What should I use for my 5.9, a 2.5" or 3" exhaust?

If you go too big, you lose torque. I've looked at many exhaust formulas, calculators, forums, etc., but they all have different answers. Most big name exhaust/muffler companies reccommend a 2.5" system, but I'm sure that's for a stock D. I see people running both 2.5" & 3" on modded Ds. Any opinions????

Vehicle specs:
'99 Durango 5.9L 4x4 mostly street driven & only tows a ski boat or small trailer.
33" tires, T/S lift, 3" Body Lift with stock 3.55 gears.
It currently has a CAI, but will soon have more goodies added....tuner, headers (CA legal), Fastman TB, etc....

 

From my searching the most common is a 3" system from the muffler back for single exhaust and 2.5" system from muffler back on a dual setup. I went with 3" pipe with my single exit magnaflow and I don't feel like I have lost any torque down low. It actually feels like I have gained some.

 

Take your pic, the results will be the same. Your manifolds and Y pipe will not flow more then the 2.5 can.

Beware though, if you add headers and a high flow cat/y pipe and a 3" exhaust and say a Flowmaster muffler you may flow to much and trigger the check engine light because of lack of back pressure for the O2 sensors.

If it were me, stock engine, intake, ECM, Injectors and manifolds - I would run no more then a 2.5" intermediate pipe.

 

If most of your driving is below 3000rpms then the 2.5 is good.

 

Run regular OEM manifolds down to the Y pipe. OEM cat flows fine. After the OEM cat go up to 3" and choise your muffler. I like the 24" MagnaFlow SS into a 3" to tail pipe.

 

OEM cat flows even better after you take a johnson bar to it....

Just remember HP is about moving as much air and fuel through your engine as you can. I like to start with the injectors/ecm then replace the manifolds with a nice set of block hugger headers. Your 3" pipe with a nice Flowmaster, Borla will round out the package. Next step would be to redo the ignition if you get a crazy injector lol. But thats from a gear head like myself, it all depends on what you want from your Durango.

 

hydrashocker - Why run OEM manifolds & not headers?
I did hear that it's better to run an OEM Cat & not a Hi-Flow Cat.

 

I would go with 2.5 just because the Magnum motor isn't really a high revving beast like some motors... If you were hitting 7,000 RPM or more, I'd say go with 3".... But that isn't the case...

 

what is a johnson bar? Never heard of that.

IndyD

 

I guess a johson bar means gutting the cat? Dunno why someone would do that... (on a durango) I ran 3" front to back.. Gets annoying real quick trust me.. now im running PPH(Headers)- PPH Y pipe- Magnaflow 3" HP(picked due to small size)-flowmaster AT40 back dual 2.5" - When i ran a full 3" to tail pipe its just too loud, yeah sounds great, but ever work on call 24/7 and get calls 2/3am in the morning? It can easily get on the neighbors nerves.