ST185: De-Cat Downpipe vs. Gutted Catalytic Converter

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ST185: De-Cat Downpipe vs. Gutted Catalytic Converter

Postby 93celicaconv » Wed Dec 08, 2021 7:33 pm

There is a thread in this forum regarding a person who acquired at GT4-RC from Japan and had questions regarding his catalytic converter. It sort of drifted into gutted catalytic converters a bit.

What I would like to know, from those who have done this and have some experience regarding an ST185, if someone wanted to gain a bit of torque and/or horsepower, and many have went to replace their OEM catalytic converters with a de-cat downpipe, but some have gutted their own OEM catalytic converters, is there any performance benefit by doing either (compared to leaving the original catalytic converter in place)?

If there is a benefit in removing the slight pressure drop across the catalytic converter, is there any benefit in a replacement de-cat pipe being installed vs. just gutting the original catalytic converters and reinstalling it? I'm thinking if both methods are about equal in torque/power benefit, why are so many buying de-cat pipes instead of just gutting the OEM catalytic converter?
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Re: ST185: De-Cat Downpipe vs. Gutted Catalytic Converter

Postby simple » Wed Dec 08, 2021 8:10 pm

The topic of gains has to be covered in depth on a MR2 site right?

Personally gutting a cat never appealed to me. It is messy and you are dealing with materials that should be recycled. I'd rather get aftermarket and sell the stock cat to someone who needs it or the scrap yard so it is processed appropriately. That is what I've done on all my turbo cars. My Subaru had 3 cats and going to a single Cobb made a difference but the Alltrac has one and it is not very restricted. At least I tell myself that so I don't buy a Berk downpipe..so far.

Going cat-free is awful in my experience. The car smells awful and pollutes massively. The smell is terrible not just to people on the road behind you but also to the owner when you stop driving. I know this personally from the old ranch truck I have.
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Re: ST185: De-Cat Downpipe vs. Gutted Catalytic Converter

Postby grip-addict » Thu Dec 09, 2021 2:53 am

yeah, better off to let the mr2oc madness handle the inevitable infighting that goes with any question over there.
Gutted or no-cat DP's are better for performance than factory but worse for the planet. That's the jist of it.
Anytime you see a gutted cat it was most likely done because it was a ''free'' mod. An aftermarket DP that is designed to flow will always outperform the factory one - intact cat or not.
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Re: ST185: De-Cat Downpipe vs. Gutted Catalytic Converter

Postby underscore » Thu Dec 09, 2021 4:03 pm

The numbers are floating around here somewhere, but a gutted cat picks up a bit of power and a proper downpipe makes more since there's less turbulence (no rough edges or diameter changes). Both pick up/restore more if your cat was clogged, being 30 years old I'd bet a lot of them are. Like simple I'm not a fan of gutting them as you're throwing out something that should be recycled and at least back in the day you could recycle the cat for nearly as much as a used downpipe cost, so I'd rather spend the extra couple bucks for more power.

As far as emissions go I've never really cared since my car is barely driven. Overall a single vehicle is insignificant in terms of global emissions, obviously that changes a bit if you're daily driving it in a smoggy area. Everyone says it should smell, but mine doesn't smell inside the car at all and a car behind can only smell it when the cold start is running. After that there's nothing. That's coming from my wife who has a very sensitive nose.

The one thing I have noticed that I haven't seen anyone mention before, and the reason I might eventually put a cat back in, is noise. I had a 3" downpipe followed by a 3" straightpipe to a 3" BWR straight through "muffler" and it was stupidly loud (my stock exhaust had exploded open at the front resonator after installing the downpipe and doing a pull). It could set off car alarms just puttering by if there were buildings. I took that off and gave it to a buddy who put it on his car but left the stock cat in and it sounded perfectly pleasant, just a little more bass-y than a normal car. Meanwhile the resonator + Blitz NUR catback + silencer I have now is still too loud for my liking.
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Re: ST185: De-Cat Downpipe vs. Gutted Catalytic Converter

Postby 93celicaconv » Thu Dec 09, 2021 4:18 pm

Thanks for all the feedback so far.

My son has two ST185's - one with the original catalytic converter (not gutted) and one with a de-cat downpipe (an older version that burned through the cold air inlet tube for the alternator). Without a doubt, the one with the de-cat downpipe has more pep, and it moves the boost gauge farther and quicker. I was thinking maybe the one with the original cat still in it had problems with the wastegate system, but now I'm thinking it may be just the original cat causing the problem. Really isn't a problem - if someone only had the ST185 with the original cat in it and couldn't compare it to another, wouldn't see anything being wrong.

My son got a new de-cat downpipe to replace the older one - the new one doesn't have quite the sweep the old one has, so it will not damage the rubber cold air inlet tube (we have a new rubber cold air tube to replace the burned through one). Based on how the new de-cat downpipe installation goes, and verifying that it can't damage that cold air inlet tube for the alternator, I may get another and install it on the car that currently has the original catalytic converter in it.

Regarding exhaust noise, both ST185's have original exhaust systems on them. The one with the de-cat downpipe is just a tad louder - but the sound I think is very appealing and fits for a car with a more powerful engine. The one with the original cat in it is a bit quieter, not much different than a Celica with the 5S-FE engine.

If anyone else has any inputs/thoughts, would appreciate if you provide them.
Last edited by 93celicaconv on Wed Jan 12, 2022 7:43 pm, edited 3 times in total.
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Re: ST185: De-Cat Downpipe vs. Gutted Catalytic Converter

Postby peteomalone » Wed Jan 12, 2022 7:16 pm

My experience with removing the front cat is that looking at the turbo outlet elbow casting, there is a significant restriction as it makes the turn. My guess is removing this has more benefit than gutting the cat. Besides the slightly louder exhaust with a downpipe, the other benefit is the improved breathing. The turbo can easily be heard spooling up via the intake. With the stock cat setup, I don't think I ever heard it.
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Re: ST185: De-Cat Downpipe vs. Gutted Catalytic Converter

Postby 93celicaconv » Wed Jan 12, 2022 7:46 pm

peteomalone wrote:My experience with removing the front cat is that looking at the turbo outlet elbow casting, there is a significant restriction as it makes the turn. My guess is removing this has more benefit than gutting the cat. Besides the slightly louder exhaust with a downpipe, the other benefit is the improved breathing. The turbo can easily be heard spooling up via the intake. With the stock cat setup, I don't think I ever heard it.

Pete, what downpipe did you go with in your 90 ST185?
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Re: ST185: De-Cat Downpipe vs. Gutted Catalytic Converter

Postby peteomalone » Wed Jan 12, 2022 8:41 pm

Jerry, on my 91 ST185 I put an Aussie downpipe. Great craftsmanship but unfortunately now out of business. For this 90 ST185 I put on one from BRD in the UK. Also great work. Large radius bends so it does get real close to the alternator duct. Right now running without it, but on my old ST185 the Aussie pipe was also close and never burned through the rubber.
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