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Discussion Starter · #1 ·
In all my noobiness i was reading the basic tuning info on http://www.spdusa.com/ and was interested by their opinion to just replace the midpipe cat with a catless pipe and leave the downpipe cat in unless you want over 280 hp or more. This seems to make sense to me since the STI has one downpipe cat and can still achieve 300 HP. Though when i search on the forums it seems everyone never talks about just replacing the midpipe and if they do they say that it is worthless and you won't get any power gains with the downpipe cat still in there. So what is the real deal on this, will you get any gains with removing the midpipe cat and leaving the downpipe cat in? All this is assuming the up-pipe cat has been removed too. I would definately be interested in just replacing the midpipe cat and leaving downpipe one in since i dont want to cause too much pollution and stinkiness and I don't want that much power. Thanks.

Eric
 

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leifericson said:
In all my noobiness i was reading the basic tuning info on http://www.spdusa.com/ and was interested by their opinion to just replace the midpipe cat with a catless pipe and leave the downpipe cat in unless you want over 280 hp or more. This seems to make sense to me since the STI has one downpipe cat and can still achieve 300 HP. Though when i search on the forums it seems everyone never talks about just replacing the midpipe and if they do they say that it is worthless and you won't get any power gains with the downpipe cat still in there. So what is the real deal on this, will you get any gains with removing the midpipe cat and leaving the downpipe cat in? All this is assuming the up-pipe cat has been removed too. I would definately be interested in just replacing the midpipe cat and leaving downpipe one in since i dont want to cause too much pollution and stinkiness and I don't want that much power. Thanks.

Eric
It is true that the both the JDM and the USDM STis make 300hp with the downpipe cat in place, but that isn't the only difference between the two cars' engines. If you are removing a catalytic convertor, you mights as well get all of them out of there, or at least the most restrictive (the downpipe cat). I am sure that you'd get some gain with just losing the midpipe cat, but it wouldn't be the same a if you were to remove the downpipe cat.

And if you don't want that much power, you should be banned.
 

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Discussion Starter · #3 ·
heh, well i just got the car and it's my first stick so right now i'm just worrying about driving it on hills, but yeh anyways, i know it's better to take out all the cats but i was just curious if anyone has just taken out their midpipe and the gains that it caused....
 

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The more power you make, the more the cat restricts you. I have seen loses over over 35 whp by adding cat but depends on how much power you are making as well. Best thing is to remove the stock units though. The diameter is far to small to alow decent flow. If you want a cat grab a Turbo XS or APS turboback exhaust which incorporate a 3 inch cat. You'll have more power and be able to handle much higher flow rates while staying green.
 

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The 3rd cat isn't that restricitive IMO. I had to retain it for STX class. I've since removed it and feel little gain. Contrast that w/ adding a catless downpipe- night and day. An aftermarket downpipe not only removes a restrictive cat but also usualy incorporates a better design as well- bell mouth, divorced or semi divorced wastegate.

There are several downpipes on the market that incorporate a hi- flow cat, those would be a vast improvement over oem.

Big Sky
 

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leifericson said:
In all my noobiness i was reading the basic tuning info on http://www.spdusa.com/ and was interested by their opinion to just replace the midpipe cat with a catless pipe and leave the downpipe cat in unless you want over 280 hp or more. This seems to make sense to me since the STI has one downpipe cat and can still achieve 300 HP. Though when i search on the forums it seems everyone never talks about just replacing the midpipe and if they do they say that it is worthless and you won't get any power gains with the downpipe cat still in there. So what is the real deal on this, will you get any gains with removing the midpipe cat and leaving the downpipe cat in? All this is assuming the up-pipe cat has been removed too. I would definately be interested in just replacing the midpipe cat and leaving downpipe one in since i dont want to cause too much pollution and stinkiness and I don't want that much power. Thanks.

Eric
Personally, I think that this is the way to go. First of all, you want to get rid of the uppipe cat to help to spool the turbo and to prevent it from breaking up and causing problems. I have heard of people with high flow cats not passing tailpipe emissions and having CEL problems. If you want to run one cat, I would decide between the downpipe cat and the midpipe cat.

Some things to think about in favor of keeping the downpipe cat...
1) If you need to reinstall the midpipe cat for emissions purposes, you can do this in approx 20 min. Reinstalling the stock downpipe would take longer.
2) The downpipe cat should be slightly more efficient since it is closer to the turbo and appears to be slightly larger.
3) You don't need a CEL fix.
4) A midpipe replacement is generally cheaper than a downpipe.
5) The stock downpipe has thick walled pipes which will keep the car quieter than most aftermarket downpipes.
6) The stock downpipe has heat shielding that may be difficult to duplicate with an aftermarket downpipe.


Things in favor of keeping the midpipe cat...
1) Although it is hard to tell because of a bend in the downpipe cat, the downpipe cat appears to be slightly larger than the midpipe cat. Using nothing more sophisticated than shining a light in each, they appear to be the same matrix type structure. The downpipe cat is probably slightly more of an obstruction than the midpipe cat.
2) For high power applications, the downpipe design with a flat plate over the wastegate isn't ideal. Of course, if you are going for huge power, you probably don't want any cats. For people who are looking for only moderate power increases, I have read that the flat plate over the wastegate opening doesn't matter.
 

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It would be nice to see if someone had the dyno time, w/ catless uppipe and catless downpipe and then another run w/ catless uppipe and catless midpipe.

Not only hp/torque #'s, but also spool up- I feel the downpipe greatly contributes to a faster spoolup.

If it was very close, then Rich outlines some very plausible reasons for going midpipe vs downpipe. If it wasn't very close than a person would have to choose the power gains over the down sides.

I've got a dyno run w/ the oem 3rd cat in (dp/up catless), does anybody have a run w/ up/mp catless w/ oem dp? It wouldn't be the most scientific comparo, but might prove some insight.

Big Sky
 
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