Compression Ignition Addict
Join Date: May 2008
Location: Cheney, Washington
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Good question. I would say as long as you have a 3" downpipe and your catalytic converter is either gutted or gone that you should have plenty of flow. I called many perfromance shops and thry told me that going to a 4" exhaust was not needed on our trucks unless they had major modifications.
I went to a 4" exhaust and really didn't notice much at all. There may be performance gains from the 4" pipe if you have super large injectors an a big turbo but, with baby swamps, I doubt you'll need it. If athetics matter to you, cut 18" off the end of your stock pipe and put a 4" or 5" tip on the end.
I'm sure others will disagree but, this is what I have found in my reasearch and results.
I am original owner of 97 Extended Cab 4x4 with 86,000 miles, Unlimited Diesel 175/30 Stage 1.5 Injectors, ATS Intercooler, Tony Wildman 6 Position Chip, 4" Turbo Back Diamond Eye Exhaust, Stealth SPR1 HPOP, Wicked Wheel 2, Hypermax Combo Boost/EGT Gauge, DIY Intake, FPR Mod, CCV Mod, IDM Mod, 203 degree T Stat, Stainless Steel High Pressure Crossover Line, Fitch Fuel Catalyst, 40,000 lb. Trucool Max Transmission Cooler. Next up a Bean's D-66, electric fuel, torque converter and a valve body.