1990 C-1500 2WD, 350. The cat is rusting out so I bought a "hi-performance" cat from Summit. Once under there I see it's time to replace the whole thing.
3" crossover pipe into a 3" cat, necked down to 2.5" into the muffler and out the back. The new pipe is 3" up to the muffler with two 2" tailpipes out the back. It's mostly noticeable when towing something, but I'm hoping for a throaty rumble and a little more ass.
You'll get a lot more rumble for sure, but I don't know if a stock '90 small block is restricted a whole lot by a single 2.5. They aren't moving a ton of air if it's still got the 2bbl TBI. A single 2.5" tube has an area of 4.9 sq in, and dual 2" tubes only give you 6.28sqin, so you're only gaining about 25% capacity.
Single in/dual out mufflers make me sad.
Take a look at a TBI head intake port. It can make more sound, but it won't make more power. Google "TBI Swirl port heads" and see that they covered half the intake valve.
You will probably gain some EMPEEGEES simply because your cats are in terrible shape. At this point I bet they are half clogged and reducing your exhaust pass through to a straw. And yes, while it does seem silly to have a larger exhaust than the intake, exhaust gases are hotter and are still busy expanding as they go through the tubing to exist the truck. A bigger diameter exhaust will probably help.
I'm running a flow thru Cat (3"in/out autozone) with a Gibson 3" cat back (1991 chev 1/2ton 350/5spd.extcab). Picked up at least 1 emmpeegee....the voice is excellent.
914 love the signature.....definitely can relate