I happened to change out the OLD spark plugs from my turbo D16Z6, and noticed the Wiseco pistons were clean on the intake side, but lots of carbon on sides/exhaust half - sort of a cresent around the plug hole (when I say the intake side is clean, I mean sparkling-new clean). Timing is fairly conservative, 50trim T3, 3" dp/stock cat & exhaust, stock head. I was wondering if that is telling me the intake side is getting too hot or getting slight detonation (shock carbon off). I always thought the intake side ran cooler, so the exhaust side would burn carbon off faster.

I'm kinda worried because of how old the plugs were. 3+ yrs old, OEM heat range, .044 gap; ~500 miles with the turbo parts on. They made 160whp at 6psi, but really broke up when I turned boost up past 12psi (not EMS, have 2.5bar sensor). I know you're all shaking your heads, but its a long story. Finally found a local place with NGK bkr7e's, so it will only see those from now on.

 

So...nobody has anything to say?

 

Put the new plugs in a leave it alone.

If you want to turn the boost up you should have it done correctly with a proper tune.

 

ATM its a very conservative CROME tune. ~11:1 in boost, 14.5 cruising, 13.5 low vac, conservative timing. Its not the tune that's bad, it was just the plugs. Engine runs 10x smoother now, pulls harder, and actually needs less fuel. No misfires with extra boost.

I'm just wondering why I'd have carbon on the exhaust side, and none between the intake valves.

 

Unless you're aware of something I'm not, my guess would be that it's simply a matter of you not having complete combustion and that incomplete combustion is being pushed out the exhaust. The mixture could be so rich that the engine doesn't get very hot/hot enough for any of the valves to be hot enough to burn off the carbon deposits. There isn't really too much of a reason for the intake valves to be covered in carbon deposits since it isn't exposed to "dirty" fuel/exhaust like the exhaust valves are/would be.