Ideas on stoppin some pingin? (long)

[Tim Rogers]

Brian Jones wrote:

>>Having said cooler is better...does EGR help to make the combustion
Cooler while it is operating?<<

As Bruce stated, yes this is true.
In regards to preventing preignition (knock,) the EGR has no effect.
Knock is caused by the mixture getting too hot too soon, via a spark from
bad ignition timing, carbon build up (carbon is a great insulator and gets
hotter than the cylinder head or piston normally would operate at,) a hot
spot in the cylinder head or piston caused by bad design or damage (a nick
or high spot in the metal will not conduct heat away as well a flat surface)
or by bad cam timing or valve lash (an exhaust valve that does not stay
seated long enough will not conduct its heat away quickly enough.)

A question for Bruce and others in the know:  I know that a lean mixture
caused high combustion temperatures, but does it directly cause preignition
as well?

A note on knock:  Many people have heard of a blown engine caused by knock,
which then melts a hole in the piston.  So do you know why this melting of a
piston does not occur under regular combustion conditions?
Under regular combustion, which is a rapid but controlled burn, the
molecules of the mixture that are in contact with the combustion chamber
surfaces do not combust because the walls of the combustion chamber keep
them too cool to burn.  In turn, these molecules insulate the combustion
chamber from the direct heat of combustion.
Conversely, in a knocking motor, instead of a controlled burn there is an
actual explosion (yes there is a difference) and this explosion knocks that
boundary layer of usually unburnt molecules away from the walls of the
combustion chamber and exposes the metal to the direct heat of the exploding
mixture.  This is more than the piston can radiate away and it melts.
So now you know.

Tim Rogers
-93 NX T-top -needs motor work
-91 NX - Racecar-to-be
-75 Silverado - Rusty Beast
-01 Astro - Work Van, Tow Beast