Do they make one, if not what good is it to put a 4" exhaust on with the orginial cat size. Turbo to cat 4" then downsizing then back to 4" after cat. Inspections stations here in Pa. says all diesels will be inspected in a year or so, its a federal thing on the cat and exhaust. One more question, some on here says there should be back pressure on turbo and others say no, have heard the 7.3s don't need the back pressure but the newer variable vane turbo from 03 and up must have the back pressure, does anyone know which answer is right.

 

Any body have an answer for this?

 

Well.... Just recently I installed the 4" Bullydog down tube on my 04' to complete the turbo back install, leaving the cat in. Tight fit at the firewall! Where the tube mates to the cat, the cat inlet flares to 4". That's were you cut the factory down tube off. Then weld or clamp the new tube on. (I welded). The exit of the cat is a little more than 3.75" ID. The stock tail is 3.5". So, all I can speak for is the BD, but the connection to the exit of the cat is an outer slip over that flares out to 4". Worst case, you could call it a 4" system with one restriction -aft cat.

As for the turbo needing some back pressure. There's a lot on this subject. Do a search through the forum and brew a pot of coffee for an evening's read. Personally, I left the cat in for two reasons. Federal law, and the warranty. I did run my system as cat back for about a month, then now for the last couple weeks as a turbo back 4", muffler deleted, cat in place. I will say that there are improvements and differences in engine performance with both installations. Catback, terrific exhaust note with two rpm ranges of 'Let's Git R Done' torque producing rumbles. Turbo sings and spools up quicker than stock. EGT's are a bit cooler when towing heavy loads. About a 75 degree drop. Turbo back, all of the above, but the two rpm ranges of power are smoother if not quicker as the turbo does seem to spool up even faster. Exhaust note has smoothed out and lost a decible or two. I haven't towed yet with the complete system, but just freeway driving I think EGT's pre turbo have dropped another 50 degrees, but with my foot into it, I think EGT's do climb more rapidly. Something to keep an eye on. Fuel economy did improve with the cat back install, About a mile to the gallon, but it's too early to tell if the complete system has made significant change.

-Steve