Do you really need the expensive stuff?
by Eric Peters
Don't be gulled by slick oil company marketing
ploys about the benefits of premium fuel: few
new cars really need it and those that don't won't
run any better from using it.
There's no mystery to it. Just
take a look at your owner's manual; it will tell
you the manufacturer's fuel
recommendations. There may also be a sticker
on the gas cap or even on the
instrument cluster under the gas gauge.
Whatever it says, abide by it.
You're only wasting your money by burning
premium fuel in a car that doesn't
require it. Higher-octane (91 and
up) gas burns more slowly, and
will actually give poorer performance
when fed to engines that were
designed to burn regular 87-octane
fuel.
But the reverse isn't always true. What happens, in
fact, if you use regular or even
mid-grade gas in a car that really does need
premium? If the car in question is a
late-model one, nothing that will cause any
permanent problems. The computer will adjust
the ignition timing and other engine parameters to
compensate for the lower-octane juice.
You may notice a slight fall-off in acceleration,
but no engine damage or drivability
problems should arise.
But with some older, pre-computer cars (model year
1981 and before) you could have a
problem. For example, a Sixties-era "muscle car"
with a high compression ratio must
have premium fuel to avoid deadly engine knock
(pre-ignition), which occurs when the
gas and air inside the engine's cylinders ignites
before the piston reaches its firing
position at "top dead center." When that happens,
the explosion tries to force the piston
down when it's coming up - and that puts enormous
strain on engine bearings,
connecting rods and the relatively fragile aluminum
pistons themselves.
Unless you want to ruin your high-compression
engine, premium fuel is an absolute must
in such cases. You may even have to add a can of
octane boost to each tank in some
cases to bring the fuel up to spec. In this case,
premium fuel prevents engine knock
because it is less volatile and hence burns more
slowly than lower grade gas. Thus it is
not as susceptible to pre-ignition.
However, even today's "ultra" premiums come nowhere
near the octane level of the
leaded premium that was available 30 years ago. In
those days, octane ratings of 100
were common; today 94 is the best you can get - and
the octane level is raised not by
lead but by the addition of "aromatics" that may
cause problems in older engines.
Fortunately, very few cars still on the road today
have high-compression engines that
need such fuel. The handful that remain have
usually had their engines rebuilt with lower
compression pistons to run on today's lower-grade
gas - and the others can avail
themselves of octane boosters readily available at
auto parts stores.
You should not buy octane boost, however, for use
in an emissions-controlled car with a
catalytic converter. Octane boosters may foul the
converter and eventually plug it up.
Besides, no factory-built produced since the early
1970s needs the stuff anyway. You're
just wasting money and buying the advertising hype.
There is one thing, though, that could cause your
late model, regular-fuel car to need a
higher grade gas: age. As an engine gets older,
carbon build-up on the tops of the
pistons effectively increases the compression ratio
- which in turn means you may find
the car knocks when you use anything but mid- or
even premium-grade gas.
This is a normal condition and nothing to worry
about. Your engine will run great for many
miles to come, so long as you feed it the stronger
juice. If you wish, there are ways to flush
the engine and purge the carbon from the tops of
the pistons, but this service is not
cheap, and the pistons will eventually get a
coating of carbon all over again anyhow. It's
typically cheaper and certainly less hassle just to
spring for mid-grade gas.
With this exception, you should stick with the fuel
recommendations of the manufacturer
and avoid being sucked into the trap of paying
extra for something you don't need.
If for some reason your car still knocks, the
ignition timing is probably off or you need a
tune-up. Don't crutch the problem by going up to
higher grade gas; have the car looked
over by your mechanic to determine what the problem
is.
-------------------------------------------
Steven St.Laurent
Test Engineer
Test Branch, GSD, MCTSSA
MARCORSYSCOM, USMC
760-725-2506 (DSN 365-2506)
Work:mailto:stlaurents@mctssa.usmc.mil
Home:mailto:saint1958@home.com
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