I am about to re-fab the exhaust back to my compound turbos. I have plenty of stock exhaust pipe and it would be really easy. Just wondering, Honestly, does a bigger exhaust such as a 4" or 5" make a big differnce in egt's or is it just a big money maker for some of these companies.

 

The difference between 3" and 4" is huge. The more air you can get in (high flow filter) and out (4 or 5" exhaust) - the better. Unlike a gas engine, a Cummins needs little if any backpressure

 

With twins you should run 5" as the bottom turbo is 5" on the exducer side.By necking it down to 4" you will create a restriction which would cause heat issues.

 

missed the twins part...

 

5 inch it is. Thank You.