Can anyone help me figure out what is needed to be done in order to gut the cats? I am putting Hedman Hedders on my 91 GTA 5.7L and am going to run true dual exhaust.

Another question, would it be better to run cats or just go w/o the cats, I am in Oklahoma and we no longer have emissions tests.

Thanks for the comments.

John

 

well if ur going with true duals it sounds easier to do away with the cats one less tihng to worry about, and since u dont even get tested then thats another plus.

i would just run the headers and worry about getting the true duals with out any cats under the car nicely.

 

I have been told that our cars have to have the cats in order to run right, because w/o them there will not be enough backpressure getting to the motor.

the only problem i have w/ running without a cat is that there is noone around that will do the exhaust w/o the cat since it already has one on it.

 

LOL, go to Autozone, rent the pipe cutter and hack them off. then drive it to the muff shop and tell them what you want done. hell, just to be safe you could hack off the whole system. that way they would never know.

just tell them you bought it like that


You don't need cats. it'll run way better w/o them then with gutted ones. a gutted cat dosen't flow very good. I've heard that because of the shape a small vortex forms in the center and can kill like 30% of the flow on a gutted cat.


just make sure you pinch off the air tube that runs to the cat.


Dave

 

Hi fellow oklahoman, what part are you in? I'm running without a cat and my car runs fine... sounds alot better then the gutted cat it had before also...

 

I am in Broken Arrow. Did you put your exhaust on yourself or did you have a shop do it for you? All of the shops around here say they can't do it because the law still requires vehicles to have them even though there is no emissions tests.

 

Run with gutted cats. All you need to do is cut the cat off, gut it, then push a 2 1/4 inch pipe through it, and then install it. This way you have the appearance of a cat on the outside and nothing but free flowing exhaust on the inside. Do a search on the subject, it has been beat to death.

 

I'm in Midwest City. I had a shop put my exhaust on since it was all original/rusted/corroded together. He kept the cat, but later I went to a shop I know will do things like this and had a piece of pipe put in place of my cat...

 

How does the car run w/o the cats? i have been told that these cars need the backpressure that is put out by the cats, is this true? the guy that told me this, said that the car will not run right if the cats are removed.

john

 

you don't need cats to run anything....ever....did care back in the day not run right because of a lack of cats? nope....also back then you could walk into a parts store and buy a "test pipe" to check to see if you're cat was clogged. So many people bought test pipes....wouldn't you know it....eventually there were some really rusted out "test pipes" on cars.

good luck and ditch that cat...

 

Drop the CAT down at the rear bolts and take a solid pry bar or something long, sharp, and thick and run it through and just suck all the broken pieces out with a vacuum or pull them out with your hands.

I did it to mine and I'm an Emissions Inspector! hehehe

 

My car runs just fine without it, and sounds alot meaner with the straight pipe versus gutted cat.

 

Backpressure - call me stupid, but isn't backpressure a BAD thing? Wouldn't it be better to reduce backpressure, to improve exhaust gas scavenging? I would think that backpressure makes the engine work harder to force the exhaust out of teh engine and through the exhaust system, thus (if my logic is correct) if you reduce backpressure, you reduce useless load on the engine, right?

 

That is the same thing that I thought. I asked my dad about it and he said if what i was told about GM products needing the backpressure was true, then why is his truck running better w/o the cat than it was before.

I think I am going to run w/o cats if i can find someone that will put the exhaust on for me and not hassle me about it being against the law to take the cats off.

 

--straight from a GM tech-----

It's not backpressure that can make or break HP curves in performance, it's exhaust velosity. You want the smoothest flowing exhaust possible however using too big of pipes will cause the scavengeing to be too much. you want to ge the exhaust away from the engine as quick as possible. too small of pipes will cause a restriction and overpressure the exhaust system, too big of pipes and the exhaust will move as a slow rate of speed. you need to choose the propper diameter for your performance level. ie: a 500HP BBC would run best with 3" duals, however a 350HP SBC would probably run best with 2.5" duals.

It's all deturmined by the setup of the pipes, headers and your engine build.

I Put 3" true duals on a stock '68 327 in a 79 regal. it ran like dog sh*t. then i swapped out the 3" for 2.25" and it ran way better.

Of course, anything over 500 or 600HP would probably run best un-capped

 

It's like this... GM and other car manufacturers say a car "runs better w/ CATs" meaning it runs CLEANER. But as far as we are concerned and as far as PERFORMANCE is concerned, flow is more important and the less resistance you have (ie. CATs, restrictive exhaust pipes) the more potential you car has to breathe allowing for it to have a little more giddy-up

 

Good, Hawaii has no smog-check (even though you can always find smog-test wonder juice at Wal-Mart...) so I'm looking to get duals w/o cats.

 

If your worried about not having the cat "appearance", couldn't you take the cat off, cut it in half, rip everything out, and weld in a pipe where the cat was, then weld the cat shells back on. Could that be done? Just a suggestion.

 

When I hollowed out my cat my car seems to lose power . I would'nt say it sounded too great either . I was told the same thing that purple82 mentioned , the bigger open chamber causes turbulance makeing velosity slow down . You want the exaust to exit as fast as possible so " scavageing " occurs . When the right amount of air is confined just enough it tends to move much faster . That also helps to pull the the intake charge through .

A friend of mine trimmed the extra pipe off my cat and streched it out just enough to put a 3" piece through and welded it . A few coats of aluminized paint and it blends in perfect . So far I've had 2 cops look under my car and I have'nt gotten a single ticket .