Have a 2006 5.9 cummins Mega cab. Is it worth the $$$ to go with a 5" exhaust to improve mileage/power or should I just bypass the cat with a straight pipe and stay with the stock 4" ??? THANKS

 

4" is all you need till you reach 500 hp no real gains to be had and you actually lose a little on the bottom end going to a 5" at stock power levels.

If you REALLY want to improve mileage......shave a pound or two off the right foot

 

^^^ This ^^^


I have seen dyno numbers of about 700hp with a full stock exhaust. They pulled the cat and the dyno numbers stayed almost identical. Save your cash and keep the 4". The biggest restriction is the muffler.

 

Did you mean the biggest restriction is the cat.....because I could barely see any light from a 200 lm stinger flashlight through my cat.....& my muffler guy said the stock mufflers were not very restrictive.

 

we used some glycerin filled gauges and oil pressure gauge installation kits to check restriction on a earlier model truck one before the cat and one between cat and muffler. the muffler was more restrictive than the cat. on our 06's we removed most of the in-cylinder egr to gain 6 mpg and keep the oil cleaner. part of that i needed to open up the exhaust to achieve the lowest possible pressure at the exhaust manifold. i like quiet, all big rigs use 5" and the components is cheaper than 4" that is the only reason we went with the 5" we set up three of the 06's with different combinations. the one we settled on was a 5" straight pipe kit from fbd a Donaldson m090072 resonator and a m090535 muffler from Ryder truck parts. total cost $400. there was another muffler a little quieter (m101181) but was like $150 more money. as stated above unless you are making a mod, you will gain nothing. also it is very easy to remove the guts of the cat and re-install it. we started doing this in the 90's thinking they were going to check to see if it was removed. we used a masonry bit and drilled holes through it then took a bar and broke it out. it drills and breaks easy. after we figured out how the next one only took about 15 minutes