If I don't have money to spent on a hight flow cat,Can I install a replacement cat bigger with big inlet-outlet or smaller with big inlet-outlet.....the muffler is a flowmaster 80..

I have a 2,5 pulley,.180 thermo,and KN cone fillter

High-flow cats are cheaper than standard replacement cats. Keep the same inlet and outlet, and get a hi-flow. They have a larger grid area (by about 10%) over OEM.

www.justmagnaflow.com

What about getting rid of the cat? like welding a straight tube in its place. Will that be good or bad for performance? In a N/A & in a */C bonne.

You may be able to dump the cat living in PR, but I wouldn't. I ran for a short time on my 95 SLE (supercharged) without one (due to it clogging and rattling, and the budget situation at the time). I lost a good amount of low-end torque. And gained very little top end.

99% of the cars on this Forum can't outflow a hi-flow cat. Which is why I put a new one on my SSEi last week. And my car flows an incredible amount of exhaust with a 2.0" pulley on it.

If I buy a cat 10% bigger,it'* not the same result .If a hight flow cat is 10% more free??

10% bigger? Not sure what application you're talking about, but alot of members here go with the Magnaflow hi-flow cats on that site. 10% bigger in outside dimensions may mean nothing when you look at the internal design of the cat, or the dimensions/spacing of the heat shield.

I suggest you price out both options. See how it comes out.

I'm just talking about a cat from a big pick-up,I just want a comparison in cfm,big inlet-outlet and more surface of filtration ....like the theorie for the air filter

I can't provide you with flow numbers, but I'd imagine going too big would net the same result as just race-piping it. Considering that, my advice to you would be to go with what we know from our experiences on this Forum.

Many members with your generation have gone with the Magnaflow Cat, and I'm sure someone will chime in with a part number, source, and price shortly.

Is't not like air filter bigger is better?I have a pickup with 7.5 liter,I suppose thet the cats flow very well to exhaust this engine

Read what I posted above. I would venture a pretty good guess that too big is the same as straight-piping it, which we KNOW kills low-end torque. So no, I wouldn't think bigger is better.