I have not tried that setup, but I have experimented with mine alot, anyone here can tell you that. Yes you need backpressure, but less than stock. I think that going to a single 3" pipe off the y-pipe into a muffler like you are talking about, then a single 3" out will be good. I expect some HP / Tq gains as well as some milage. Bigger than that you will start to lose milage. When I first did mine, my milage dropped by 2 mpg, but I did true dual 2.5, with an x-pipe (way overkill for these engines). SInce then I have been adding reducers. Now I have the same setup with the ens narrowing to 1.75" a piece. I think a tighter tube all the way would be better. So, I think you have a good setup idea, and would love to hear the results as that is what i think would be really good. The only change I owuld make would be dual exhaust out of the muffer or a reverse Y and dual pipes. I just like that look better.

 

smoke actually i'm assuming there are still two pipes to the muffler. I'm leaving those stock, installing a big block flowmaster with one 3" out the side where the factory pipe is now. The reverse of what your talking about. Did that to a 86 with a 305, but only had a single2" out, but had the smaller muffler. Sounded very loud, but did the job. I just don't want to open it up too much

 

I also have an 04 andI only had the one pipe running into the muffler. But the cats come after the headers and befor the Y so i would imagin that you could have it changed to the two pipes running into the muffler. And it dose sound like one hell of an idea if you are able to get the two pipes ran down.

 

I can see gaining mpg, but not so much around here. I could afford too loose SOME backpressure, but not alot of it. I know that the hills around here (with too much of a loss of backpressure) would cause more downshifting and therefore, higher rpms and less mpg. BUT, when I was down South with my truck I could drive it all day and it would never downshift unless passing or starting from a stop. And let me tell you that was soooo freakin nice for a change! All the roads were flat and open and (for the most part) the South has fewer red lights and stop signs per square mile compared to where I live. SO....I can see that a considerable loss in back pressure would not hurt and in some cases help mpg, unless you stomped the gas all the time. Around here, even in the country side it is up, down, up, down, stop, go, stop go, it BLOWS!

 

hey man you said you lost power and fuel economy... sounds like you might have an exhaust leak somewhere. the custom exhaust should boost both power and fuel economy. you should check for a leaking manifold or somewhere in your pipe