B-V8ers,
Here is my 2 cents if anyone wants it.
Detonation / Preignition (abnormal combustion) does cause major damage and can
cause it very quickly so it is very important to find the cause and correct it
as soon as possible. Abnormal combustion can occur at any RPM and most any
driving condition. It is not always heard. Most people hear the normal ones
like "run-on" after the ignition is turned off or the "pinging" on a quick
throttle opening from idle or the "pinging" or misnamed "valve rattle" on
heavy loading. The damage of abnormal combustion is very easily identified on
an examination of the piston, but as should be you are concerned with
preventing the damage.
First you must identify the cause. You know you have the "run-on preignition"
so you should look at all possible causes in that area. They include,
Excessive chunky carbon in the combustion chamber, Over advanced ignition
timing at idle, Lean mixture at idle, Spark plug heat range too hot. Sharp
edges in combustion chambers like a steel head gasket protruding inside of the
chamber, Octane too low for the compression ratio of that engine, Engine
temperature too hot. Over heated air entering the carb. Oil in the combustion
chamber by whatever means. and even an idle RPM set too high. Any one or
combination of the causes can be at fault.
All except the gasket protrusion can be diagnosed and corrected from outside
the engine. It is relatively easy to test each one. Most of you have large
sums of money invested in your engines. "Inaudible detonation" can also do
extensive damage so it is very important to prevent "Abnormal Combustion" at
any RPM. A good tool is the MSD "engine knock alert" It gives you a visual
display in LEDs of knock that you normally could not hear thus alerting you to
correct the problem. If you have the "Run-on" preignition there is a good
chance you have the abnormal combustion at other RPMs even if you don't hear
them. This is especially true on any engine that is modified in any way. I
don't work for MSD but have seen this item used on a friends V8 that was
highly modified and I have one for my BGT V8 conversion. The MSD part # is
8964 if anyone is interested.
HMF
MGB GT V8 conversion
TR-7
XJ-6 Jag
www.mg-tri-jag.net
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