On Fri, 13 Nov 2009 17:37:33 -0600, AZ Nomad
Post by AZ Nomad Post by email@example.com Post by firstname.lastname@example.org
It most definitely hurts, and can lead to an expensive rebuild.
Post by email@example.com
Well, I have several (4) decades of experience as a mechanic that
re-enforces what I have been taught - high octane fuel will NOT damage
an engine. High LEAD fuel can damage an engine. High lead fuel is high
octane, but high octane does NOT need to be high lead.
Propane is 115 minimum AKI and unless it is run too lean it will NOT
harm an engine (if the engine has hardened valve seats designed for
High octane unleaded motor fuel without ethanol likewise will not harm
any engine designed to run on regular unleaded gasoline.
agreed. octane is basically the fuel's resistance to preignition.
Going higher than necessary is a waste of money, nothing more.
You need to understand the difference between detonation and
pre-ignition. Pre-ignition is ignition instigated BEFORE the spark
fires - generally from a hot spot in the cyl. This can be a sharp
valve edge, glowing carbon, overheated spark plug, etc.
THAT is not detonation, and higher octane fuel will NOT prevent, or
even reduce it.
Detonation is the "explosion" of the destabilised end gasses, usually
farthest from the spark plug, in static pockets, due to high heat and
pressure causing the hydrocarbons to dis-associate.
Octane rating indicates the ability to resist this spontaneous,
uncontrolled burning after ignition has occurred., and a faster
burning fuel is less likely to detonate than a slow burning fuel
(natural octane, I like to call it) This is also why detonation
GENERALLY occurs at lower speeds and under high load, not at higher
Detonation can CAUSE pre-ignition, but is not detonation.
Detonation can also CAUSE pre-ignition. If cyl head temperature
increases and exhaust gas temperature drops, that is the surest sigh
that detonation has occured,
How, or why, you may well ask?
When detonation accurs, it disturbs the layer of air directly against
the surface of the combustion chamber, and "scrubs" it off. This layer
acts as an insulator, preventing the total heat of combustion from
being transfereed to the cyl head and piston. Whenit is disturbed,
much of the heat of the exhaust is absorbed bu the piston and cyl
head, reducing the exhaust gas temperature and raising the cyl head
Now, when this happens, parts of the cyl head and/or piston, and/or
the spark plug, will overheat, and there becomes a high probability
that the fresh charge of air/fuel mix will ignite spontaneously before
the plug fires - classic pre-ignition.
If pre-ignition happens as a precursor to detonation, it is because
the spark, occuring too early in the cycle, causes cyl pressures to
increase MUCH higher than normal (expansion against a rising piston
instead of against a descending piston) and that pressure and heat
acts on the end gasses for a longer time, making the disassosiation
and detonation more likely.
Pre-ignition causes a normal, controlled conflargation in the cyl -
just at the wrong time. Gives you the same effect as "spark knock"
caused by over-advanced timing - which again is NOT detonation - but
can cause detonation.
Pre-ignition and "spark knock" can be hard on bearings and pistons -
causing causing cracked or broken pistons and/or pounded out bearings
and/or bent rods etc.
Detonation, on the other hand, causes burned pistons and/or metal
transfer to the spark plug, and/or cratered surfaces in the combustion
chamber (looks like small sharp bits have been pecking away at the
roof of the combustion chamber, or the top of the piston ) and
aluminum "spray" on the plug tip, and CAN cause fractured pistons,
damaged bearings, bent rods, etc along with the other signs of
Detonation causes overheating as well as being excaberated by
Pre-ignition is most often caused BY overheating.
Detonation as a precursor to pre-ignition is more common than the
other way, but both are possible.