Hi All, Just got a used 04 forester XT with 55,000 miles. Question, is
it really necessary to use prem. gas or will mid grade or regular do? I
will probably buy the prem. but I put on a lot of miles and saving some
$$$ would help. Question, with the Toyota Rav4 I traded in at 190,000 I
changed oil at 6,000 is this ok to do with the turbo? I need to get at
least 200,000 mile on this car. Any advice on Subaru would be most
welcome. Thanks
Sorry - this is a long quote from wikipedia;
Detonation
The fuel/air mixture is normally ignited slightly before the point of
maximum compression to allow a small time for the flame-front of the
burning fuel to expand throughout the mixture so that maximum pressure
occurs at the optimum point. The flame-front moves at roughly 33.5
m/second (110 feet/second) during normal combustion. It is only when the
remaining unburned mixture is heated and pressurized by the advancing
flame front for a certain length of time that the detonation occurs. It
is caused by an instantaneous ignition of the remaining fuel/air mixture
in the form of an explosion. The cylinder pressure rises dramatically
beyond its design limits and if allowed to persist detonation will
damage or destroy engine parts.
Detonation can be prevented by:
* The use of a fuel with higher octane rating
* The addition of octane-increasing "lead," isooctane, or other
fuel additives.
* Increasing the amount of fuel injected/inducted (resulting in
lower Air to Fuel Ratio)
* Reduction of cylinder pressure by increasing the engine
revolutions (lower gear), decreasing the manifold pressure (throttle
opening) or reducing the load on the engine, or any combination.
* Reduction of charge (in-cylinder) temperatures (such as through
cooling, water injection or compression ratio reduction).
* Retardation of spark plug ignition.
* Improved combustion chamber design that concentrates mixture near
the spark plug and generates high turbulence to promote fast even burning.
* Use of a spark plug of colder heat range in cases where the spark
plug insulator has become a source of pre-ignition leading to detonation.
Correct ignition timing is essential for optimum engine performance and
fuel efficiency. Modern automotive and small-boat engines have sensors
that can detect knock and retard (delay) the ignition (spark plug
firing) to prevent it, allowing engines to safely use petrol of
below-design octane rating, with the consequence of reduced power and
efficiency.
A knock sensor consists of a small piezoelectric microphone, on the
engine block, connected to the engine's ECU. Spectral analysis is used
to detect the trademark frequency produced by detonation at various RPM.
When detonation is detected the ignition timing is retarded, reducing
the knocking and protecting the engine. See also Automatic Performance
Control (APC).
[edit] Pre-ignition
Pre-ignition is a different phenomenon from detonation, explained above,
and occurs when the air/fuel mixture in the cylinder (or even just
entering the cylinder) ignites before the spark plug fires. Pre-ignition
is caused by an ignition source other than the spark. Heat or hot spots
can buildup in engine intake or cylinder components due to improper
design, for example, spark plugs with heat range too hot for the
conditions, or due to carbon deposits in the combustion chamber. Spark
plugs with a high heat range will run hot enough to burn off deposits
that lead to plug fouling in a worn engine, but the electrode of the
plug itself can occasionally heat soak, and begin glowing hot enough to
become an uncontrolled ignition source on its own. Bits of carbon that
build up in a combustion chamber can also heat soak to the point where
they also are glowing hot and ignite the air-fuel mixture before the
proper time.
Pre-ignition and "dieseling" or "run on" are the same phenomenon, except
in the latter case the engine continues to run after the ignition is
shut off with a hot spot as an ignition source. Pre-ignition might cause
rough running due to the advanced and erratic effective igniton timing
and may cause noise if it leads to detonation. It may also cause
"rumble" which is fast and premature but not detonating combustion.
This heat buildup can only be prevented by eliminating the overheating
(through redesign or cleaning) or the compression effects (by reducing
the load on the engine or temperature of intake air). As such, if
pre-ignition is allowed to continue for any length of time, power output
and fuel economy is reduced and engine damage may result.
Pre-ignition may lead to detonation and detonation may lead to
pre-ignition or either may exist separately.
Carl
