Does anyone know via a dyno run or other reliable source what the hp gains would be for a total exhaust overhaul? Specifically, shorty headers, high flow cats, and cat back exhaust...I live in NW Indiana near Chicago....So emissions isn't going to let me get away with deleting the cats altogether, or I would.

 

The 4.0 gains little from exhaust work including shorties; even LT's on the 4.0 provide minor gains. I'm not well versed in dyno info on the 4.0 but since everything exhaust BUT headers on the 4.0 yields little to no gain (tough to discern between pulls), WITH shorties, my guess would be 5-10 RWHP and I'm likely being gracious. On both the 4.0 and the 4.6, Ford did it right with a fairly free-flowing system.

If you want any substantial power gain on the 4.0 without going FI or Nitrous, you have the CAI & custom tune combo for about 15-20 RWHP on the V6.