I have yet no experience of my own, but another finnish turbo-slanter told that NGK PBR6FS has worked good. Better than 7. He has newer style head with small plugs, but heat range should be same.
Hmm, Should have mentioned that I'm running the old style head. Any idea what that would translate into a gasket style plug?
OK I might have answered my own question...
from NGk's plug number breakdown it looks like I need a
B - 14mm thread
P - projected tip
R - resistor type
6 - heat range ( maybe a 7 )
E - 3/4" thread reach
S - standard electrode 2.5mm
Does this look right?
SO..if all of this is correct
one step hotter would be BPR4ES
ZFR5N should = BPR5ES
one step colder should be BPR6ES