Quote:
Originally Posted by Jordo!
Probably because the motor is high compression and tends to run on the hotter side given the occasional oil temp issues.
Hotter plugs are probably prone to some pinging. Although, if you run 93 AKI (or higher) you would probably be fine with slightly hotter plugs -- remember, the engine is tuned and set up to run on 91.
That said, read p. 9-4 of the owner's manual... they claim some light knock under load acellerating up hills is normal 
You could also try playing with the gap, I suppose, but if it runs good, why mess with it? |
Honestly I've been building and tuning boosted engines for the past 5 years... I really don't think this setup needs 8 heat range plugs. A 13:1 engine might, though. There must be something about the cylinder head design that allows Nissan to get away with running such a cold plug with no fouling issues. That's more of the info I was interested in getting - I just read most of the service manual today, and nothing there.
The heat range of a plug dictates how much heat the spark plug can carry away from itself and the area surrounding the electrode. Really, it's just a way to prevent detonation and pre-ignition by eliminating the spark plug as a potential hot spot in the cylinder. I haven't spent much time looking at stock AF curves, but maybe the motor runs lean enough stock to warrant this extra protection.
I know my Evo X has basically the same plug design, and heat range (8) stock. But it essentially runs 20+ CR (boost
PR * combustion chamber CR) under full boost stock (19-ish psi). At 32 psi, I was only needing a 9 heat range plug. Similar modern combustion chamber design, but I know it's not the same engine at all.
But you have a good point with the owner's manual comment and such. A 3.7 L V6 making 332 bhp on 91 octane stock is pretty much pushing the limits to begin with.
This is just a question of curiosity and thinking about boosted applications on the Z, not looking to mess with anything; so I'm not sure how plug gaps are relevant. I don't really like messing with gaps on iridium-tipped plugs anyway; too much room for mistakes and potential broken plugs inside the combustion chamber!