Re[2]: Who can that be knocking at my...piston?

[Toby Erkson <Toby_Erkson@ccm2.hf.intel.com>]

I think I'll add to the foray...  This was my understanding of preignition and 
detonation:

Preignition:  The fuel mixture is ignited at a location within the cylinder 
other than the spark plug.  A common preignition point would be a sharp point 
within the combustion chamber, such as exposed threads of a spark plug or the 
exposed threads of the spark plug HOLE.  Bits of carbon deposit that are jagged 
and very hot can be a preignition source.  It's as the name implies; the fuel 
mixture begins burning before proper (spark plug) ignition.  This condition will
create a loss of power for sure but I don't have a clue if it makes any 
particular sound.

Detonation:  This is due to the fuel mixture EXPLODING and not BURNING like it 
should be.  For those who don't know, the fuel mixture actually burns in the 
combustion chamber.  I'm going from my (weak) memory here, but I believe I read 
that gasoline burns at about 100 feet per second; when gasoline explodes it is 
"burning" at about 1000 feet per second (roughly three American football fields 
in length).  Now, imagine that 1000ft/sec occurring inside the cylinder -- 
that's the knocking you hear!  Little explosions going off inside your engine.  
The fuel mixture is expanding SO FAST that it is placing tremendous forces 
against the piston, connecting rods and crank (well, and the little guys that 
hold them all together).  Hmmm, from what I can remember the difference between 
a low octane and high octane fuel is it's ability to resist dieseling (the 
ability to ignite a fuel mixture by compression, without a spark or glow plug). 
The more the fuel mixture becomes compressed the greater it's chance to 
spontaneously ignite.  This is why most high compression engines require higher 
octane fuel.  I have the book that I read this from at home, somewhere, and I'll
try to bring it with me tomorrow.

     Toby Erkson
     air_cooled_nut@pobox.com
     '72 VW Squareback 1.6L bored and stroked to 2.0L
     '75 Porsche 914 stock 1.8L for sale
     Portland, Oregon, USA