| My stumbling and hesitation problems at WOT are cured, after switching from NGK PFR6G-11 plugs ("G" plugs) to NGK PFR6B-11B plugs ("B" plugs). The B plugs are what are listed in the owner's manual and the service manual as the "colder" plug for our cars. The G plugs are being sold by the dealers as replacements for our TT cars. I'm told these plugs will work great in a NA, but not TT's. The photo shows the difference in lengths between the two plugs. The new B plug is on the left, the slightly used G plug is on the right. 
I am holding the plugs evenly, as shown by the yellow line at the base of the insulator. The red and blue lines show the difference in lengths marked at the center electrode tip. You might be able to barely make out that the B plugs have a raised nubbin of platinum on the underside of the curved, ground electrode. The G plugs supposedly contain platinum too, but it is flush in the electrode. I installed the G plugs less than 10K miles ago. The car ran perfectly for a short while, but then developed a hesitation, stumbling, and sometimes backfiring problem at high RPM's in the higher gears. The problem was getting noticeably worse with time. The parts guy at the Nissan dealer swore the G plugs were the right ones for the car when he sold them to me. I even phoned him the first time I put the G plugs in because they looked so different from the plugs that were coming out. He said the G plugs were matched by VIN number to my car. I can't really blame him if that's what his computer is telling him, but his computer appears to be wrong. For now, my Z is running perfectly again, pulling strong to redline, idling steady as a rock, and pegging the stock boost gauge. It feels so good to have her back. I'm going to book some track time at Sebring, FL in June to celebrate :c)
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Some important things I learned from switching spark plugs. - 
