[XCRN]

I have a 5.3 LS and after I switched headers with larger primaries, from 1-5/8” to 1-3/4” I noticed my fuel flow increased cruising at 55mph, low end torque felt like it dropped, and also noticed a more than 10% drop in mpg calculating at the pump. The 1-5/8 headers were made of the cheapest of chineasium and the header ports did not line up great with the cylinder head, the blending in the ports looked to have been done with a rock and chisel and just poor quality and tolerances overall. The headers were just 2 different ones from 2 different sets and I had to rig up a terrible down pipe poorly welded to get driving due to an unforeseen circumstance that required me to get this driving ASAP. The new headers are a much higher quality, close matching ports, tight tolerances, and very nicely blended and smoothed over welds in the primary ports. This mates perfectly and smoothly with the rest of the exhaust, no janky rigging required.

So with what I thought should have improved efficiency dropped it. The only difference I did during the header swap was swap the oil pan to a lower clearance oil pan which required changing the pickup tube and O-ring that was apparently torn up. So now my oil pressure did increase which I am sure is adding some drag to the motor but to drop it 10% seems a bit much for increased oil pressure. But other than that everything is the same from the header back. AFR stayed the same, but I eventually had to richen it back up to regain some lost low end power.

So is this a matter of some back pressure might help? My muffler is a straight through with no cat. Would replacing the straight through muffler with chambered muffler maybe help or adding a cat?