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Old 05-17-2009, 04:46 PM   #1
ScubaruImpreza05
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Question High Flow cat pipe questions

It's that time of season again, the mod bug is attacking I do a little bit of modding every summer, and this summer I'm thinking of doing something about my restrictive catpipe and getting some struts. I already have STi springs and I opened the muffler and gutted it last summer, as well as made Williaty's intake and new plugs

Anyways, to start off, are the only 2 companies making high flow cats Stromung and Random Tech? If so, is Random tech really worth the extra $200 premium over Stromung?

Also, would it be difficult to make your own catpipe assuming you had these tools at your disposal: Mandril bender, Welder and appropriate piping? There is an exhaust shop not 4 minutes from my house, but I'm cheap and the old man has an industrial welder and access to a mandril bender. If I got my hands on an appropriate High Flow Cat, think I could mod me up a custom cat? Or is it a bad idea?

As for the high flow cat, if I got one that was correct diameter and made for....let's say a 200 HP car in stock trim, would that suffice? I'm going off of the thought 'More HP means more air' so the flow would be good I assume?

Thanks for any ideas
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Old 05-17-2009, 05:10 PM   #2
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Quote:
Originally Posted by ScubaruImpreza05 View Post
It's that time of season again, the mod bug is attacking I do a little bit of modding every summer, and this summer I'm thinking of doing something about my restrictive catpipe and getting some struts. I already have STi springs and I opened the muffler and gutted it last summer, as well as made Williaty's intake and new plugs

Anyways, to start off, are the only 2 companies making high flow cats Stromung and Random Tech? If so, is Random tech really worth the extra $200 premium over Stromung?

Also, would it be difficult to make your own catpipe assuming you had these tools at your disposal: Mandril bender, Welder and appropriate piping? There is an exhaust shop not 4 minutes from my house, but I'm cheap and the old man has an industrial welder and access to a mandril bender. If I got my hands on an appropriate High Flow Cat, think I could mod me up a custom cat? Or is it a bad idea?

As for the high flow cat, if I got one that was correct diameter and made for....let's say a 200 HP car in stock trim, would that suffice? I'm going off of the thought 'More HP means more air' so the flow would be good I assume?

Thanks for any ideas
You could use any cat with the appropriate ID, 2.25" being the accepted best compromise for good velocity and high flow volume on a non built EJ253. But if you have stock header still then there is a cat in there so replacing the second cat would be to very little gain. It would also require custom bending because no one makes a bolt up catpipe for the 06+ impreza. If you have already changed headers then this would not be a concern.

I would search for more info on this because I think the stock cat(s) is not a restriction unless you have significantly increased the flow rate of the engine via cams, etc. I could be thinking of the muffler, though.

Also, magnaflow and dynomax make generic HFC's that you could fab up a piece with, too. I'm sure there are more companies too
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Old 05-17-2009, 05:28 PM   #3
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Originally Posted by RaceFaceXC View Post
You could use any cat with the appropriate ID, 2.25" being the accepted best compromise for good velocity and high flow volume on a non built EJ253. But if you have stock header still then there is a cat in there so replacing the second cat would be to very little gain. It would also require custom bending because no one makes a bolt up catpipe for the 06+ impreza. If you have already changed headers then this would not be a concern.
Ahh, I thought the cats were on the same pipe, my mistake. My searching wasn't good enough. I'll get a look in under my car when I rotate my tires Friday, but all those stupid heat guards under there make it impossible to get an idea of where stuff is at, and i don't have a lift to just pull stuff off easily

Quote:
I would search for more info on this because I think the stock cat(s) is not a restriction unless you have significantly increased the flow rate of the engine via cams, etc. I could be thinking of the muffler, though.
When I gutted my muffler I posted on the forums here and everyone said it was basically pointless to gut it without replacing the cats in front of it. I'm doing this to free up some efficiency, hp is just a side effect to me

Quote:
Also, magnaflow and dynomax make generic HFC's that you could fab up a piece with, too. I'm sure there are more companies too
Ya, I did some initial research and Vibrant claims CFM of up to 578, but I think that is for their more expensive race HFC. I want to have something around 500 CFM or more.
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Old 05-17-2009, 06:24 PM   #4
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random tech and stromung are the only "bolt-on" cats out there. i would save the money and do the cat and header together. if u get the random tech one, u probably won't throw a CEL, where the stromung is more likely to b/c it is only 1 cat. this is the easier, more expensive way.

if u do "frankenstein" it, u need a 2.25" inlet and outlet diameter and 2 cats to not throw a CEL. then u have to have the shop bend the pipes in the right way and add flanges so it will bolt up to the stock header. this is the harder, cheaper way.

i did the "bolt-on" way, although more expensive. the sound is sick, especially if u have a catback on the car. mine is the stromung single tip and definitely recommend it if u don't have a CBE yet.
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Old 05-17-2009, 07:10 PM   #5
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Ya, I did some initial research and Vibrant claims CFM of up to 578, but I think that is for their more expensive race HFC. I want to have something around 500 CFM or more.
Yes, but what is the CFM that your stock cats flow and how much does yor engine flow. Putting on a HFC that has a much higher CFM without needing that additional flow potential is pointless. These are the things you should investigate and see if your stock cats are even limiting you in the first place.

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random tech and stromung are the only "bolt-on" cats out there. i would save the money and do the cat and header together. if u get the random tech one, u probably won't throw a CEL, where the stromung is more likely to b/c it is only 1 cat. this is the easier, more expensive way.
No, the randomtech and stromung are not bolt on cats... you need to change the header to an 05 and previous impreza RS collector flange location header (borla, replicas, TWE) before these will "bolt on". The midpipe flange has to be rotated too, if I remember correctly. And they will both throw a CEL, the random tech is not going to not throw a CEL under the same conditions, i guarantee it. You should look into why the random tech has two humps and the construction of the stromung.
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Old 05-17-2009, 08:58 PM   #6
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if your concerned about smog test, if you have one you likely wont pass. As the extra flow prevents it.

If no smog then do what you like.
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Old 05-17-2009, 09:58 PM   #7
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if your concerned about smog test, if you have one you likely wont pass. As the extra flow prevents it.

If no smog then do what you like.
I believe you are saying that with a high flow it will not pass the smog emissions test, but if I do not care, then it will work?

I was hoping both cats would be on the catpipe, because then I could swap out before I would go for an inspection, but if I need to mess with the UEL headers to make a high flow system work, I'm not sure if I will go through with it. UEL will cost more and I'll have the same flow as the EL stockers.



PS - Cant find crap on stock 2.5i CFM anywhere.
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Old 05-17-2009, 10:11 PM   #8
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Here's a thread by williaty which includes his conclusions regarding track pipes and HFCs on a stock engine.

http://forums.nasioc.com/forums/show....php?t=1698802
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Old 05-18-2009, 06:43 AM   #9
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Here's a thread by williaty which includes his conclusions regarding track pipes and HFCs on a stock engine.

http://forums.nasioc.com/forums/show....php?t=1698802
Excellent post, unfortunately, as I read it, it is talking about the 2.5RS UEL + SOHC cam engine. From what I have read, the 2.5i heads flow very well, although now looking at it, we have the SOHC. Another thing I liked seeing (though I don't know if it was for the 2.5i cats):

Quote:
The cat part isn't surprising. There are some flow bench testing data of various cats floating around Nasioc and Google. In stock form, the cats have little problem flowing 200-250cfm before becoming resistive to flow.
Then stealing Pat's graphs and post

Quote:
Originally Posted by Patrick Olsen View Post
As a counterpoint, I'll throw up my dyno comparison of a header/cat install. This was on my then-stock 2.5GT (N/A DOHC), same day dyno, going from the stock header/cat/mid-pipe to an MRT header/cat/mid-pipe. Not only did peak power jump ~14whp, it also shifted to the right ~400rpm. I don't have the Excel file here at work, but as I recall power was up +18whp at redline. Peak torque didn't go up nearly as much, but you can see it also shifted well to the right.



I've also posted in the past this comparison of a stock SOHC 2.5RS (I think it was a 2000) vs. my stock 2.5GT.



While I think it is generally accepted that the SOHC heads flow nearly as well as the DOHC heads, and the intake manifolds and headers on these two cars were essentially identical, clearly the SOHC is much more optimized for day-to-day driving midrange. Based on process of elimination, the difference in the torque curves on the two engines comes down to (1) ECU and (2) cams. It would appear to me that the SOHC cams/tuning are such a limitation that even when you free up the intake and exhaust you just can't get the engine to breathe more. That was definitely not the case with my DOHC, even with the stock cams.

Pat Olsen
Quote:
Originally Posted by williaty View Post
I think that's probably a pretty valid conclusion to draw. I knew that the DOHC engines had a reputation for breathing better but then I had seen flowbench numbers showing the SOHC and DOHC heads basically right on top of each other. That pretty much leaves just the differences in cams to account for the differences in breathing.

I'll make the first post clearer than I'm talking about the SOHC engines.
Then, my 90% asleep brain decided to make a quick math problem to figure out my necessary CFM for my engine, thus need or not need for cat upgrade. As follows:

Quote:
1 HP approx equals 1.45 CFM
1 CFM approx equals 0.0745 lb of air/min
0.108 Lb/min approx equals 1 hp
1 Meter cubed/sec = 35.314 CFS = 2118.867 CFM
1 KG/sec = 132 lbs/min approx equals 1771.812 CFM

power coversions:
1 PS = 0.9859 HP = 75 Kgf m/sec
1.3405 HP = 1 KW
1 HP = 746 watts
(Source is DSMtalk.com .... )

Using this formula, and the stock hp rating of my engine, it is this: 173hp (crank) x 1.45cfm = 250.85 CFM.

So, in conclusion, not knowing if the first quote above applies to all Subaru Cats or just 2.5RS cats, the stock set up is at it's limits just at stock form. I'm no physicist, so I'm sure there are far more variables, but if you want to start getting some numbers above stock form, a cat is not as bad an idea as it may seem. Not disproving Williaty at all because he knows about 500 times more than I do about these engines, but if I'm paper racing it seems there is some leniency to the cat debate. I'll probably edit this shortly with head flow benches because this post is big enough already and my computer likes to randomly refresh pages, thus losing posts

EDIT#2: I cannot find flow specs for the 2.5i heads. They may be the same as older models, but I'd like to know, and for sure. It may very well be the heads flow less than the exhaust at this time, and I'd need to know so i can adjust my mods appropriately.

EDIT#3: Hell, I can't even find a peak CFM for our stock exhaust systems, but I found that 2.25" STRAIGHT PIPE is 490cfm.

Last edited by ScubaruImpreza05; 05-18-2009 at 07:51 AM.
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Old 05-18-2009, 07:02 PM   #10
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Really though, I think hfc are a waste of money.

Expensive HP with the risk of a P0420 code.
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Old 05-19-2009, 07:21 PM   #11
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I did some comparing and if the heads are maxed at 250 CF/M, then the system is flowing at it's max because you need that to hit 173 HP crank atleast. So it isn't worth the HFC until you have atleast headwork and/or cams. I R

On the bright side, now I'm getting Tokico D-specs
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Old 05-19-2009, 08:33 PM   #12
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^ Money well spent. What springs you gonna get for em? Some STI takeoffs?
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Old 05-19-2009, 09:30 PM   #13
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Already have the USDM STi springs on. My stock shocks are starting to bounce more, I was planning on new shocks by 75k miles anyhow
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Old 05-19-2009, 09:36 PM   #14
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I would not worry about flow on any aftermarket cat. I would worry if the cat will heat up enough for your O2 sensor to work properly.
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Old 05-20-2009, 02:23 PM   #15
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racefacefx... when i said bolt-on i meant that there was no cutting or welding needed. do u have the borla with random tech cat? where is ur answer based?

on a 2006, the header and cats are one unit now, where the older ones, the header and cats were seperate. u need these to be changed at the same time. the dual converter on the random will most likely not throw a CEL. my car didn't throw one with the header. the stromung cat will throw the CEL. if a TWE EL header is added to either, there will be CEL all the time.
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Old 05-20-2009, 02:42 PM   #16
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Even better news, I bought rims

(P.S. - what outback said. If it was just a separate pipe with the cats on the stock setup it'd be alot easier...and cheaper, but unfortunately it is not so.)
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Old 05-20-2009, 03:48 PM   #17
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Really though, I think hfc are a waste of money.

Expensive HP with the risk of a P0420 code.
Since mention of the P0420 code was made in this thread, I thought I would throw in a quick summary of experience with my OEM converter.

My car threw the P0420 code with stock configuration, and Subaru replaced the catalytic converter under emission system warrantee. This happened with about 65k-70k miles on the odometer. I also requested new O2 sensors with the cat replacment - but I had to foot the bill for these; couldn't get them covered under the warrantee bill.

I got the new cat, then bought a Borla header and Stromung CBE. Got the Borla coated - with ceramic-type stuff - hoping to keep the heat in the exhaust and avoid the CEL-P0420. Removed the entire system, then added the new pipes and kept the new OEM cat in service as well. The new exhaust worked great for about 10k miles, then I got another P0420 code.

The current code/CEL has come and gone (many times) over several hundred miles, and now has been off for a couple thousand miles. I haven't checked to see if the code is still stored, but I fully expect it to be there, and I also expect that it will return - eventually - after clearing. It seems that the OEM design won't last without killing either the converter or the O2 sensors, at least with my car.

Why shouldn't I now go ahead and try a high flow converter - can't be any more problematic than keeping the stock exhaust system?

Related question - does the rich-running nature of these cars cause any problem with catalytic converters? Can the excess fuel (with periodic afterfire) destroy the element? If so, then it's likely to happen with a HFC also - right? Does that make the track pipe a longer-lived solution?

Thanks,
John
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Old 05-20-2009, 05:41 PM   #18
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A track(test) pipe has NO catalytic converter, a high flow does. Unless you mean a HFC that's referred to as a track pipe?. So, you'd have the code always, but nothing would break
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Old 05-20-2009, 05:55 PM   #19
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Originally Posted by PA-Outback2000 View Post
racefacefx... when i said bolt-on i meant that there was no cutting or welding needed. do u have the borla with random tech cat? where is ur answer based?

on a 2006, the header and cats are one unit now, where the older ones, the header and cats were seperate. u need these to be changed at the same time. the dual converter on the random will most likely not throw a CEL. my car didn't throw one with the header. the stromung cat will throw the CEL. if a TWE EL header is added to either, there will be CEL all the time.
I said they are not bolt on because they do not "bolt on" to a factory header for the OP's impreza. So some type of fabrication is needed unless he replaces his headers.

So a more efficient header design that gets much more heat to the cat will cause the cats to be less efficient and throw a code but an inefficient header, will not throw a code? That doesn't make any sense. There will still be a P0420 code even with factory headers and a HFC. the headers have nothing to do with it. The fact that you havn't gotten a code floors me.
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Old 05-20-2009, 06:53 PM   #20
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Originally Posted by JohnSellers View Post

Why shouldn't I now go ahead and try a high flow converter - can't be any more problematic than keeping the stock exhaust system?

Related question - does the rich-running nature of these cars cause any problem with catalytic converters? Can the excess fuel (with periodic afterfire) destroy the element? If so, then it's likely to happen with a HFC also - right? Does that make the track pipe a longer-lived solution?

Thanks,
John
Don't take my word for it, but this is what I heard:
Running rich will melt the cats eventually.

I also read a related issue of people's cats melting because of untuned BOVs.

OBX has resonated test pipes, and your exhaust will be loud and raspy still.
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Old 05-20-2009, 10:41 PM   #21
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raceface.."So a more efficient header design that gets much more heat to the cat will cause the cats to be less efficient and throw a code but an inefficient header, will not throw a code? That doesn't make any sense." i got lost.

the random tech dual cat was designed not to have a CEL. there is a very rare chance that one will come on.
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