I hear a lot here and other 3800 places that a 3" free flow exhaust is needed when you start going down on pulley size and then at some point headers are the only way. What I do not understand well is why the 3" number using the stock manifolds. I remember measuring the opening on mine and it was about 2.5". Maybe can be opened a little bit but I'm not sure it will go to 3". So how do you accomplish that? You just plug a 3" pipe and then go down to the muffler? Could you show some examples of your setups using stock manifolds (not interested in headers as it is obvious)? Tks.

What I did...
https://www.fiero.nl/forum/Forum2/HTML/115264.html

I do not think you will get all the bennifits of 3" exhaust when using the stock manifolds; as you pointed out, they have restrictions...

quote Originally posted by ALJR: I do not think you will get all the bennifits of 3" exhaust when using the stock manifolds; as you pointed out, they have restrictions...

Even though the stock rear manifold dump isn't a true 3" internal diameter, you will want to consider that any bend you install in an exhaust system downstream of the manifold is a potential restriction. Since it is impossible to run an exhaust system in a Fiero without bends, you are going to want to use the biggest diameter pipe and mandrel bends you can (within reason).

I can attest that 3" mandrel bent exhaust on a 3800 SC using stock manifolds outperforms the same engine (and stock manifolds) using an exhaust system built out of 2.5" mandrel bent pipe. That's at least TO the muffler. If you are running a single-in, dual-out muffler, then the outlet pipe size can be smaller since you are dividing flow between two outlets.









-ryan

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quote Originally posted by Darth Fiero: Even though the stock rear manifold dump isn't a true 3" internal diameter, you will want to consider that any bend you install in an exhaust system downstream of the manifold is a potential restriction. ... -ryan

I agree with that. The 3" will help that tight bend. I want to do this soon. I'll fix my wires and test again. I'm getting 2-4° of KR only on heavy load WOT around 3.5-4.5k rpm almost all in 2nd (don't have road to get to high rpm in 3rd!). Few times gets to 5-6° briskly if I do the WOTs consecutively.

WCF Typical 3"

quote Originally posted by MountainHiBlue87GT: WCF Typical 3"

Does WCF now make a 3" Y-pipe for their 3800 headers? All the ones I've seen are 2.5" at the outlet after the two pipes merge into one on the Y-pipe.

Here is the simple exhaust I made for my old 88GT L67

Flowtech muffler and flex coupling from Amazon.com and 3" bends from Advanced Auto Parts.