Forum member V-C provided me with a set of Denso IKH20TT iridium plugs pulled from his '08 manual-transmission FJC with about 94,000km (58.4K miles) on them.
Shockingly, the iridium tips on the center electrodes were MISSING on two of the plugs, and several other plugs looked like they were eroding so severely that they too would have lost their iridium tips if they had continued to be run in that engine.
These are the correct TT-series plugs for a 2008 1GR-FE engine.
1. All external markings and the general plug construction leads me to believe these may be genuine Denso plugs. HOWEVER, I'll forward some of the photos to Desno and ask them to verify authenticity.
2. The laser-welded iridium alloy center electrode tip was completely missing from two of the plugs, resulting in a spark gap of over 2.1mm (.083") in the worst case.
3. On the remaining plugs that still had their iridium tips, the gap was ~(TBD)mm ( TBD ").
4. All plugs still had the platinum tip present on the ground electrode.
5. No unusual deposits were seen on the ceramic insulators; there was a smattering of tiny fused-on black particles on the ground electrodes.
6. There was no evidence of overheating of the ground electrode bodies or unusual erosion of the platinum tips on the ground electrodes.
7. There was no mechanical damage to either the center or ground electrodes.
8. On most of the plugs that still had their iridium tip, there was evidence of melting of the iridium tip. Rather than a general melting of a homogenous piece of metal alloy into a large hemisphere (like the tip of a tungsten electrode in a TIG welder), the melted areas had the appearance of hundreds of tiny spheres of melted material, leading me to believe that the iridium tip may be a tiny slug of sintered powdered-metal rather than a rod of solid iridium alloy.
9. On most of the plugs that still had their iridium tip, the point at which the tip was welded to the core center electrode was greatly eroded and under-cut. It would appear that with continued use, these plugs would also have continued to lose material from the weld area and have eventually become under-cut to the point where the iridium tips would have broken off.
10. All plugs showed some residual anti-seize paste in the threads of the plug body.
DENSO TT INFO
Denso states that the electrode material in their iridium-tipped plugs is an alloy of 90% iridium and 10% rhodium.
The 'TT" plug nomenclature stands for 'Twin Tip': there is a 0.7mm diameter platinum alloy tip attached to the ground electrode, and a 0.4mm diameter iridium alloy tip laser-welded to the nickel-alloy center electrode core.
Denso claims that: "The patented formula has the highest concentration of Iridium of any spark plug and outperforms in the most severe engine temperatures resisting oxidation and voltage wear for over 100,000 miles
In their installation guide, Denso stresses repeatedly that iridium plugs do not need to have their gaps adjusted, and it is crucial that no tool of any kind (gap measurement or gap adjustment) ever contacts the iridium tip.
Lots of technical insight into Denso's TT spark plug design and construction can be found in their patent filings at: US6885137B2 - Spark plug and its manufacturing method - Google Patents
Photo 1: Denso plug model ID markings on ceramic insulator
Photo 2: Denso iridium TT markings.
Photo 3: Laser-engraved Denso code on plug body.
Photo 4: Plug missing iridium tip from center electrode; platinum ground tip is intact.
Photo 5: Another plug missing its iridium tip.
Photo 6: Gap on one of the plugs missing its iridium tip was ~2.1mm.
Photo 7: The plugs that still had their iridium tips intact showed various degrees of undercutting and erosion that would likely have caused them to eventually lose their iridium tips.
Photo 8: Higher magnification revealed the iridium tips to be covered with tiny spherical globules of melted metal, looking more like a sintered or powdered-metal part rather than a solid rod of iridium alloy.
Photo 9: A different plug had a similar appearance of the iridium tip: under-cutting and tiny spherical globules. This tip was also not accurately 'centered' on the center electrode body.
I'm not drawing any conclusions here (yet), just presenting what I saw.