Originally Posted by dawg
ok I had this issue since last year before putting the car away for winter.
I changed coil, installed new distributor (I think i blew the module when i started car with battery buddy hooked up by mistake).
but old one was fine.
took out and hooked up vacum gauge.
needle fluctuates between 17" and 19".
normal is a steady 18".
I readjusted the idle mixture but still has same fluctuations and part throttle ping.
only thing left in the ignition system is the MSD 6AL box and the moroso ultra wires.
btw all cylinders are firing (checked with water spray and temp gun)
pulling the spark plugs and all were black and sooty.
Looked at the pictures that's quite an ouch! This is classic preignition rather than detonation. It took advantage of the usually hot and distorted location between the adjacent exhausts in the middle of the head.
Is this a 400 I see cooling holes in the gasket and head?
Needless to say preignition and detonation are issues requiring immediate attention. These two conditions can do a lot of damage in very little time. The classic causes for either are pretty similar as are the cures.
Considering that as hot rodders we tend to push the compression limit after all that's where the power and efficiency are. But this can be a thin edge that tends to wear away with age. I see combustion deposits, these shrink chamber volume which increases the compression ratio, it may not be much, but we pushed the limit in the first place if you remember (detonation). They also insulate the chamber running temps up or the rough edges of the carbon may become incandescent. Either of these can be hot enough to light off the mixture before the spark happens (pre-ignition).
Ping under moderate load and RPM indicates too much advance for the condition. Fixing this becomes a matter of adjusting the rate and amount of advance in specific locations of the RPM curve. You really need to use a vacuum advance or scrap all the vacuum and mechanical advance systems and upgrade the electronic box to one that includes programmable electronic advance. The advantage of using vacuum and mechanical is that along with base the overall system can be adjusted for the different operating zones of the engine. For example the vacuum advance allows a high amount of advance under cruise conditions with low RPM and light load on the crankshaft that is reduced as the load on the crankshaft is increased but RPMs may not necessarily be increased. When an engine is operated without vacuum advance there is a tendency to put in too much base and/or too much centrifugal to cover the lightly loaded cruise zone. The problem this causes is for example you come to a hill which increases crankshaft loading while not changing the RPMs. This can easily arrive at a condition where there is too much advance for the combination of load and RPM on the crankshaft. This is an open invitation to detonation.
Certainly preignition and detonation can be controlled with intake temperature, engine temperature, mixture ratio, etc. but these are more global solutions to the problem which remain what they are regardless of operating conditions. This can lead to inefficient and high wear operation of the engine which one way or another gets into your wallet more than necessary.
I have no issue with the 6AL box nor your wires, the mixture only cares about being lit off, a hotter spark and certainly multi-sparks improve the chances of starting the fire. But once lit, it doesn't burn faster or hotter than if a weaker spark started it. But a consideration is that with a greater chance to get burn going, it may prove beneficial to reduce the advance a bit. Shows to go ya how many times a weak sparked engine misfires which has the effect of reducing chamber temperature, therefore, the risk of detonation and or pre-ignition occurring.