Do I need a downpipe with a 4" exhaust system?

[Erwin8r]

Ok, I tried this in the general section, but it was probably the wrong place for it. I want to get a "Turbo Back" 4" exhaust system, and am wondering if it is worth the hassle to use a 4" down pipe (or a 3.5" aftermarket) instead of just keeping my stock down pipe and welding the 4" system to it. What do you all think? I understand that the stock one is 3.5", so what would the advantage of a, say MBRP 3.5" down pipe be? I want to lower my EGT's, and of course, maybe get a small bump in ponies. Thanks!

[Karls03]

My OEM down pipe was 3 inches, so the 3.5 did help. A 4 inch will work, but it's mucho tighto in there when you try going 4 inches.

[Erwin8r]

Originally Posted by Karls03

My OEM down pipe was 3 inches, so the 3.5 did help. A 4 inch will work, but it's mucho tighto in there when you try going 4 inches.

Did your 4" system come with the 3.5" down pipe? What model did you go with? Thanks!

[bob harris]

I BOUGHT MBRP 4 INCH PERFORMANCE SYSTEM BUT THE 4 INCH DOWNPIPE WAS FOULING ON GEARBOX. FITTED THE ORIGINAL 31/2 DOWNPIPE THEN INTO THE 4 INCH PIPES AND IT WORKS JUST FINE

[Erwin8r]

Originally Posted by bob harris

I BOUGHT MBRP 4 INCH PERFORMANCE SYSTEM BUT THE 4 INCH DOWNPIPE WAS FOULING ON GEARBOX. FITTED THE ORIGINAL 31/2 DOWNPIPE THEN INTO THE 4 INCH PIPES AND IT WORKS JUST FINE

Thanks for the info. So I guess my next question is, do some trucks have 3" vs. 3.5" down pipes stock?

[snafu!]

I got flowmaster and it came with a 4" down pipe, didn't really have any problems sliding it up to the turbo had to turn it just right and it went right up there. Kind of like a puzzle. All truck are a little different though, I've heard of people having trouble with the 3.5" down pipe.

The only reason I got flowmaster is because mbrp was 3 to 4 weeks out when I ordered mine and I'm impatient. Lol

[xjeeper]

I have run all 3 ways (stock, 4" flowmaster, and now 3.5" mbrp), I felt that the stock downpipe spooled better in normal driving, the 3.5" is good all around, and the 4" was a pain to fit in the truck.

[Karls03]

If I'm not mistaken, exhaust tubing is measured on inside diameter. All I know is that I checked the box for a 3.5 MRBP down tube and it was larger than the OEM tube on my 2003. I already had a 4" Dynomax cat back and I connected the 2. A friend has a 4 inch on his 03 and it's a tight fit.

Why did I buy exhaust twice? Because we miss measured and thought that the down tube was 3.5 inches. Turns out it was 3 inches into the resonator, then 3.5 from cat to tail pipe.

[Erwin8r]

Sounds like A 4" MBRP with a 3.5" down pipe is the way to go; and maybe the earlier 6.0's had less space to work with?

[JIM_C]

You definitely want to get rid of the stock downpipe on an '03 6.0 as they are not mandrel bent. Meaning it's a crimped up POS where it turns to go to the cat.