Sarah Gallagher <sarah@sprynet.co.uk> writes:
>David Schwarze wrote:
>
>>I run my bus close to 0 degrees as well.  It shouldn't hurt anything, as
>>long as the engine isn't pinging (it will be more likely to ping with
>>advanced timing)
>
>What, why and how is this mysterious phenomenon?
>
It's a sound a bit like marbles being shook in a coffee can.  It's caused
by the air/fuel mixture being ignited too soon by the spark plug.  As the
piston is moving up the cylinder the exploding air/fuel tries to push it
back down.  The pinging (rattling) results, and is very hard on the piston
and valves.  It usually occurs more often when the engine is under a load,
like climbing a hill.  Higher octane gas reduces pinging, as does setting
the timing back.  Generally, an engine performs best (power and fuel economy)
with the timing advanced as much as possible, but not so far as to produce
pinging or overheating.

Pat Walsh, '71 Westy, walshp@camry.nawc-ad-indy.navy.mil