Header Primary Size Increase MPG Loss
 

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?

1 3/4” headers on a 5.3L is strictly for high rpm with a cam change for horsepower. Economy build would be focused on smaller tubes and cam for maximum efficiency at your cruise rpm.

'primaries'
 

Even on a 355-cubic-inch small-block Chevy, HOT ROD Magazine would not go above 1 -5/8 inch primaries for exactly the reasons you're experiencing.

Oh you only ever want like 1.5 to 1.62 inch primary headers.
Heck I have 1.62 inch headers for my 454 that's been punched out to 8L and 11:1 compression.
1.75 primary headers on a small block are for like high rpm nitrous engines.

So my header size choice is not ideal I gather. Unfortunately it is made specifically for LS swaps into Jeeps and is part of a entirely bolt in exhaust system and my previous exhaust setups had interference issues with the front drive shaft. I will say my first exhaust was 1-5/8 primaries with 2in dual into a 2.25 single, when I went to the new header back exhaust with dual 2.5in to 3in single and different but CHEAP 1-5/8 headers, that really did improve efficiency by 10% and the new 1-3/4 headers with same as the previous exhaust system just dropped me back. So instead of changing the headers, can I add a restriction further up the system, or does it not work like that?

adding restriction
 

The only thing I've seen is Yamaha's, 1992, computer-controlled, variable-restriction valve, which is located at the end of the collector.
The engine is run at every conceivable load and rpm in a test cell, and all the performance 'histories' are stored in an internal database.
Depending on the load, sensor input, and operator intent, the ECU selects the most efficient valve position known for the engine.
It seems WAY beyond the scope of a shade tree mechanic like myself.:(

I don't think the problem is with restriction, or backpressure, or whatever. The problem is the pipe diameter. The larger diameter primaries will reduce the exhaust velocity, which in turn weakens the scavenging effect. And as a result, it reduces low-end torque and part-throttle fuel economy.

Think about how you tune intake runners. Long thin runners are better for low-end torque, and short fat ones are better for high RPM breathing. Exhaust tuning is similar.

Long story short, you need smaller headers.

I say let the cylinder head exhaust port determine primary size. For my riding lawn mower it's got 590cc engine and it has 1 inch OD header. For some reason the 459cc on my stump grinder engine has a 1.25 inch pipe.
Not sure why the smaller engine has a bigger pipe. Both engines run at about the same top speed. They make around 30hp per liter but only at 3,600rpm.
My 1L per cylinder suburban got 1.62 inch headers, it makes about 60hp per L but it also spins a good bit faster than a lawn mower engine, not double the speed but close.

If this is in a truck, you want Thorley Tri-Y headers. I have had them on 2 different small block powered vans. My G20 van and my Express van have both had Tri-Ys. Noticeable torque gains from idle.

I would say you are just adding to excess flow and it probably requires the throttle body to close more to maintain 55 mph now without speeding up. There's a reason why the prius uses EGR to control RPMs in the mid range on the prius. Allows the car to open the throttle body just about wide open the entire time and pump the intake manifold full of an enert gas to essentially "throttle" the engine as if it had a small wide open throttle body. Doing smooth transition 180 degree itty bitty headers would work the best i assume but people typically aren't going to go to all of that work. They will just do a cheap short tube header with the oem cat and move on if they care about low end efficiency.
So go small to increase torque down low for mpg or go large to increase up top for horsepower (torque x rpm/5252 = hp). They are inverses. The try y's and smooth bends help efficiency.
This reminds me of the guy with the lawn mower carb on the v8.
Also, this is the reason why people try and pulse and glide at the highest percent load they can get in closed loop without going to open loop.