High Flow Vs No Cat

09suby · 08-20-2010, 07:03 AM

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?

* Registered users of the site do not see these ads.

grafton · 08-20-2010, 07:32 AM

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

TFBHTMFSNE · 08-20-2010, 08:24 AM

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.

formula91 · 08-20-2010, 03:57 PM

Quote:

Originally Posted by TFBHTMFSNE

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?

hellajdm · 08-20-2010, 04:27 PM

Quote:

Originally Posted by formula91

Not worth the hassle to change the cat in the first place. Are you even using different cams?

uhhhh... what?

formula91 · 08-20-2010, 06:00 PM

Quote:

Originally Posted by hellajdm

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.

RaceFaceXC · 08-20-2010, 06:04 PM

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.

Back Road Runner · 08-20-2010, 07:27 PM

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.

suex · 08-20-2010, 09:28 PM

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..

08-20-2010, 07:03 AM	#1
09suby Scooby Newbie Member#: 222590 Join Date: Sep 2009 Chapter/Region: South East Location: Clermont,Fl Vehicle: 2009 Impreza 2.5i Spark Silver Metallic	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? * Registered users of the site do not see these ads.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
High flow cat VS No cats...none...	lavid2002	Newbies & FAQs	32	11-09-2009 10:05 PM
High flow down pipe catted on 06?	Skin Irritant	Newbies & FAQs	11	09-11-2008 02:21 PM
High Flow vs. OEM fuel pump?	Spitulski	Normally Aspirated Powertrain	4	10-12-2007 11:31 AM
Cobb high flow vs. cattless	nineleft	Newbies & FAQs	33	11-10-2006 12:48 PM
Hi flow cat vs No cat?	WillysPU	Factory 2.0L Turbo Powertrain	39	11-21-2005 08:56 PM

08-20-2010, 07:32 AM	#2
grafton Scooby Specialist Member#: 187936 Join Date: Aug 2008 Chapter/Region: MAIC Location: Fairmont WV Vehicle: 97 Legacy w12:1CR 05 LL Bean outback	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

08-20-2010, 08:24 AM	#3
TFBHTMFSNE Scooby Specialist Member#: 156086 Join Date: Aug 2007	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.

08-20-2010, 06:04 PM	#7
RaceFaceXC Scooby Specialist Member#: 165749 Join Date: Dec 2007 Chapter/Region: South East Location: stupid Florida Vehicle: 2007 No YELLING on the bus!	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.

08-20-2010, 07:27 PM	#8
Back Road Runner Scooby Guru Member#: 60082 Join Date: Apr 2004 Chapter/Region: MWSOC Location: Minnesota Vehicle: 2004 Forester STI Silver	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.

08-20-2010, 09:28 PM	#9
suex Scooby Newbie Member#: 243393 Join Date: Apr 2010 Chapter/Region: NESIC Location: Foresdale Vehicle: o6 impreza rs silver	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..