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Old 08-20-2010, 07:03 AM   #1
09suby
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Default High Flow Vs No Cat

I plan on changing my exhaust on my car. I plan on putting a high flow cat with and agency power exhaust. Would this throw any check engine lights? Also, what if I was to go cat-less would this throw a check engine light?

Also if the catalytic converter that I get is a 2.25" inlet/outlet and my exhaust is 2.5" piping would I still be able to weld the catalytic converter on? Or does it have to be a 2.5" outlet cat?
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Old 08-20-2010, 07:32 AM   #2
grafton
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there is some dyno testing on rs25 i believe that shows no noticeable difference between high flow cat and catless in terms of hp & torque

you want to get a ceramic high flow cat to avoid a CEL, and a good exhaust shop shouldn't have any issues stepping up the pipe size
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Old 08-20-2010, 08:24 AM   #3
TFBHTMFSNE
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If you're running on the street, go with the high flow cat. Running catless can get you in trouble with the authorities in some areas. You know, tampering with and/or removing emissions equipment. It's not worth the hassle if you get caught.
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Old 08-20-2010, 03:57 PM   #4
formula91
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Quote:
Originally Posted by TFBHTMFSNE View Post
If you're running on the street, go with the high flow cat. Running catless can get you in trouble with the authorities in some areas. You know, tampering with and/or removing emissions equipment. It's not worth the hassle if you get caught.
Not worth the hassle to change the cat in the first place. Are you even using different cams?
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Old 08-20-2010, 04:27 PM   #5
hellajdm
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Quote:
Originally Posted by formula91 View Post
Not worth the hassle to change the cat in the first place. Are you even using different cams?
uhhhh... what?
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Old 08-20-2010, 06:00 PM   #6
formula91
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Quote:
Originally Posted by hellajdm View Post
uhhhh... what?
What I meant by that was.

The max CFM allowed by the stock cat is more than enough for an engine with stock cams and internals.

The major restriction is in the cylinder head, not anywhere. An intake can free up "some" power, but it's definitely not even close enough to having enough flow to justify getting a high flow cat.
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Old 08-20-2010, 06:04 PM   #7
RaceFaceXC
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double what Formula91 said, especially with stock exhaust manifold. I would only ever change to a HFC if fitment required it or engine work required it. A catback will possibly be louder with a HFC, likely more raspy and buzzy. track pipe will intensify these effects and guarantee a CEL.
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Old 08-20-2010, 07:27 PM   #8
Back Road Runner
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Yeah, there's a range of test data out there for both the EJ25 engine and about exhaust in general including specifically cat flow and resulting HP differences.

Without digging around for a while, the gist is the stock cat fits the stock engine well enough. A high flow cat is only a minimal improvement and still requires a full exhaust upgrade and good header if you cared at all about power. The difference between a high flow cat and no cat is negligible. It's not even that much of a difference with a lower flowing cat. Realize that the limitation is mainly on the top end, and our engines are already rolling off in power at that point. You don't really worry about flow until the engine is first capable of pumping that much in the first place, the whole engine: intake, throttle body, intake manifold, heads, cams, header, exhaust pipe, and muffler along with the cat you're so worried about.
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Old 08-20-2010, 09:28 PM   #9
suex
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I agree, unless your installing headers, a hi-flow cat is a waste of money, and yes, you will get a cel on the hi-way... I do every time...lol..
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