a Mo--the M3 has a 11.7 to 1 compression ratio and the slob was turbo'ed (making the compression variable by boost pressures).

Good article.


1 Posts
Cars with exhaust gas turbocharger turbine housing are single entrance, which means using only the energy of the exhaust vent pressure, without the use of other auxiliary energy. Due to the large car engine speed range, and therefore must have exhaust turbocharger adjustment device, so the engine can be obtained relatively constant boost pressure within a certain speed range. In addition, gasoline is ignited ignition, its compression ratio is a certain limited range, too high will lead to exploding. Therefore, also have knock detection and control mechanism, always adjust the ignition timing. Your pick of the bargains for AutoParts
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