I also prefer to use a non-projected tip on high boost applications.
Why is that? What problems have you encountered?
This never got answered publicly, so for anyone who's interested here's my answer from our PM conversation:
Hi. I found protruding BPxxxx plugs fine for most of my Coupe ownership, in fact I ran normal BPR7 plugs through all of the car's power 'hikes', right up until 280bhp and only had to start experimenting with plugs and gaps when trying to pass 300 bhp.
I found that the BPxxxx plugs suffered spark blow-out more than the same plug but non-protruding.
My take on this is that the BP nose is situated more in the centre of the combustion chamber where the fuel mixture is densest. It's harder for a spark to jump across a gap filled with dense fuel mixture, so the spark struggled to jump the gap.
Whereas the non-protruding plug is situated closer to the cylinder head, where the mixture is less dense - and probably a bit cooler too.
My theory seems to be borne out in reality too. I know that there are/were surface-spark plugs out there for real hardcore motors. I think we're talking big boost + big n20 motorbikes.
