[s-cars] Engine Knock, was: I love the dealer's story

[Airbil at aol.com]

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[ Picked text/plain from multipart/alternative ]
Rich (not Dave, though Im sure dave would love a new motor too)
Early last Sept I had a slight engine knock @WOT due to old sprk plugs and
insufficient rs2 FPR.  Here's an excerp of what I learned from a Z car page
WRT engine knock: (I dont know how it will appear here, due to aol nuttiness)
or here's the whole article.
http://zhome.com/ZCMnL/PICS/detonation/detonation.html
Makes me believe that an engine running at all is a miracle.

"Causes of Detonation

Trying to isolate a detonation problem in a highly modified or even slightly
modified engine requires you to determine if the combustion problem is being
caused by a change you made to the engine, is inherent to a design flaw, or
is derived from an external cause such as carbon deposits or poor fuel.
Obviously the area with the largest variable is the fuel quality. Octane
determines a fuel's ability to resist auto-ignition, or in lay terms, its
ability to wait for a spark to initiate a flame. For our purposes we will
explore knock independently of fuel octane rating.
When knock occurs, there is uncontrolled combustion with multiple flame
fronts, with only one of them being initiated by the spark plug. During
knock, cylinder pressures experience an extremely rapid rise and spontaneous
burning and release of the end gases' energy. It is not uncommon to see a
pressure rise of up to 8 bars per degree of crankshaft angle, or up to 50,000
bars per second. Anything that will either increase heat or retain heat in
the combustion chamber will promote auto-ignition.
Common problem areas are sharp edges of metal either on the piston or in the
combustion chamber. For instance, if the piston has a valve relief cut into
it, there is usually a very defined edge that the cutting tool leaves. This
sharp edge is greatly prone to super heating and will actually retain enough
heat that it will start to glow. If the fuel should hit this glowing edge
either prior to the lighting of the spark plug or even after ignition, it is
very likely that another flame front will initiate. If this unintended
ignition occurs very early in the compression stroke, then the piston will be
forced up against the increased pressure of the burning gas and will result
in a form of abnormal combustion referred to as preignition. When this
happens, the end result, if severe enough, is that the connecting rods bend.
Turbocharged engines are specifically prone this. Carbon deposits that build
up in the combustion chamber or on the piston top will have the same effect,
along with the decreased volume at TDC, which in turn raises the compression
ratio.
As stated earlier, anything that will raise temperatures in the cylinder will
help promote detonation. This rise in temperature could occur many different
ways: very lean air/fuel ratios, heating of the charge air prior to entering
the combustion chamber, compression of the air molecules from forced
induction, or poor heat dissipation through the cooling system. Increase of
cylinder pressure will also prompt detonation. Camshaft profile, compression
ratio and quench area all come into play. Design criteria as to combustion
chamber shape and mixture motion, spark plug location and piston design can
all make an engine more or less inherently prone to abnormal combustion.
Naturally, spark timing and fuel quality round off the list.
My .0002 for tonite.
Bill m