I found a smoking deal on 4" stainless steel exhaust and muffler for my truck. I am running original 3" pipe with muffler and cat removed, so straight threw. Would it make any difference on performance or fuel mileage to go to this new system?

 

I really doubt it. Stock exhaust is ok to maybe 350 hp.
bigger exhaust can reduce spool-up. Velocity is a key here and the biggest restriction is the turbo exhaust housing.

 

Performance - NO!
Sound - Yes
And... with a 7 yr old exhaust and a "smoking deal".... it might be a good time to replace rusty parts and upgrade?
I have never subscribed to the reduced spoolup theory...but with a factory turbo... it's no biggy either way.

RJ

 

I don't understand! Why would the exhaust companies tell us that we'll gain 25hp, 50 ft-lbs of torque, and 4 mpg? They wouldn't intentionally mislead us, would they?

On that note, there are some really good deals on SS exhaust out there right now. I just ordered a 4" Magnaflow Pro Series SS system for $330 delivered off Ebay.

 

You would get that much with conjunction of other mods, air intake, chip, etc. It also has to be quality exhaust and same diameter all the way through, some of these exhausts advertise 4" but downpipe and muffler can be as narrow as 3"
Definitely go with bigger exhaust as it will be better in long run when you do other upgrades, it will also lower your EGT's.

 

Sorry, I was being a bit sarcastic, but there isn't a 'wink' smiley.

I got tired of my straight-piped stock exhaust. Too loud for me. I'm going with the 4-inch to make room for future improvements. It's cheap, and it certainly can't hurt!

 

I didn't notice a thing going from a straight pipe/stock to 4" turbo back. Mine rusted out where it goes over the diff, so I replaced it with the 4".

 

I like my ebay 5" exhaust. Its quieter than my old 4" from Summit Racing. I didn't notice any reduction in EGT's with either exhaust. In my experience with turbo cars a larger exhaust is always better. The whole "you need a little back pressure" argument that kind of works with naturally aspirated engines goes out the window with a turbo because the turbo itself creates a ton of back pressure. Your main objective is to make it as easy as possible for the exhaust to get out of the turbo and a bigger exhaust should do that.

John

 

I have done just the 4" on a number of 12 valve trucks and YES it will make a little difference going to the 4" and getting away from the 3 or 3 1/2 stock grap. I even had a 91 that I took the complete tail pipe off of my 03 truck and used and it along with a 4" straight pip and it helped it. You will get more by improving your air intake!

 

Its all bout flow:
1) bigger exhaust on a turbo vehicle is better due to more flow
2) the more air moviing in or out of the motor the better (thats y we buy turbos, intakes, exhaust manifolds and etc)
3) better air flow means more or the potential for more hp (which is where manufacturers get their ratings because their trucks may already have the simple bolt-ons like an intake or a programmer that typical consumer already has on their vehicle
4) more hp (especially with diesels) the less the engine has to work
5) the less the engine has to work the more mpg