I took the muffler off, still have the cat on. I've noticed other cummins are alot louder than mine. Does the cat muffle that much sound?

 

On a diesel I've never had one so I cant answer. But I know one a Gasser it does a lot.

 

Take the cat off unless your in cali. No use on there, big mpg diff too. And another reason is most trucks are 4 inch exhaust not 3 inch like stock.

 

Yea it'll be coming off, I am in cali unfortunately. But on the fortunate side I fall into the emissions/smog exemption category because it's a pre 97 diesel.

 

Perfect, rip all that emissions bs off there. I'd look into up grading turbos before buying a down pipe for a hx35 if you do plan to upgrade in the future. Many companies include a down pipe in turbo kits which use a hx40 downpipe

 

You could do like several people do: pull the cat off, beat out the innards, and stick it back on there. Pass any inspections and still get great mileage plus sound like a beast.