I think this has turned into a big debate in some places so I want to know what you guys think (without the debate lol). Power has a lot to do with this of course so lets keep this to saying we have sane power levels (<400HP) and how exhaust would effect the truck. First we have the stock exhaust. Is that 3" with the muffler and all that really too much of a restriction when at stock power levels? I don't think it is but would you gain efficiency by straight piping it, leaving the power at stock? Most people have around 350-400HP lately because of the cost to get much more out of them without having egt issues or something. What size exhaust do you need for that? I know Mike seems to be doing fine with a 3" straight pipe but even cummins manuals for cummins 5.9 gensets with the same or lesser power say 4" minimum. So what gives!? Is there really a benefit to going bigger or is that folklore? I think it has been shown to reduce EGT's by 100F going from 3" to 4", but if the exhaust housing exit is only 3" then wouldn't you not notice anything because of that bottleneck? There is a lot of people going both ways with this so I would like to see some proof as to which is fact and fiction, from personal experience to actual testing.

What little tidbit I can provide here is that I know most people with 4" exhaust on mild moded engis say <450HP has done quite well. But as for 5" its debatable but some say too big of a pipe causes the exhaust to cool too fasts and hang in the pipe causing a loss of performance. (Heresay)

I have no saved data, nor did I document this with instruments. My truck is still sort of stock.I travel to the west coast to see my grandkids. That's 2,000 miles one way. I do a hand calc each fill up and averaged 17 mpg on the way west. While there, my son-in-law knows someone at a muffler shop and I changed out my 4" exhaust to a 5" and resonator, for cost (and a 6 pack over a b-b-q that night). On the way back, I hand calc each tank and got 20 mpg. Since i have been back, I noticed my in town has increased by 2 mpg.I do not know the stats or root cause of this, but I believe there is some back pressure with the 4" I had, and basically nothing with the 5". This has to assist in the results. Expensive equipment change, but improvement I like.The old guys from my yesteryears always told me: open the back end and in crease the flow in, minimum for improvement on performance, mpg and engine life of a diesel. So far, even with todays equipment, it works for me.

Did the 4" have a muffler or was it straight pipe? That is really interesting how a stock truck could notice a 2mpg diff going from 4-5", both of which are seemingly way overkill on a stock cummins. Gotta hear more on this Did you change anything else at all other than that exhaust? Fix anything?

I did a little experiment of my own to see if I could get an improvement in mpg over the stock configuration of my truck. I have kept copious mpg numbers since the day I purchased my truck so my thinking was that I could compare any changes to my historical data. I first installed a K&N hi-flow filter in the stock filter housing. I couldn't tell any difference in performance or mpg using my driving style and with out any towing(I am pretty soft on the pedal and drive 65mph max on the freeway). Next I installed a 60hp chip and could really feel the hp add. After extended miles over several months (and no towing) I was able to say I got a 2mpg improvement in my highway driving and almost the same in my city driving. In stock configuration and 100k on the truck I was averaging 18mpg in town and 20 mpg on the road. With the chip my mileage in the city moved up to 19.7 and my hiway driving went to 22mpg (this is spring and summer time driving). I still had the K&N filter in place during this test. I then removed the chip after I had about 5k miles on it and then I installed a 4" turbo back exhaust system with a straight thru resonater type muffler ( my truck did not come with a catalytic converter). Again I put 5k miles on the truck (with no towing) and I saw the similar 2mpg improvement over the stock configuration in both city and highway driving. The K&N filter was still place for this test and the driving for this test was summer and early fall. At this point I was thinking I had hit the mother lode because all I had to do was put the chip back on the truck and I would have a net gain of 4mpg and better truck performance. It didn't happen that way. When I installed the chip in conjunction with the 4"" exhaust system and K&N hi-flow filter I still only saw the 2 mpg increase over the original stock configuration. So in the end the sum total of what I got was a performance improvement (60hp chip) and a 2 mpg improvement.

my first 100k a got between 18 and 20 mpg with everything bone stock. the milage depended on how fast i drove. there was no towing involved. i checked fairly regular, but did not keep any records. i then added the banks stinger plus. 4"exhasut, with straight thru muffle, quick turbo, ottomind tuner. i got pretty good power ou of it, with no change in the milage. i could hot rod aroun or just drive normally and it stayed the same. the only differance i ever have is from speed. now this is for highway driving and around town iusually get 14 to 15.my milage has been very consistant since i bought it, before and after the banks. for a brief period i was getting 20mpgwhen i installed my new vp running between 70 and 80 mph.

Did the 4" have a muffler or was it straight pipe? That is really interesting how a stock truck could notice a 2mpg diff going from 4-5", both of which are seemingly way overkill on a stock cummins. Gotta hear more on this Did you change anything else at all other than that exhaust? Fix anything?

If your question was directed to me .... No, it was a stock exhaust system on the trip out. I did have a K&N drop in air filter installed 10K miles before the trip and used it the entire trip. But the exhaust was the standard system with no other changes. Also, outside of about 400 pound of stuff the wife wanted to take to the grandkids, and nothing excessive back, it was an empty load. I have hand calculated mileage from day one and so far have only capped the tank each fill. But like a fool, have not documented each fill. (I do now). The new exhaust is from the stock CAT on back, resonator and no muffler. Stainless steel system. I have added the items stated in my sig after that trip. (by the way, that trip was two years ago, sorry 'bout that). Nothing has improved my mpg like the exhaust change out. I will say, the Banks CIA system on the front end has improved the exhaust where it is way less soot on the back bumper verses the stock. But I believe any system would have done that for me. I feel with the stock 4" system, I should have changed out the muffler to a resonator first to see if this would have improved anything and save money, but getting the 5" at cost, I couldn't turn it down. The exhaust gases are cooler verses the stock system (I check by feel, I don't have a temp gauge yet) and I don't understand the theory of it being so cold it prohibits the flow. If you have a constant flow, cool or hot, what is the issue ? When I pull my trailer loaded, it's the same before and after the change out. I'm an old coot, work with me people, please !

I got to thinking about this some more and I think maybe a smaller exhaust has a bit of resistance when you get into higher HP. The gases start piling up on each other and it ends up having to force it's way out of the exhaust. As you get bigger, there is still the bottleneck in the exhaust housing but maybe the bigger free flowing exhaust also has an effect of pulling the air out of the engine, aiding it. This might not be as true when you're at WOT but when you're trying to get mpg's then the exhaust is very large and as the air flow changes per each exhaust valve opening/pulsation, I wonder if each pulse helps the next one out, sucking air out of the engine. This would be through momentum of the previous air pulsation. Kinda like when you shut a door, the momentum of the air carries itself to the other end of the room and is felt by whomever is over there. I need to get some books on this stuff

When you have an enclosed area, like the inner diameter of a pipe, items like exhaust is considered a mass volume, and can restrict flow. Also the total length of the restricted area, any additions of curves and bends (like 45 or 90 degree bend) add to the restriction, or so called back pressure. This is somewhat constant in air, gases and liquid.Now add higher amounts of volume, (like exhaust from a higher horse power engine opened up), will increase material density within the restriction and adding pressure to the system. I'm far from an engineer, but I suggest, if you determine your system should be increase to accommodate higher horse power, I'd try less restriction in your system now, (muffler to resonator or a straight pipe) and if this isn't enough, I wouldn't go over an inch in diameter from your stock size, (there is a reason for the original size by the engineers in the first place). Above that, you might consider going straight to atmosphere.

I'm going to get some books on this today so I can stop guessing. You are right I just want to get some real solid data going.