There's quite a few jeep guys on here, you may want to ask questions in the 4x4/off road section to see if you can get any nibbles...

 

It's a little known fact but automotive companies spend millions on exhaust design for each system. In fact, "exit turbulance" is one prime factor in loss of efficiency that is farily little-known. Unless you are really interested in taking a course on flow dynamics, I would simple stick to what works and ignore the marketing schemes. Even though they do sound nice!

 

It was kind of lost in all the other posts in this thread, but the whole point of my earlier post was to point out that you can get most of the benefit of a 4" exhaust simply by replacing the restrictive stock muffler with pipe. In my case I used the one Geno's Garage sells, but any muffler shop can do the same thing.

It's far cheaper, and if you retain the stock resonator there's no drone.

 

^^That's a point that was made to me by a diesel mechanic I know in Montana. His point is the turbo outlet is the limiting factor regarding diameter of pipe. He said removing the stock muffler was the equivalent of an aftermarket 4" on a stock motor, until you power it up, you don't really need a larger pipe.

 

Yes and no..... I've also heard some crazy examples of knowledge regarding exhaust from even exhaust shops. Just because someones in the industry doesn't automatically mean they're fully knowledgeable in the field. I'm sure we've all seen examples of that.

So considering the smallest portion of the exhaust as maximum ability to flow is correct, but this would depend on what someone was referring to and where in the exhaust this restriction was. If this rule always applied to anything within the exhaust then recognizing that the head ports are miniscule compared to the turbo would mean that the exhaust ports are tiny.....and because not all cylinders fire at the same time, all exhaust there after can be 1.5 in? Nope.

The best example I can think of for an analogy is.....make roughly a 1/4" small hole a piece of paper and put it up to your mouth. Breathing through the small hole, you should be able to breath fairly easily without feeling to suffocated. But now try that same thing with a drinking straw. The hole size is approximately the same size but the additional length of the straw adds friction and resistance to the air moving through it making it much more difficult to breath. Same principle rule applies to exhaust. Exhaust diameter cant be too small otherwise you create a constriction, but make it to big and you allow the exhaust gases to cool too rapidly and slow down before they exit whereby those slow gases get in the way of the hot gases and cause turbulence. So tuning an exhaust to the engine size and HP is important.
If I remember right, I understand that Cummins wanted a 3.5" exhaust system but Dodge used 3".

 

Not to mention that there are 6 exhaust ports and one turbo

 

Those are good points guys, I actually tried the paper hole straw thing!!

Excellent info.