When I did all this over the summer I went with race headers from hooker. I also did the the trual duals before the axl and did 2 1/2 pipes. In my expirence do it right, do it once. If you want to do true duals go full out. Due the headers and get them coated. Then use this as an excuse to do a full tune up, wires, plugs, O2 sensors, and maybe a 160 T-stat. Now if you don't have the funds to do this, im not saying don't do it i'm just saying when you want more later you have to go through the whole process again.

From what I have heard 3" is for big HP application and 2 1/2 will flow just fine for stock or near stock engines. I may be wrong but thats what I always went by.

Hope it helps, and good luck with your decision.
~Jim~

 

so if he has dual 3 inch your telling me he will have just as much torque as dual 2 1/2?

i know the manifolds are are gonna provide the back pressure, but what happens if he goes longtubes later on?

maybe i've been wrong, but i have always thought that u have to have some backpressure to keep the torque,

why would he not need 2 1/2? that supports some pretty big power.

 

with longtubes and 3" true duals...... would 2 1/2" duals be better then 3" or would it make nearly no difference?

 

this just depends on ur plans for the rest of the car...if you dont plan on doing MAJOR work (internal engine work/cam etc) then just go 2.5"....but if your gonna go major to it...go 3"


By the way...i read above that backpressure isnt needed...i believe that to an extent...but i think the LT1 does need a slight bit of backpressure...

Cory

 

No car needs backpressure. backpressure is always bad. You will lose some torque from the larger pipes though due to less scavenging because of the now slower moving and quicker cooling air.


edit: go with 2.5" man.

 

spend money on headers first!!!!!!!!!1

 

Most people dont go true duals. The ones who do usually are pretty big into modding the car. If he plans to do a big heads and cam, or N20, or Supercharger, something along those lines, then 3" would be nice.

I really dont think there would be that big of a differebce between the 2.5" and 3" though, unrtil you start getting into some pretty wild set ups. But as far as that goes, I dont think you will see that big of a difference in low end TQ either between two 2.5" pipes and two 3" pipes.

You probably should go with dual 2.5's though ...

 

i am planning on eventually getting my heads and intake ported and going with a bigger and better cam......so this 3" dual system i am aiming to buy is appropiate? btw i am not gonna install the true duals until i have my coated longtubes thats just the way i roll

 

You might notice some loss in very low RPM TQ, but for everyday driving, your not gonna see it.

I say go for it.

But you wouldnt be going wrong with true 2.5's either.

In all honesty, even with heads and cam, using the same mufflers, your looking at probably a 2-3 HP peak difference.

The more I think about it, inless you are going FI, I would go with the 2.5's

 

Thought I'd chime in.
3" duals do not kill torque. I would think the small manifolds would kill tq more than anything else. My '94 is lowered with 3" true duals and I have NO clearance issues. It even had cats on it but I ended up gutting them.Depends on who fabs it up and how much they care.
BTW I did mine in stainless with the X-pipe and bullet mufflers and it cost me about $200 in parts. I put it up myself. Took awhile but I'm confident I got it routed better and tighter with no rubs or anything better than anyone could have done.
Cats will quiet the car down, but if you get them because of backpressure concerns, get GOOD ones. Turns out I didn't need them at all. My car is very quiet for a stroked car dumping in front of the axles(until you step on it).
In all honesty, you dont really need to go 3" until you get close to the 400HP mark IMO... But it's nice to have so it's one less thing to worry about it. Gains over my previos longtube/borla setup was amazing.
Oh, and get the headers. I noticed the step up from manifolds to shorties was about the same as the step up from shorties to longtubes. It all depends on how much you wanna spend and how fast you wanna go, really.

 

my cousin has a 73 montycarlo with a 350 it had 450 HP before he rebuilt it he has 2.5"in. pipes it is the loudest street car I'v ever heard but it is dumped before the axle. I also have a buddy w/ a 2000 mustang GT he has 420 HP and 3"in. exhaust it is ungodly loud and sounds like total [ edit - do not override the language filter ] he did have 2.5"in. it sounded good but when he went to 3"in. it made no diff in HP and now it sounds bad. Last but not least another buddy of mine has a 89 5.0 mustang and he is faster than any one, and I would say a stock LT1 w/ a cheep cat back is way louder than he is, but he is running mid 12.s in the 1/4
So if I was you I would go with 2.5"in. it sounds way better and who ever puts the exhaust on will like it a lot better than 3"

 

Thought I would chime in here. 3" don't "kill" torque they move it higher in the powerband. So in reality you don't lose total torque at all; in fact you may gain more total with 3" than with 2.5" but probably not much. The difference is that you will not make as much torque through the rpm range (up to ~4-5k) as you would with 2.5". The often untalked about aspect.

As stated scavenging makes a huge difference in terms of torque production in exhaust systems. The other often unspoken term is "venturi" effect which is effectively what causes scavenging. Smaller pipes also promote less turbulence and provide for a better "swirling" of the gases which pulls them from the exhaust ports. In all reality a 1 5/8 header long tube would give our cars much better street performance vs. 1 3/4. The thing is that 1 5/8 will become a restriction at some point but it will be rather high in the rpm's. From what I know 1 5/8 make better average tq/hp than 1 3/4 but 1 3/4 make more topend effectively perpetuating the "I gained 30 hp from headers alone" statements. There are not many people talking about average hp/tq and let me tell ya fella's that is what it is all about- especially in a street car.

Now what to do with your car? If you are staying with the stock manifolds
3" would most certainly be overkill. There is no doubt in my mind. I know I guy who swapped out his y-pipe for only a borla 2 3/4 y and his lowend was definately not as peppy as before and he had more difficulty launching(have more examples but you get the point). Many people go for the "bigger is better" in exhaust because "someone gained xx hp" doing this. If you are getting ported heads and a nasty cam that will move a lot of air the 1 3/4 will definately be the way to go as the engine will be moving much more air than a stockish car. As stated heads/cam change the picture and as such you would certainly upgrade the headers. I think 2.5" duals would be more than adequate to handle heads/cam with longtubes later on if you go that route. stick with 2.5"

In my opinion exhaust is like cam choice. Some go for a cam that gives them 2 hp more than a smaller car that gives better avg. hope this helps

RRR

 

so 2.5" duals r better then 3"?

 

For the minimum mods you have done, YES.

 

like some others have said...if you want all out top end power= 3"...if you want more usable street power=2.5"

i ditched the cat on my car and there was a noticable difference in lowend power...now being that i don't race my car all the time and top-end power is not my top priority i miss that low end...it was most noticable when i have the cut-out uncovered...originally when i had the cut-out uncovered w/the cat still in place it still had a decent bit of tq...i keep the cut out covered now and the car still has a good bit of driveable tq...

also..the stock exhaust i believe is 2 3/4"...and that is total diameter...if you went to dual 3" pipes that obviously more than doubles the size of the exhaust... bigger the pipe=less low end power...not to be confused with less top end power...the more the motor can flow the more top end power it will make..