Head will be coming off for further investigation, but is this plug damage telltale of a lean condition or detonation?

 

Looks like detonation which could be caused by being lean. The little bumps probably are aluminum from the top of the piston.

 

That looks like pre-ignition. It can becaused by wrong plug temp range, timing is too advanced, or lean mixture.

 

Little bumps are likely melted electrode. you went way hot on the plug tips. why is the not so simple part. sorry for the cut and paste but why type it when this says it well. you could have too hot a plug that it turn caused pre ignition, Octane too low for conditions and one or more of many tuinggs that can make the combustion temps go too high.

Pre-ignition
Defined as: ignition of the air/fuel mixture before the pre-set ignition timing mark
Caused by hot spots in the combustion chamber...can be caused (or amplified) by over advanced timing, too hot a spark plug, low octane fuel, lean air/fuel mixture, too high compression, or insufficient engine cooling
A change to a higher octane fuel, a colder plug, richer fuel mixture, or lower compression may be in order
You may also need to retard ignition timing, and check vehicle's cooling system
Pre-ignition usually leads to detonation; pre-ignition and detonation are two separate events
Detonation
The spark plug's worst enemy! (besides fouling)
Can break insulators or break off ground electrodes
Pre-ignition most often leads to detonation
Plug tip temperatures can spike to over 3000°F during the combustion process (in a racing engine)
Most frequently caused by hot spots in the combustion chamber.
Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is being forced upward by mechanical action of the connecting rod, the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the force of the premature explosion) and it can't go down (because of the upward motion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. This is detonation.
Most of the damage that an engine sustains when "detonating" is from excessive heat
The spark plug is damaged by both the elevated temperatures and the accompanying shock wave, or concussion

 

Usually it's detonation that kills electrodes. And the bumps are part of your piston. Smart to pull the head.

 

steel pistons?

 

No brainer on pulling the head. HG is likely shot (missing some coolant), and its running rough at idle even w/ new plugs in it. I am expecting to find some piston damage, cracked rings, etc. Will run a compression or leakdown test in the next couple days to see if that gives me a quick idea of other damage.

Factory forged alum pistons, Swain coated. WR5DC Bosch plugs.

EGTs were not looking abnormal. Factory knock control sensor/system apparently did not react (not a surprise, I do not trust the 30 year old bosch system to do much of anything). Coolant and oil temps were low, ambient temp was in the high 60's, low 70's.

Relatively similar setup that I have been running for several years, so just trying to determine if I developed a fuel delivery/pressure problem (lean), or an octane/bad fuel mix (detonation).

 

As mentioned in earlier posts, ignition timing is also a factor. If the spark is too advanced you get these issues. Back in the day, Champion spark plugs would give you a fresh set of plugs and have you run a few laps and shutdown the engine. Their techs would then examine the plugs with a magnifying glass and look for traces of aluminum. If any were found you enriched the mixture slightly and went through the procedure again. If these signs are ignored, eventually a hole will burn in the top of the piston. I have had first hand experience with that issue.

 

If this is a sudden-onset change (i.e. the engine ran fine until this suddenly started happening), you might look into bad fuel, or possibly the ignition timing somehow got way advanced.

 

Hope your pistons don't look like this:

 

Do all plugs look like that or just one? If just one I'd be looking at a bad injector, running one cyl out lean. This is from a turbo car? Do you log boost? The turbo adds another level of complexity to the equation...

 

me too...

All the plugs are similar in color and appearance, but only the one pictured has the tip blown off. Yes, its a turbo motor. Boost is not logged, but I do pay attention to the gage.

 

If you can pay attention to the boost guage you are not driving hard enough.

 

Fair enough...!

Home track has a 5000 ft front straight, so there is some "down" time to take a peek at gages, at least once a lap.

In car video shows the boost gage clearly, so I can check on it after the fact to see if there were any boost spikes. And actually works very well as a throttle position indicator, to see where in corners I am coming off and getting back on the gas.



Will have the head off probably tonight, so will follow up with pics if there is anything interesting to see.

Thanks for all the comments. I appreciate it.

 

Head and piston pics. Head is shown with #1 to the right. Piston pics are shown #4 & 3, and #2 & 1.

Didn't find any significant damage. The piston pics are a little hard to see, the rough surface on the pistons is carbon build up, not pitting.

The broken plug was in #2. Possible #3 did have some coolant in it, but not sure. Couldn't find an obvious leak point in the head gasket. All better than I expected to find.