downpipe question

[davidstereo]

I recently have gotten some money to put into my truck and was wondering a few things? The downpipe is still the stock 2.25" but the exhaust after the downpipe is 3". I want to get gauges now before I do anything else, but need to know if I should replace that downpipe, as they will be in there anyway, also with a new three inch one. Also should I get a bigger exhaust? I gues what I need to know is, what are the performance differences between 3" downpipe and 2.25", and differences between 3" pipe, after downpipe, and 4" or 5" pipe, after downpipe? Thanks, David

[Dvldog8793]

Howdy
A 3 inch downpipe is the largest that will fit, size is not the main issue with the downpipe. The real problem is the dents that GM puts in the pipe. It might be to control the back pressure or who knows??? BUT some downpipes are REALLY mangled in the factory, others are not so bad.
Sooo, YES replace the downpipe. If you go to a 3" you might need to push in or modify the firewall and floor pan to get clearance. try to get as much room aroun d that pipe as you can. If it touches anything you will hear it!

The rest of the exhuast will really shine at 4inch, I've heard that anything above that is overkill, but I'm not sure how SCIENTIFIC that reasoning is....
Unless you don't like loud trucks then don't waiste money on a muffler...
My truck has a 3.5" open exhaust and it was the most noticeable performance modification that I did. And it sounds like it!!!

L8r
Conley

[DennisG01]

Here's a pic of my stock downpipe. I replaced it with a 3" madrel bent pipe - no clearance issues on my '98. The lower stock pipe in the picture is crimped where the pipe goes past (I think) the rear heater or AC lines for my Suburban. The new exhaust pipe has NO crimps anywhere in it and there were NO clearance issues anywhere - I replaced the whole exhaust system. The system I bought is a 4" one - steps up from 3" to 3.5" to 4".

http://www.thedieselpageforums.com/photopost/data/519/medium/downpipe.JPG

[JeffsTowTruck]

I did the 3" exhaust a few years ago. I have never done the down pipe yet. I want to soon. Also I'm going to do away with the cat. You did not say if you had one. The combination of the cat and down pipe at one time will make a diff. for me I know. I have not done the gauge thing yet but want to do that also. The better the air comes in and leaves will help, on any engine.. !
The biggest thing I noticed so far was reworking the comp. and the intercooler.
I'm trying to do suttle things to leave the engine stock but help my power. I pull a little too much weight and need all the help I can get. I did not want to harm the engine or my wallet!!

[davidstereo]

So I guess that I will put the three inch downpipe and the four inch exhaust no cat and no muffler for the best performance. And there goes the money I had to put the gauges in. Oh well I guess i'll do the pyro now and wait on the boost until later. Gotta love credit card debt. Thanks, David

[Dvldog8793]

Howdy
The Exhuast kits are nice, but not the only option. There are people that have taken the used factory systems from Fords or Dodges and made them work just fine. It really depends on your ability and how much messing around you want to do. I am a welder/fabricator but didn't really want to spend the time so I spent the money instead! Allot can be gained just by eliminating the factory "obstructions" and keeping the rest of the factory system with the exception of the downpipe. That needs to be replaced, even a factory "un-squashed" pipe will be better than what is normally found.

I would put gauges first before any major mods as it is REAL nice to know what it going on in the engine bay!
L8r
Conley

[6.5 Detroit Diesel]

Replacing the stock downpipe is the way to go. When I pulled mine, it was mangled beyond belief. As I was already building my exhaust, I was able to remove the down pipe fairly easily. Just had to reach around the turbo and get the belly clamp off. Then a bit of wiggling and it came out and the new one went back in. Added that to the 5" exhaust and yes, it is really loud, and you can hear the turbo winding up really easy. :D It sounds like a gasser at top rpms. You can fit the 5" in, it will just fit under the box. I put mine out the side in front of the tires, as there is no lift, and getting a bend in the 5" inch was going to be very co$tly.