Is true dual exhaust a good idea for a 3800SC? (Page 2/4)
dobey SEP 12, 01:54 PM

quote
Originally posted by phonedawgz:

+1 on running just a single O2 sensor. Not the optimum way to do it but it will work fine.



If I was going to run dual exhaust on a PCM that only supported one, I'd make a circuit to use two, and average the signal so it looks like one, but both banks impact the reading in a similar way to having one O2 just after the Y before the cat in the factory setup.
fieroaddicted SEP 12, 02:18 PM
the mustang header i am using, came with a place to mount a second o2 sensor. I bought a gauge to monitor it from the dash. As for the muffler, I am going to be running the same 355 muffler. My trunk is cut already, so i am hoping i can make it work without a whole lot of trouble. I would like to use the third port also, with a vacuum operated butterfly mounted in a similar fashion to the ferrari setup.
troy
Darth Fiero SEP 12, 02:31 PM
The single upstream (of the cat) o2 sensor the 3800 PCMs use only really needs to see one bank of (3) cylidners. Is it better if it can monitor both banks (ie: all 6 cylinders)? Sure. But it will work fine only monitoring 3, and I certainly wouldn't try to combine two separate o2 sensor signals to feed a single o2 sensor input to the PCM. Doing that would be asking for trouble.

That having been said, I'm not sure if running true dual exhaust on a 3800 is going to make it sound any better. However, I can't say that I've ever heard one so the jury is still out on that. But you should be aware that keeping the two exhaust banks completely separate with no balance pipe or merge will probably cost you some power, at least under the curve (usually more prevalent in the mid RPM range).

-ryan

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[This message has been edited by Darth Fiero (edited 09-12-2014).]

dobey SEP 12, 02:37 PM

quote
Originally posted by fieroaddicted:

the mustang header i am using, came with a place to mount a second o2 sensor. I bought a gauge to monitor it from the dash. As for the muffler, I am going to be running the same 355 muffler. My trunk is cut already, so i am hoping i can make it work without a whole lot of trouble. I would like to use the third port also, with a vacuum operated butterfly mounted in a similar fashion to the ferrari setup.
troy



You guys realize it won't sound anything like a Ferrari by just copying what the Ferrari exhaust setup is like, with a 3800, right?

I think the best option would probably be a Y-pipe exhaust with a single cat, and after the cat, a custom collector is made to balance the output between the 3 ports. A standard Y for the two lower muffler inputs for normal exhaust flow, and a third port with an electronic (or vacuum if you really want) controlled butterfly valve, that goes straight up to the third input on the muffler. That should give the best balance between the 3800 and what the muffler was designed for, and should allow for a reasonable exhaust system in the somewhat confined space of the Fiero.
fieroguru SEP 12, 03:26 PM
I would first understand how the 3 pipes flow through the muffler.

From their location, I would assume the center top inlet goes through the muffler portion (lots of room between inlet and outlets). The two lower inlets in the corners are right by the outlet pipes and likely just dump right to the tail pipe cavity, which would be the same as running w/o a muffler. If those are the only two you hook up, you might find the exhaust to be super loud. This also makes sense for a high flow exhaust with valves - you need a small amount of flow to be highly muffled (center inlet) so it will be quiet at idle and cruise. At WOT you need a large amount of flow with very little muffling/restriction (dual corner inlets) for maximum power.

I would snake a light through the tail pipe section and look if you can see the light from those corner inlets. If you can, then those two corner inlets will likely do very little to quiet the exhaust down.
zmcdonal SEP 12, 03:42 PM

quote
Originally posted by Darth Fiero:

That having been said, I'm not sure if running true dual exhaust on a 3800 is going to make it sound any better. However, I can't say that I've ever heard one so the jury is still out on that. But you should be aware that keeping the two exhaust banks completely separate with no balance pipe or merge will probably cost you some power, at least under the curve (usually more prevalent in the mid RPM range).

-ryan



My idea behind having a true dual setup was more for ease of install and potentially take up less space. I didn't see a point in having it all sent into one collector only to split it up again for the dual inlets in to the muffler.


quote
Originally posted by dobey:


You guys realize it won't sound anything like a Ferrari by just copying what the Ferrari exhaust setup is like, with a 3800, right?
.



I am not necessarily going for the Ferrari sound, but my brother and I figured that exotics have the same issue that the Fiero has, muffling a lot of exhaust in a short area. So I thought it should do a pretty good job toning it down, and if it ends up sounding sort of exotic at the same time, that would be a plus too.


quote
Originally posted by fieroguru:

I would first understand how the 3 pipes flow through the muffler.

.


Surprisingly the top center hole of the muffler was the one that vacuum operated on the Ferrari. I was considering putting electric cut outs in there as well that would divert to the top hole if I wanted to make more noise

Here is a diagram of how it's setup on a 355

[This message has been edited by zmcdonal (edited 09-12-2014).]

dobey SEP 12, 04:09 PM

quote
Originally posted by fieroguru:
I would first understand how the 3 pipes flow through the muffler.

From their location, I would assume the center top inlet goes through the muffler portion (lots of room between inlet and outlets). The two lower inlets in the corners are right by the outlet pipes and likely just dump right to the tail pipe cavity, which would be the same as running w/o a muffler. If those are the only two you hook up, you might find the exhaust to be super loud. This also makes sense for a high flow exhaust with valves - you need a small amount of flow to be highly muffled (center inlet) so it will be quiet at idle and cruise. At WOT you need a large amount of flow with very little muffling/restriction (dual corner inlets) for maximum power.

I would snake a light through the tail pipe section and look if you can see the light from those corner inlets. If you can, then those two corner inlets will likely do very little to quiet the exhaust down.



All of the inputs have some muffling. The two lower inputs are 1-in and 1-out ports. The third upper input is a single in, dual-out, system. On the Ferrari, The two lower inputs on the muffler are from exhaust flow going through cats. The upper cat gets a Y from both headers, that bypasses the cats, and it splits into a Y internally in the muffler, to the additional tip on each side. I think the two lower inputs result in a pipe which goes upward into the muflfer, makes a 180, comes back down, and then goes out the tip. The F355 is not that loud at idle, really. You can hear it get louder when the butterfly opens though, and it only opens after about 3000 RPM.

https://www.youtube.com/wat...e&v=whznJ8Gi_SM#t=30
dobey SEP 12, 04:24 PM

quote
Originally posted by zmcdonal:
My idea behind having a true dual setup was more for ease of install and potentially take up less space. I didn't see a point in having it all sent into one collector only to split it up again for the dual inlets in to the muffler.

I am not necessarily going for the Ferrari sound, but my brother and I figured that exotics have the same issue that the Fiero has, muffling a lot of exhaust in a short area. So I thought it should do a pretty good job toning it down, and if it ends up sounding sort of exotic at the same time, that would be a plus too.



A true dual setup won't save space with a transverse engine (or in generally, really). Two 3" pipes that are 36" long is twice as much space as one takes. Then if you need to put dual cats in, those take up more space, as they are larger than 3" diameter. True dual in a Fiero will take up lots of space. This is why I mentioned it would be much better for a car with an Audi V8/Audi trans swap, for example, because that's longitudinal, so you can get away with a short true dual exhaust much easier there.

That said, the Fiero doesn't really have a problem muffling the exhaust. The stock exhaust routing provides plenty of exhaust length to get all the needed emissions equipment in, and muffle the sound. It's actually about the same length as the exhaust in a Firebird. The problem with swapped engines like the 3800, LS4, etc… is that nobody makes exhaust designed to fit the Fiero and those engines. The Ferrari V8 is a 5 valve DOHC engine, and makes peak HP at 8250 RPM. That's much higher than you're going to be running any stock or even mildly modified 3800 SC2 at. The F355 engine needs to be able to flow a LOT more air at that RPM.

I think the best way to get an exotic-ish sound out of that muffler with a 3800 in a Fiero, is to run a Y-piped exhaust that splits before the muffler. This really isn't any different than have a muffler with a single inlet, and dual outlets, like the stock Fiero 2.8 muffler. You're just choosing to use a dual inlet muffler.
fieroaddicted SEP 12, 06:12 PM
here is a pic of the cutaway i found
LFiero67 SEP 12, 06:20 PM
Tinytuner has the option to turn on the second o2 sensor, never tried it, but the switch is there.

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