Join Date: Mar 2007
Location: San Antonio, Texas
I encountered a similar pinging problem with my first 2 liter hot-rod build. In the mid-to-late 80's the best pump gas we could get in Texas was 91 octane. At the time I was running 10.4 Borgos with cams that were similar to what we're talking about here. I was using my trusty, re-curved MarelliPlex that I'm now running on my new motor. Here's a couple of factoids from that period.
First a definition of terms: "Knocking is more commonly known as pinging. But it differs somewhat from detonation. Knocking / pinging is caused by the peak cylinder pressure event coming too early because the flame front from the spark plug initiated too soon." --- from the internet.
Especially in 100 degree Texas summer weather, my high-compression motor would ping under even a light load. At the time, I had recently discovered David Vizard's various books on engine building and was reading everything of his that I could find. In one of this tuning books Vizard mentioned that a common characteristic of Otto cycle engines is a knock/pinging event at about 2400 rpm. With my "all in at 3k" advance curve this is exactly what I was experiencing. I surmised that that I was experiencing the flame front problem described above.
Vizard was also enthusiastic about MSD CD ignitions which, as we know, were designed to have longer firing events at lower speeds. I put two and two together and decided to see if the MSD's multi-firing over about 30 crankshaft degrees design would reduce or eliminate the pinging I was getting. To shorten a long story, the experiment worked. With the MSD installed, my 10.4 (actually about .5 or .6) was completely eliminated. It was really surprising to drive in 100 degree stop and go traffic and not experience pinging. Even with the 91 octane gas, my hot-rod 2 liter was a smooth as could be. So what was the MSD ignition doing? It appears that the multi-firing design was doing a better job of controlling the ignition evet, despite the limitations of the 91 octane fuel.
The culprit, of course, was the crappy 91 octane gas which was all that was available at the time. With my newly built 2 liter and 93 octane gas, I'm comfortable running the same MarelliPlex and no MSD.
Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series