I put in an extrude honed manifold in my dyno-tuned CAT and turbo boost and fuel mileage immediately dropped. I'm doing another Dyno soon, but was wondering if anyone ever upgraded the manifold and what the effects were. Before the manifold upgrade the CAT was tuned for maximum performance of 550 HP at 1,425 RPM and got 6.2 MPG with 45,000 lbs loads cruising at 59 MPH with 3.73 ratio. Now it fell down to 5.5 MPG with same payloads doing 62 MPH with 3.42 ratio. The axle ratio was changed at the same time as the manifold.

 

Since the manifold was just changed, that's the first place I'd look. Check for leaks.

 

Did that. There were a couple of leaks in the air-to-air hose, but they were fixed, MPG stayed the same: 5.5 loaded, 6 empty. Before it was at least 0.5 MPG higher