I've a vehicle that's been doing my head in. It is a 6.6L cast iron Bottom end LS1 with a ProCharger 15 psi and a 235/245 cam on 116. It starts, idles and cruises well at light throttle in closed loop but under load fouls plugs and fumes noticeably on the dyno. I can foul plugs at 13:1 at 2000 rpm around atmosphere, under WOT at 12.5:1 it doesn't foul plugs but they read very rich and fumes black smoke in quite large amounts. I have tried running CL up to about 100 rwkw only going into OL and 13:1 as boost commences and I can get the plugs clean enough to not foul but it's still fumy. The engine is new. We had to replace the intake with a 4bbl single throttle as the FAST manifold had very poor distribution under boost(plugs read poorly previously). It makes 575 rwkW, I have scaled the tune by 30% so I have timing on the MAP. I have tried two different Wide bands, the dyno and a wideband commander and both agree on mixture. The car has a substandard exhaust in a twin 2 1/2. I'm thinking possibly exhaust back pressure too high. The engine is quite inefficient at part throttles, it takes a lot of timing with no detonation, 35 degrees at 2000 rpm at atmosphere. Open to any suggestions. Can't really post a log or the tune as its tuned in HP, I have both, this was already tuned before it came to me with HP but wasn't scaled and had a different engine.

Any thoughts appreciated