i'd give the 5's a go first,the difference between 5's & 6's doesn't appear to be much in bp_es',so try the hotter plugs first.if there is a problem,then try 6's or maybe even 7's,i used to use bp7ev's in my engines,with no fouling problems,except when a leanout chewed the side out of number 3 piston one time.i cheated my way round that by swapping plugs cylinder to cylinder,so the fouled plugs cleaned themselves up.it got me from tamworth to sydney,which is what i needed.

Hi .
I am looking at getting some ngk iridium plugs for my car. the only thing is, on the website they recommend the 5, which is a hotter plug than the 6 they say for the L series. In light that I have a slightly modified engine and it would be.. seeing more revs on a daily basis - would it be smarter to go down a heat range from that recommended?