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Well I know a ton of people have commented about this in the past, but when I did a search, I wasn't finding the information I wanted. I gutted my cats yesterday, so I thought I'd post my experience and give a bit of detail that I wish I had had before starting to make it easier.

First off, you'll have to remove the 6 nuts holding the downpipe to the car, and then the one smaller bolt from the exhaust flange. I got really lucky, my car doesn't have a million miles on it, and the previous owner literally never drove it when it rained. So my bolts were not stuck at all, but I have heard that some people have a devil of a time getting them off. At any rate, I put lithium grease on each stud before putting the bolts back on to make future removal easy.

After the downpipe is removed, you can then remove the front cat completely. The rear one I left bolted to the engine, like most people do. The tools to use are simple, an electric drill, 2 drill bits, a 1 inch flat wood bit, and I also used a 3/8 inch metal bit to punch a few holes through the material and weaken it. It's important to note that the precat material you are cutting through is only about 3.5-4 inches thick. You do not need a very long bit to accomplish this, just enough to poke through about an inch. I drilled the holes in the front precat with the metal bit, spaced evenly in a triangle around the center. Then I attacked the center with the wood bit, and punched through the middle with it. When the center of the cat is hollowed out fairly well, you can take a screwdriver to the sides of the cat, and break the seal against the sides of the pipe. It takes time, but on the front precat, it is really no big deal.

Finally, you will start breaking out huge chunks of material, I went back and used the wood bit sporadically here, because it catches on the metal and rips it apart and breaks it up. Once you start breaking out big pieces, your work is almost over. When you are done, I took a 2.5 inch wire wheel with an extension to the insides of the cat, polishing it smooth. You may need to scrape the sides with a screwdriver and hammer to get to this point if it did not get ripped out too cleanly. But, this is easily done. So, with the front precat done... take a break.

The rear cat is a PITA! But, if you love your car enough to even consider doing this, the payoff is worth it.

Using the same tools and technique, I had the rear cat gutted in about 3 hours, polished and ready to go. A lot of people say that after you do this mod, you will hear popping sounds from your exhaust... that is the left over crap that they didn't remove, there are metal filings everywhere. One important thing... you MUST wear eyegoggles, a face mask, and be sure to cover your hair completely. Trust me on this. You really can't do the rear precat without doing this. You will be under the car and this stuff just rains down on you.

Don't worry about damaging the sensor behind the cat, if you use a drill bit that will only extend about 5 inches or so, you will never hit it. Anyway, when the sides are very smooth, (I am a perfectionist, and I was also worried that too many filings would gunk up the main cat down the line) use the wire wheel and polish it out. Then I took a heavy duty shop vac and sucked all the dust out of both cats.

The end result was two very thoroughly gutted precats, with smooth inside walls, and about 15 horses I'd say. You really can feel the difference. I hope this helps anyone planning on doing this. Plan on spending a day for this, longer if you have problems getting the downpipe bolts off. But when everything is bolted back up, and if you have done a great job in removing all the dust and material, you won't hear any "popping noises", just the usual exhaust tone, maybe slightly louder at WOT.

Good luck!

Todd

PS I happened to race a Supra turbo that night after all was said and done. I could really tell an improvement, my car is not stock, but it definitely adds more punch!
 

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thanks for the thorough walkthrough
 

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I guess I’m missing something here. Wouldn’t it be a crap load easier to just remove the pre-cats and replace them with straight pipes? Is there a reason one would want to keep the pre-cat when it was gutted and therefore, useless?

Pardon my ignorance but it seems like your going through moor trouble then you need to. :confused:
 

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Discussion Starter #5
LOL, well, you can buy precat eliminators, but they run about $450, plus it is a major job to unbolt that rear precat and install the eliminator anyway. Buying a downpipe eliminates the front cat, (if you go with the Stillen) but does nothing with the rear cat. Hollowing out the precats essentially gives you a straight pipe. There are a few weld spots, but I really don't think using a straight pipe would be much better than gutting the cats... maybe an extra 2-3 horses at most. Basically, doing this saves you $450.
 

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Me Tarzan..u Jane
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Actually the Precat Eliminators do alot better job then just gutting the precats. They get rid of pretty much the most restrictive part of the exhaust, the O2 Sensor housing. Put those on and a good downpipe with a free flowing main cat or straight pipe with a custom Cat back system and your looking at about 70-90 hp increase easy. well worth it to get the eliminator kit.
 

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Well, I've got a 91 turbo, so the front cat doesn't have an O2 sensor, it is just a straight pipe. So the eliminator doesn't do much better than gutting that one. The rear one, yeah, but it still won't give you that much more horsepower than just gutting the rear cat, not to mention what a pain it would be to install it. A downpipe can give up to 40 horses, or has been claimed anyway, that is without doing anything to the precats. Taking out the main cat and running an aftermarket catback exhaust gives you another 30-40. Gutting the cats gives you about 10-15, so the precat eliminators couldn't add much more than that.
 

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UTRacerX9 said:
Well, I've got a 91 turbo, so the front cat doesn't have an O2 sensor, it is just a straight pipe. So the eliminator doesn't do much better than gutting that one. The rear one, yeah, but it still won't give you that much more horsepower than just gutting the rear cat, not to mention what a pain it would be to install it. A downpipe can give up to 40 horses, or has been claimed anyway, that is without doing anything to the precats. Taking out the main cat and running an aftermarket catback exhaust gives you another 30-40. Gutting the cats gives you about 10-15, so the precat eliminators couldn't add much more than that.
I have heard that a DownPipe will give you around 30 horses on a Dyno Sheet but I dont know how true that is, cuz they have a Dyno Sheet for the Tornado Intake that shows it giving a car 8 horses. HEHE:D. Also a Cat - Back will not give you 30 - 40 horses, not even if you cried for a year about it. :D . Also gutting your CAT will give your car 5 horses at the max and there you are pushing it. HEHE:D. With a good Cat - Back and a Hallowed CAT you will be lucky to get about 15 horses.
;)

Also you will not get 70 - 90 horses off a Pre - Cat Eliminator Kit. And if you did gain all that from your exhaust system than your car would have lost sooo much Low - End that you wouldnt be able to take off, off the line like you used to stock. HEHE:D
 

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Me Tarzan..u Jane
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Actually I was not just talking about 70-90 for the ENTIRE exhaust from the turbo back. And we TURBO guys dont have to worry about losing low end torque cause the turbos alone provide enough. we could run with no exhaust if we wanted...be loud as hell but ~0 backpressure.....:D :D :D
 

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Good post Todd. I just got done gutting my pre-cats. I'm so glad I'm done. That's got to be the worst job in the world. I haven't noticed a difference with the seat of my pants meter, but I'm going to see if I can get a better time out of my G-tech. I'll post my results.

Later,

Dustin
 

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I'd like to see an exhaust system that will net you 70 horses, let alone 40. Gutting your precats will net you 15-25, gutting your main cat, maybe forty if you include the precats. You also wont' be street legal, and you'll end up with a check engine light. Yes, the turbos maintain all the backpressure you'll ever need. I would be very surprised if you can pull even pull 30 with a 3" or 4" turbo to tip all the way back.
 

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I have seen the cat straight pipe replacements on Ebay for $35 and there a direct bolt up. This way you can keep your cat around in working order if you ever need to sell the car or have it E.T.'ed.
 
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I have to agree, it was a PAIN IN THE ASS job. Sweaty, Dirty, HOT METAL FLAKES flying around noisey PAIN IN MY A$$....but i would do it again. She revs a little freer and sounds so much sweeter. My BUTOMETER says it gave me HP. Not 40hp....maybe 10-15hp.

70-90 with exhuast...I am severly doubting that....
 

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Now 90 ponies from an exhaust system swap… :eek: That would be awesome. I would love to see some member’s dyno numbers stock and then disconnect the exhaust from the turbos…

A big pain in the ass but I would like to see it none the less.
 
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Hey Cameron...I see you have a VR4 and an Explorer..That's funny...

So DO I!!
 

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Me Tarzan..u Jane
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Well I dont have solid numbers on that to back up the 70-90 hp gain, just going by the numbers I've found on doing some research. 40-50 from downpipe, 10 from Precat Eliminator kit, 5-10 from high flow cat/test pipe, 10-20 from a cat back....even using the low estimates it should give ya around 65 hp gain. But I'll get some numbers when I get mine done.
 

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I think those estimates are per component. Although manufactures lead you to believe you can just add them together for a total HP gain, it doesn’t always work that way. For example, if a down pipe will net you 20 HP gain and a high flow cat will net you 20 HP, doing both together may only net you 30 HP and not the 40 HP you would think. I don’t know exactly how it works but this is what I have been told. :confused:
 
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Rule of thumb with exhaust stuff is NOT to add numbers of all seperate additions for a grand total. if you take 3 mods that give 20hp each and do all three, chances are you will net 30hp not 60hp. Otherwise we would all be running aroun with 500hp with BPU and full exhuast. Not going to happen...

Point of deminishing returns....And efficiency of a given system peaks out
 

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Why would guys get dirty gutting the pre-cats when you could get some dowpipe to eliminate the precats. I'm sure theres more power in that anyway.

late
 

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I asked that same question earlier in this thread. Essentially, it’s to save about $450 and to give the appearance to someone inspecting your exhaust that the cats are still there.
 
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