Spark Plugs - Projected Tips vs Standard?

[Captain Obvious]

It appears that Datsun switched plugs from the projected tip BP6ES (where the "P" means "Projected Tip") to the standard non-projected tipped B6ES (no "P") in 75? But what's interesting is that the small amount of armchair research I've done seems to indicate that the farther down into the cylinder you get the spark, the better the burn, and projected tips help in that way.

Anyone know WHY they switched to the standard non-projected tips?

Any reason I should not use the older projected tip BP6ES plugs in my 77?

Here's why I ask...

I've got projected tips in there now from my previous owner and they looked perfectly normal to me (after looking at 260 plugs for so long). They aren't hitting anything. They aren't burning up or showing any signs of overheating. They appear to be just fine and are loving life.

So... If I've got the room for them, and the current projected tips aren't indicating any problems whatsoever, then is there really any reason I should switch back to the recommended non-projected tips? Maybe I should take advantage of the potentially better burn and stay with the projected tips even though it's not what Datsun recommends?

What's the downside?

[Zed Head]

Might be an emissions thing. I've read that projected tip plugs have an effect similar to advancing the timing about 4 degrees, because the flame is initiated closer to the center of the combustion area. Reducing timing seems to be one of the ways that the engineers clean up the emissions gas mix. So, projected tip plugs = "advanced" timing effect = bad gases out the tail pipe.

Just conjecture...

They went back the projected tip with the P79/F54 combo also in 1980, to add to the puzzle.

Edited October 6, 2012 by Zed Head

[240260280z]

In theory, with projected plugs, the flame front will be closer to the encroaching piston face so it may act like advance. The flame front will also travel down towards the piston, sideways towards the cylinder wall and back towards the dome roof. It may give the intake valve a wee bit more time to close, or even help it close.

With the a non projected plug, the flame front will be guided partly by the dome roof and travel forward along the curved roof and also and outward.

That's as much as I have thought it through.

[John Coffey]

Projected tip plugs also slightly increase the CR. Most likely the change was a cost issue. The 280Z was getting pretty expensive in 1975.

[Captain Obvious]

The projected tips don't stick down that far... I can't imagine there is any significant increase in CR. Could be wrong, but I would have to see the numbers to believe it.

Cost? Who knows? Don't know if it has always been the case, but today they all cost the same.

I'm at a loss... The most perplexing additional info is Zed Head bringing to light that they went back the projected tip with the P79/F54 combo in 1980. What made them want to pull that spark back towards the hole from 1975 through 1979?