I have a stock 9:1 compression ratio Weber 32/36'd engine. It is able to pass smog test on W8DC Bosch plugs with the timing advanced "a hair" above the stock recommended settings. The engine runs slightly lean and the plugs are always very white to light gray. If the weather gets hot the engine has a tendency to ping on heavy acceleration. I have tried regular and hi octane as well as moroso octane booster without any marked difference.

Without changing any settings other than the installation of slightly colder W7DC plugs - the pinging disappears. However, if I pull the plugs the insulators will be much darker in color and the inside shell of all 4 plugs will be black - evidencing much unburned carbon. The idle with these colder plugs seems slightly rougher too.

Here is my question: which plugs should I use?

The car gets mixed daily use traffic and highway. Common sense tells me the W8DC's burn cleaner, but the pinging is pretty bad when it gets hot. Also, the timing needs to be substantially retarded (below the recommended specs) to eliminate the pinging. This, of course, noticeably retards the performance of the engine. I have also tried BP6ES and BP5ES with similar results. Is there another plug with a wider heat range that might work? A couple of friends suggest trying the silber plugs, or even iridiums, but I am not interested in throwing more good money away. By the way, I am using a jetting just slightly richer than CD's prescription, and the exhaust gas analyzer says we are burning clean, so I am reluctant to change what "seems" to be working.

TIA