4.0 Headers on a 2.9?

[RonD]

Exhaust systems don't "add" power to an engine, they "leave" power in the engine

Each engine will have an exhaust flow Velocity that can be calculated by RPM
With no exhaust manifold at all the piston must push that exhaust out, which takes power away from the crank
The power it takes to push out the exhaust is related to the exhaust valve size and exhaust port size
AND outside air pressure

Exhaust manifolds(headers) are designed to change the Velocity of the flow into lower pressure
If air(or fluid) flows from a smaller pipe into a larger pipe there will be a pressure drop in the larger pipe which causes air to be PULLED from the smaller pipe
There is Math involved, so feel free to look it up, lol

So headers are designed to create the lowest pressure at a specific RPM, "power band", low, mid, or high but only one of them
The low pressure means exhaust is SUCKED out of a cylinder's exhaust valve leaving more power on the crank
Its as simple and as complicated as that

Factory exhaust on most engines use mid-band, since the 1950s
And this is where the MYTH of 4-stroke engines needing back pressure came from
People would install headers with larger pipes(free flow), and LOSE POWER, "WTF!!!"
They lost the mid-range power from the factory exhaust, because they didn't understand how the exhaust system works
So they wrongly assumed engine needed back pressure, no 4-stroke engine needs back pressure

Most 3rd party headers are setup for low-band power, off the line power, but you lose the mid-band
Racing headers change it to high-band for lowest pressure

So the header design is for specific engine size and for lowest pressure in one RPM band, so 4.0l designed headers on a 2.9l would not get good results
As said the 1998-2000 4.0l heads had the smaller exhaust ports to heat up the Cats faster, so 4.0l headers for those years "might" work better on a 2.9l, but no way to tell unless you try it

Single exhaust pipes also have a design parameter because small pipe to larger pipe(the outside, lol) can generate a low pressure pulse as the high velocity exhaust exits the end of the pipe
So the size and length of that one pipe can create a lower pressure at the exhaust valve when it opens sucking out exhaust and leaving more power on the crank, and its tuned for engine size and RPM band

"H" and "X" pipes are used for the same reason
On dual exhaust systems there are opposing pulses of cylinder's exhaust
If you connect the two pipes together it creates that same low pressure pulse pulling more exhaust out and leaving more power on the crank

 

[gaz]

@RonD ,
I appreciate that you took the time to give a brief explanation of how the relationship between exhaust ports, stock manifolds, headers and the rest of the exhaust components contribute to exhaust gas flow