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.
