Hi Guys, sorry saw this thread late.
The revised plug is for warranty reasons and pretty much nothing more to my knowledge from what I have heard from talking to Ford engineers. The problem as referenced in the TSB that was posted is mostly an issue with the 5.4 engines. But because the castings are the same between both 4.6 engines and 5.4 engines the new design will carry forward on all the castings. The issue is that carbon builds up between the small space between the plug shaft and the hole that it goes through. This results in either stripping the threads or breaking spark plugs and having part of the plug stuck in the head.
So the castings are revised to fix this. The spark plug threads now go all the way to the chamber to eliminate this issue. There is also a change to the valve cover rail and corresponding valve cover. This change is a reduction in the amount of bolts that hold the covers down. This means that only new style heads fit the new style valve covers. The covers are supposedly slightly thicker and have some additional ribbing. I believe this is for NVH reasons.
The 2005-2006 and most of the 2007 cars have the early style valve covers and spark plugs to my knowledge. Only the late 2007 and 2008 cars have the new style plugs and valve covers. This is causing us a bit of a headache now because we now will have to carry two styles of head castings to be sure we have coverage. It will also mean that we will need to know what style head is currently on the engine if you are upgrading to our CNC ported heads because we will need to make sure you get the right castings that way everything is compatible. Currently we are just starting to see the new style castings but it is rare still at this point. In most cases because the heads are so new there are not a lot of them out there for us to purchase as compared to the older style. The only positive thing out of this is that there is no performance difference between the castings. The change in spark plug style does not cause any gain or loss in combustion effciency or power.
If anybody is interested in more info or anything just let me know.
Thanks
Mike Schropp
Livernois