I have been straight piped with and without the cat, had an aftermarket muffler, and then went back to the stock setup on my 600. I gave it all a chance to see a difference in mileage and there is none. I think I have another problem with my truck but this is my experience. It may be different on the 600 since the stocker is 4" in and out.

I feel that the main benefit with exhaust is lower EGT and SOUND which is personal preference. No increase in mileage on the 600....This is my observation/opinion.... I just wish I had known this before I spent all the unnecessary $$$

 

Unfortunately if you had done a search you would have uncovered many posts about this subject. You usually don't see any gains with exhausts until you are making 400hp or so except for those which you described (egts and sound). Personally, both of those items are worth the $200.00 I spent on my muffler....

 

I have seen alot of posts about exhaust,intake,filters,and synthetic oil claiming to help mileage. I guess you don't know what to beleive until you try it. Tire pressure makes more difference than anything mentioned above. I have also seen the hundreds of posts stating: "wait till it breaks in".....well I think that occurred for me after the first 1000 to 1500 miles when my mpg INCREASED from 11.3 to 13.8(hwy). It is a pretty consistent 13+ for the last 6000 or so miles.... so I am afraid that is all she is gonna do...until I can figure out what they did different to the newer 600's and the "lucky ones" who had to "wait" till they corrected the "problem". I would have let mine sit in a railcar for a few months if I'd known it would have gotten better mileage.

 

Your mileage will be in a state of influx until the engine breaks in. The Cummings does not start to set (ie break in) rings for 7000 to 20,000 mi (There is a large debate on when they really are set in).
Many on the forum state not to install synthetic oil until 15,000 mi due to this fact. Ams oil recommends 10,000 mi, and my diesel shop recommends 40k. My mileage was a joke @ 13, the jumped to 14.6 with the install of my Jegs SS 4" exhaust. Was it the exhaust or the engine beginning to set up. I am one that run his rig with respect and conceded to be gentle. But when duty calls and a deadline must be met, the hammer will fall. Wow 10K on the speedo & mileage went to 16.8 with extended hard run 2000 mi with fully loaded trailer. On the return trip no trailer and speeds in excess of the limit 18.2. Filled up yesterday with mostly in town stop & start and the famous I can get ahead of you to the next stop light back to 16.6(hand calculated and the tank filled to the brim). Leave on a run to Orlando this afternoon and hope to see if she returns to the high numbers . I have my bypass system installed but not hooked up for my Idea of being set up (broke in) @ 15K. Then the real numbers and the bombing will begin, Can we say Piers BOMBER LITE, I don't know how those H2 rims got on the truck sweetie, no those are not bigger tires, it is a optical illusion .. Good luck with the new truck

 

Dwhite, I want to mod my exhaust too, I am curious about what you used to do this. do you have any more info or pictures. also where did you get the parts? thanks

 

There was a post about this subject by jannetty racing . They stated that dyno results showed exaust and intake mods actually hurt performance because of engine defueling. The only way you would see the benifits of these mods. is with some type of boost fooler.

 

Usedtostrokeit,
I think any muffler shop that handles 4" pipe should be able to "replace" your muffler. As for the other...it will probably have to be gone when you get there. I wanted to try it (breifly) to see if it made a difference. Not a noticeable difference in mpg. I will say this. The cat acts as a muffler and the "straight piped" sound with the cat is not the same as w/o. I just like it quieter so I put mine back all stock. Still getting the same mileage and power doesn't seem any different.

My muffler shops usually charge about 50 -60 dollars to do it.
Lets see...how many gallons of diesel could I have bought with the 305.00 I spent in this "experiment".