I want to open up my exhaust I want to do it myself but I dont have a welder so can leave the down pipe from the engine to the cat the same I think it is 1 3/4 in. and from the cat back 2 1/4 in. would leaving 1 3/4 down pipe defeat the whole purpose of opening the exhaust up?

 

Not at all, one of the first mods anyone does on any vehicle is a "cat-back" after-market exhaust, and most all vehicles see at least some gain, be it mpg, more power, and/or cool sound!
I did a custom cat-back on my '93 extra-cab v6/5spd with a Flowmaster 40 series muffler and custom bent 2 1/4"tailpipe that fits and looks just like factory.
If you aren't going to weld I am not sure how you plan on fitting up a bigger pipe, you will have to find a flange for the cat to mate up but with a bigger diameter pipe than stock and I never could find a tailpipe from any auto parts store that would fit the factory contours and use factory mounting points and was a bigger diameter.
It definitely depends on what kinda look you want, I wanted it to have all the benefits and appear stock.
I have noticed a small (2-3mpg) gain in mileage, increased power (doesn't lug uphill, no more downshifting), and it has a real nice rumble sound to it.
Good luck and have fun!

 

you can usually find apropriate adapters, etc, and use clamps to hold everything together. I would still recommend getting it welded after you get it all together though. Call around to a few local muffler shops and ask them if they'll weld it for ya. Oh and muffler shops are a good place to get the pipe too as most autoparts stores i've been to only cary short (1ft) pieces, and direct fit replacements for stock.

ps. I had a friend with a rusty '77 cutlass that had his exhaust muffler clamped (he liked to change it around about once a week lol) and one time he took it to a car show, parked it in the grass, and had the whole exhaust fall off because it had melted through the rubber muffler hangers lol. It's always nice to have your exhaust system fall off while showing the car at a carshow!

 

It's not going to defeat the purpose of a better cat-back exhaust, it just won't give you as much in gains as an aftermarket header/secondary would.

 

Does anyone know of a aftermarket Yota company that makes a short pipe with the O2 Flange on it for the +- '91 after Cat. O2 Sensor ??????
Preferably in 2"-2 1/4" .

 

it's not hard to just buy a bung (most muffler shops sell 'em) and have someone weld a bung onto the size pipe you want.

 

Cool , wasn't sure if it was available like that , welding is not a problem , have access to exhaust shop .