I have a 2007 dodge dually megacab 5.9l diesel. Truck is stock except the air filter, i have installed the airaid. Im thinking of changing out the stock exhaust to a (mbrp cat-back) but unsure of which size is better 4" or 5" and why? I've heard diff. Reasons on both sizes. Any good answers would be great, thanks

 

For mild mod's, stick with the 4".

Until you start upgrading turbos and injectors,you don't need bigger than 4"

Also, instead of doing the whole MBRP system, consider doing the muffler delete from Geno's. It sounds AWESOME on the 3rd gen trucks.

It's not that loud with the cat in place.

It REALLY soudns good and is a helluva lot cheaper than the full MBRP system.

 

coal train says stick 2 the 4" but y? is it betr, same, or worse than the 5" thats what im trying 2 find out. besides the exhaust the only other plan is 2 add the banks intercooler and ram air intake. mpbr cat-bak exhaust system is $340.00 4 the 4" and $366.00 4 the 5" not much price diff. just trying 2 find out wich is best ( performance wise) 4 my 2007 dodge dually 1 ton 2wd megacab diesel 5.9l and y. p.s. whats geno's thnks.

 

Performance wise, the 4" is plenty for most trucks in all but long, extended, full boost conditions. (i.e. towing a 5th wheel up a long, steep grade).

A 4" system is capable of efficiently flowing 1900 CFM of air. A Cummins, with a stock turbo at the peak of its compressor efficiency makes approx 2100 CFM.

Now, a 5" exhaust is capable of being efficient up to 2300 CFM of air. This sounds like it would be good, but the low end drone you'll get is pretty annoying. You end up with air tumbling under most throttle conditions rather than moving smoothly and efficiently.

If you are on a stock turbo, I say stick to 4". Step up to an aftermarket turbo that is bigger than 57mm and you can start looking at 5".

You need to match the exhaust to your driving style.

 

Coal Train, where did you get those numbers, of both output and how much an exhaust can handle. I'm sure there is a mathematical equation somewhere?

 

I was a little off on the numbers, I was trying to remember them off hand. Here's the actual numbers, by year and engine type.

 

Nice, thanks!

EDIT*

Looking at getting the Aero muffler/tip system myself. I'm curious if it's worth it to save a little money here and put it elsewhere, or go with a full MBRP(or similar) exhaust system? My plans for my truck are I/E, tranny build-up work then a chip.

PS I am definately going with an AFE Intake, probably the Stage II/PG7