I have a 2007 5.9 cummins with a 6 speed. I use my truck to haul a 53 ft car trailer and haul 4 cars all day. My question is, if I take my muffler and cat off, will I lose torque??? I always heard you lose back pressure and don't want to lose any of my torque. will this be a good idea for me since i use my truck everyday for pulling??? Please let me know I am a newbie to this site and love it so far.

 

Diesels don't rely on back pressure like a gas engine does. In fact, until you get into the 5" exhaust range you're helping your truck breathe better and should reduce exhaust temps. I've straight piped three trucks and noticed gains in power and not a reduction. So in short, I like straight pipes and recommend them as long as you can stand the extraexhaust noise.

 

I have the opposite to say here... not to argue but to give a different opinion based on my experince with my truck, an '05. I removed my cat & put in a 30" magnaflow straight thru muffler and lost alot of part throttlelow end torque. mileage went down and I wasnt very happy with the setup. I then added a 4040 aero muffler where the cat used to be and it's design provided enough backpressure (IMO) to help spool the turbo quicker and bring back the low end torque lost w/o it.egt's were still reasonable and better than stock. if your looking for quick 0-60 times or 1/4 mile times and dont tow, sure straight pipe it and you'll be on your way. for towing, I think you need some restriction in there to get you off the line better and the 4040 where the cat was will help with that. it is a compromise between straight thru and not so straight thru. go to the aeroturbine website and have a look at their mufflers and you'll see what I mean.

 

any other comments on steve's post. I got a 97 that i just ordered a straight pipe for. I commute on a daily basis but i haul wood and do alot of side work that requires heavy towing. on weekends. Would this be the choice for me.

 

with yours being a 12v I think you have a 3.5" exhaust... not 100% sure since I dont own one but going to a straight may benifit you more than it would a 4" setup, I'd wait to here what the experts have to say about the older CTD's...

 

I don't think either of you two are wrong. It depends on your exact setup, a stock 03 with muffler delete (no cat stock) will have different characteristics then an 05 with muffler and cat delete, since they are both different exhaust diameters and therefore different back pressures. Back pressure is a good thing for low engine speeds and torque (to a point that is); however, back pressure is bad for higher engine speeds and horsepower. As Steve said, in his setup he noticed a lack of torque at low engine speeds and Silverline has seen multiple times a gain in power. This is all a characterisitic of how they drive, Steve (correct me if I am wrong) probably spends more time in either lower RPM ranges or utilizes his entire engine band(and transmission will make a HUGE difference on this), where as Silverline probably uses higher engine speeds.
Getting back to the questions of straight pipes for your specific applications: if you use low end torque a lot, and you don't necessarily do JUST because you tow (depends on how you drive and gearing), you want back pressure. If you use higher engine speeds typically then it is probably good to go with straight pipes. Again, this all depends on your setup, since things like engine tuning will change these effects drastically.
As Silverline said, a more free flowing exhaust will cause lower EGT's, which can help for towing...
This is not a simple one answer situation... I suggest doing what Steve did, try it and if it doesn't do what you want then go with something else. Just remember the "butt dyno" can lie to you and without monitoring equipment for stuff like EGT's then you will have issues.

BTW Steve: Back pressure hurts turbo spooling... but it does help exhaust flow characteristics at slow speeds (prevents flows from hurting in-cylinder combustion charactersitics such as in-cylinder pressures)

Oh, and had to add this:
I am not trying to step on anyone's toes, just trying to clarify exactly what has already been said and put my 2 cents in as well

 

I do have low gears 4.10 or 4.11?? with it being a manual the first 3 gears are slow and short to shift through so i guess i would say i use alot of low end torque correct. My prob is guys is i got a 4" turbo back straight pipe coming UPS. be here thurs, fri. INSTALL or SEND BACK or just give her a shot. another thing is is i have no gauges and i'm a little to broke to buy them now.

 

I would say try it and if it doesn't work out then save up the money to get the Aero 4040 or something similar.Just let everyone know how it turns out[8D] I would imagine that the drop in torque would be annoying at best, not something that would really kill your ability to tow (we aren't talking about 100 ft-lbs drops or anything)

 

that's not what I saw on my truck when I was messing with the exhaust... the open exhaust took more time to spool the turbo (part throttle), at WOT however it did spool quick, response was very quick. the thing is at wot your dumping alot of fuel which will get the turbo going. without the aero in place, I dropped approx 1~1.25 mpg, almost the exact same result when I opened up the exhaust on my 2000 gasser. closed it up some and the bottom end came back as well as the mileage. on the gasser I had dual 2.5's, brought it down to dual 2.25's and if you do the math, is just slightly more area of flow than a single 3"...

 

one thing to touch on. . .with the truck being a stick, I doubt you would see the loss as much as steve did with his auto. mainly from the G56 6spd being geard so low.