The Bosch 4 prong plugs are sad imititations of Surefire plugs. These were invented by an American whose son was killed in a WW2 fighter crash due to bad plugs. The evil spark plug industry sat on this idea till the patent expired & now a small company are making them. They’re kind of hard to describe. Instead of one prong, they’ve got 8 tabs. They’re self cleaning & never need gapping. They come with a lifetime guarentee and are fit & forget, a boon if like me, you own two V12’s. Down side ? Not cheap. They cost 4 pounds each (last time I looked). Ask the wife for a set for christmas.
On Sun, 14 Nov 1999 20:22:07 -0800, Cannara firstname.lastname@example.org said:
Paul, I personally think they’re a waste of time. Take one and connect it up
outside the engine and crank the engine over – if you see more that one arc
across the gaps, have another drink! Only one gap will break down at a time,
typically, so there’s no advantage as far as increased exposure to mixture.
In fact, the geometry of the plugs like these that I’ve seen actually reduce
exposure. Someone else may have had another experience, of course.
The way we had it explained by the Bosch chap was as follows:
With a conventional plug, you’ll get some carbon build-up on the casing of
the plug. Over time, as the gap widens, the spark may find it easier to
travel along the carbon instead of jumping the gap, causing misfires etc.
With four prongs (and the way they’re shaped and positioned), there are 8
possible ways the spark can happen. If carbon starts building up, it build
up near the inside edge of the prong, the gap there will close, the spark
will jump there, and the carbon will end up being burnt off, while still
This is the Bosch +4 plugs. Pity they don’t fit XK 3.4, 3.8 or 4.2 engines
(or the V12 either, according to their guide).