Crimedog

I recently went from a homemade 5" downpipe to a 4" downpipe and thought I might share what I found to help some folks out who are debating exaust size.

Originally I went from a 3" straight pipe to 5" stacks, everything was 5", including the downpipe (tight squeeze on the firewall!). I noticed a little decrease in spool up, and the reason I did it was to try and get the EGT's as cool as possible for truck pulling. I didn't notice any EGT improvement. They stayed exactly the same, this was with stock injectors, NOT the 370's. I figured that the exhaust was cooling too fast to build more boost. This weekend I went to a 4" downpipe coming off the cast elbow at the turbo. I have about 1.5 ft. of 4" before it hits 5". There was a noticable increase in spoolup, and EGTs very slightly increased but it's hard to tell and who knows if it's accurate. I also didn't notice any change in max boost, but it does hit sooner. So for those of you who are debating going with 4" or the big 5" sewer pipe unless you plan on running up around 450 HP or higher, definitely go with the 4". The 5" may look cool and sound hollower, but my truck really picked up and went after just 1.5 feet of pipe change.

Mike Holmen

Sounds odd. Mine spooled faster with no exhaust than with it on. 5in the turbo spooled faster than the 4in. Egt's I forgot to check that. Crank up your advance and see how much more you can take before egt's and smoke shut you down.

01smoker

Thats probably why all 5" exhaust systems come with a 4" downpipe like mine. I really could'nt tell much of difference in power and I didn't have gauges then so I don't know about EGT's. But I love the deep, loud sound that it lets out. Sounds much better than the common 4" IMO.