<VV> Predetonation (sic) woes

[Jim Burkhard]

Lloyd -

It would help if you better described the problem. Firstly, it's 
*either* pre-ignition or detonation (spark knock) -- there's no such 
thing as "pre-detonation".  You can have both, but that is when 
prolonged spark knock heats up the chamber enough to cause 
pre-ignition.  At that point, you may have wrecked something, anyhow.

Does the problem occur at low engine speed (say 2000 rpm) and high 
load (foot deep into the pedal)? Such would be the case if you were 
gently cruising around town and then it knocked going up a hill at low 
engine speed.
--- If so, it is probably spark knock. Start thinking better gas 
(which you have tried) or ignition overadvance.

Does the problem occur at high engine speed with a hot engine?
--- It is probably pre-ignition. Start thinking hot spots in chamber.

Does the problem get at all better when you retard the ignition timing?
--- If so, it is spark knock.

Is there any evidence of engine run-on (dieseling) after shutdown?
--- If so, you've got pre-ignition and retarding the timing will not 
help, because teh chamber lights off BEFORE ignition (under 
PRE-ignition, messing with spark timing doesn't help, because the 
chamber autoignites before the spark ever happens ... it doesn't need 
a spark!)

Please answer you these questions two

1. Under what conditions (be very specific: engine speed, load, 
temperature)does this occur?

2. What sort of spark plugs are you running -- the wrong plugs will 
easily cause pre-ignition.

Once we know better exactly what sort of abnormal combustion you have, 
it gets easier to solve.

best regards-
Jim Burkhard
CORSA


lloyd biggerstaff wrote:
> Does anybody have suggestions for my predetonation problems on my 102 h.p. engine? I have been using premium with a little success and have even resorted to retarding it to 8 degrees btdc.Is this problem common for this engine or am I missing something? Thanks in advance.