The higher the octane, the less power is contained in a volume of gas.
The additives used to increase the octane rating don't add any power,
and they essentially take up space. What they do is to allow the gas
to burn rather than explode in high compression situations.
burning gas is good for power, exploding gas is bad.
I'm not sure I buy this. The "additives used to increase the octane
rating," as you put it, are combustible hydrocarbons just like those that
*decrease* the octane rating. Actually, "octane" is a specific hydrocarbon
chain that shows excellent pre-ignition resistance characteristics. The
"octane rating" is actually named after this hydrocarbon. A fuel having an
"octane rating" of 100 has anti-knock characteristics identical to the
hydrocarbon named "octane." Because high-octane fuel by its very nature is
less volatile than low-octane fuel it can sometimes lead to harder cold
starts. Perhaps you're confusing this characteristic with a lack of "power"
contained in the fuel. And most modern gasolines don't actually use much
octane in them, preferring to use other hydrocarbons (such as toluene) that
actually have even greater knock-resistance than octane, and therefore
octane ratings of greater than 100 in their pure form. (Pure toluene's
octane rating is 114, IIRC.)
Engines designed for high octane gas definitely make more power on
high octane gas. But only if it's designed for it. An engine
designed for regular (87 in my part of the US) gas won't get any
power benefit from
This is 100% correct. An engine that experiences relatively low combustion
chamber pressures and temperatures won't be as susceptible to abnormal fuel
ignition, and so won't benefit from higher octane fuel. Higher-performance
engines will experience greater combustion chamber pressures and
temperatures (especially so when driven hard), and will *require*
high-octane fuel to prevent damaging engine knock from occurring. And it's
important to note that very high fuel economy can be seen as a different
type of "high performance" than very high power generation, and that
extremely efficient engines *may* still require premium. In fact, I'm
convinced that the biggest reason that "economy" cars are almost universally
designed to run on 87 octane gasoline is the impression by engineers that
the buyers of these cars aren't going to want to pay for premium fuel. I
expect that even better fuel economy could be achieved by designing their
engines to use premium. But I'm just an enthusiast and not an automotive
engineer, so this opinion should be taken with the appropriate grain of
salt.
93. High-test often has more detergents, and a good cleaning on a
engine made for 87 can sometimes restore power that the engine had
when new.
This was absolutely true 20 years ago when I started driving. My folks used
to run a tankful of premium through their grocery-getters once every couple
of months for this reason. I'm not so sure anymore, however. In the race
to portray each distributor's gasoline as "better" than their competitors,
each company has started adding detergents to its entire line of gasolines.
Start listening to the claims made by Shell and Exxon and the others on
their television commercials and you'll see what I'm talking about.
That said, 93 octane Mobil and Exxon is all that I let into my '02 WRX
wagon. I'm moving cross country next week, and I remember in the high
elevations that sometimes premium only has a 89 or 90 octane rating.
I guess the gas companies think that higher elevation means lower air
pressure and therefore less oxygen in a given cylinder charge. But
I'm thinking that turbo engines can pack just as much air into the
cylinder at high elevation as it can at sea level, at least away from
redline. Does anyone know for sure? Hopefully the guy using 87 in
his WRX lives high in the rockies.
I, too, allow only 93-octane into my Forester. The manual calls for 91+,
but the choices in my area are limited to 87, 89, and 93. If I ever come
across a pump with 91-octane in it, I'll probably use that. After I
"upgrade" my Forester's boost level, however, I will from that point forward
use only 93+ in the car. My wife's grocery-getter van, however, has never
seen anything but 87-octane in its entire life. And the stuff about lower
octane being acceptable at higher elevations is questionable, as well --
especially in a turbo-ed engine. Short of elevations in the stratosphere, a
turbocharged engine will experience no change in performance with changing
elevations. This is the exact reason that turbochargers and superchargers
were first put on piston-driven aircraft. If you're at an elevation where
your turbo can't hit max boost, then you're probably also breathing from an
oxygen tank.
Bottom line is this: If you elect to buy a high-performance car such as the
WRX or STi or XT, you're buying increased long-term costs in addition to the
higher sticker price. These increased long-term costs include higher
insurance premiums, likely shorter lifespan of engine and drivetrain
components and _the cost of buying better gasoline_. All of these long-term
costs must be considered before selecting a high-performance car. For many
of us, these costs are acceptable in order to get the kind of driving
experience we want. If these costs are not acceptable to you or if you
cannot afford them, then you should consider a different car. There's
really no excuse for buying a car that needs premium and then putting
something less into it. IMHO, it's no different than skipping oil changes
just to save money. If your budget is stretched that tight, then you've
bought the wrong car.
- Greg Reed
