[Madaaron06]

I want to redo the exhaust on my 85 poncho. I'm debating on a 3 inch single or true duals. Now many of you know that the boxys didn't come with a dual hump cross member. My thing is duals at my local shop were gonna cost me $600 with no cats $800 with. I've heard that a full 3" will flow as well as duals. Also i'm hoping that it would be cheaper. I do play on dropping a stroker in the car eventually. Currently it has a 350 with stock manifolds. Also I live in florida and there is no emissions. Should I run a cat or no cat. If i run no cat will i lose back pressure?

 

[MrBuick (Magnum)]

You don't need back pressure. This is a common myth, that seems it won't go away. The entire point of headers, and a better exhaust, is to reduce back pressure.

Duals, are not terribly hard to do. All you need to do, is hack out a section of the cross member, nad weld a plate on top. If you plan on doing a bigger motor down the line, I would spring for the 2.5" duals now. A single 3" wouldn't be enough for it, if you ask me.

 

[SEOK]

^^I agree, if you get a single 3 now, your just going to end up redoing it all over again when you drop the motor in.

 

[Bigdawg70]

That sounds expensive what kind of mufflers and tailpipes are the goin to use. I would wait and get headers before you do the exhaust if not you will be paying more to have them cut and make it fit

 

[350]

Single 3" is far from restrictive on my 6.0 LS2. You don't have to go duals, but it's cooler to say it has true duals.

 

[Marky Dissod]

Once the available flow exceeds about 2.2 cfm per hp, the gains possible by increasing muffler capacity drop to less than 1 percent.

… 2.2 cfm per horsepower …

A 2.75” Single is good for a 310hp engine with zero loss …
A 3.00” Single is good for a 370hp engine with zero loss …
A 3.50” Single is good for a 503hp engine with zero loss …
A 4.00” Single is good for a 657hp engine with zero loss …

However, each side of a Dual system only needs to flow 1.1cfm per horse, so …

A 2.25” Dual is good for a 457hp engine with zero loss …
A 2.50” Dual is good for a 513hp engine with zero loss …
A 3.00” Dual is good for a 812hp engine with zero loss …

Regarding the OP's question … 3.50" Single or 2.50" Dual.

Oh, by the way …

The first misconception that needs to be cleared up is that a header relieves backpressure, but a certain amount of backpressure is needed for optimum performance.

Just the opposite is true.

A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system. When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder. This is what the term "Scavenging" means.

 

[Madaaron06]

thanks for the info. Looks like duals is the way to go. The whole motor swap won't be for a while. Car is daily driven and i dont feel like messing with headers right now. The car has all the smog stuff plus whatever else 80's bs is on the motor. When the 305 was swapped everything was swapped on the 350. My next question is should i go cats or no cats?? i plan on running spintech prostreets and almuni piping also debating on going with a tubular crossmember.

 

[CrystalSS]

thanks for the info. Looks like duals is the way to go. The whole motor swap won't be for a while. Car is daily driven and i dont feel like messing with headers right now. The car has all the smog stuff plus whatever else 80's bs is on the motor. When the 305 was swapped everything was swapped on the 350. My next question is should i go cats or no cats?? i plan on running spintech prostreets and almuni piping also debating on going with a tubular crossmember.

Or you could use a system like the Turbo regal/monteSS its a single that goes into a dual after the crossmember. But if your shop is not up to the task of fabbing a system like this it may cost the same???

http://www.torquetechexh.com/Web Photos/GM G C11.jpg