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Old 05-17-2005, 02:14 PM   #1
tonyakavw
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Default How much added HP for removing the cat?

How much power gain am I looking at by removing the cat alltogether? I'm putting a 2001 2.5RS engine into a Porsche 914 (yes I know this isn't the conversion thread but its specific to the NA 2.5 engines)... Anyway, the car doesn't need to be smogged and I am definitely not putting in a cat.

The engine I bought came with an AEM cold air intake and no exhaust or headers. I've looked through the FAQ on headers and exhausts and it seems like most people are using the Borla setup. The MRT setup looks good, but I think I may try something custom with two mufflers. Any reccomendations here?

-Tony
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Old 05-17-2005, 02:21 PM   #2
STi-Spec C
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without the cat it's about a 12hp gain
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Old 05-17-2005, 02:28 PM   #3
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When you say twon mufflers do you mean, one pipe split axle back, or 2 pipes from the engine? If the later is the case, Id talk to TWE about possibly making you a custom header. Borlas are the popular setup cause theyre cheap, and have a good quality build. If your looking for more power, I believe the equal length headers from TWE or COBB will net you more. Borlas help the engine rev faster, but the gains arent quite as great as most hope. And this is all coming from a very happy borla user.
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Old 05-17-2005, 02:41 PM   #4
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Well, I was thinking about having two seperate headers with their own pipe going to a muffler for each side in the back. Thats how the 914s originally did it on their aircooled flat 4, but they merged the two in the back into a single muffler.

I'll see what TWE will do, equal length headers seem like a good idea.


-Tony
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Old 05-17-2005, 03:18 PM   #5
omahasubaru
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Quote:
Originally Posted by STi-Spec C
without the cat it's about a 12hp gain
I think he meant to say 1 - 2 hp gain.

But since you're not even on a stock RS powertrain.. you could see some gains.

I'd pick up a TWE header and then just bolt a turn down on it or run your piping where ever you need to from there. More than likely most off-the-shelf headers won't fit your application... just have one made, or just put a pipe going out from each exhaust port in the head.
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Old 05-17-2005, 03:22 PM   #6
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Hrm, you'd have to bolt the headers on reversed. I think this is doable, because I know you can do this with the intake manifold. Interesting... And yeah, TWE is your best bet. You will, however, have to confirm clearance with it on reversed...
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Old 05-17-2005, 05:19 PM   #7
tonyakavw
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Actually the exhaust will bolt on the same way it does in an Imprezza. The 914 is a mid-engine car, so the engine sits directly behind the seats in front of the transmission which sits between the rear wheels. I'm curious if there will be clearance issues though because of the low ground clearance. I'm going to be chopping the oil pan to get the engine to sit even lower.

I will have to turn the intake around and mount the alternator in another spot, but I will be removing the AC compressor and the power steering pump as well.


-Tony
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Old 05-17-2005, 05:47 PM   #8
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Got it. Yes, I would most certainly guarantee there will be ground clearance issues with the headers...
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Old 05-17-2005, 05:58 PM   #9
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we should talk.

I'm putting a turbo EJ25 into a 1982 911SC.

I bet we'll have a lot of the same issues. Where are you getting your subframe?
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Old 05-17-2005, 06:03 PM   #10
tonyakavw
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I'm actually going to fabricate my own engine mounting bar. The 914 engine and 911 as well (if I'm not mistaken) mount at the front of the engine, whereas the EJ25 mounts at the output end. So for the 914 you either have to make some crazy looking subframe, or just make a large U-shaped bar that goes around the engine and mounts to the suspension console on either side of the engine bay. The 911 might be trickier, might be easier, I'm not sure.

I'll PM you with a coupel URLs since I'm such a newbie here I can't post them..

-Tony
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Old 05-17-2005, 07:13 PM   #11
Matt Monson
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I am going to be doing the same swap into a '74 914 next winter. I am planning on a full custom exhaust, and actually using the turbo headers as a basis and going from there. The 225hp EJ251 that is currently in my RS should be the engine I am going to use. I am going to mate it to the 40mm Webers already on the 914. Should be a beast...

Kennedy Engineered Products makes mounts to weld in, as well as does the flywheel conversion. And this swap is going to be very different from putting one into a 911. Same concept, and a few of the same issues, but very different as well.

As for the topic of this thread. 3-5hp max for going catless...
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Old 05-17-2005, 11:18 PM   #12
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why would you go catless on n/a you need back pressure
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Old 05-17-2005, 11:35 PM   #13
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Quote:
Originally Posted by 99subi
why would you go catless on n/a you need back pressure
i do beleive that there is too much to begin with. so eliminating the cat(s) helps.
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Old 05-18-2005, 11:08 AM   #14
Matt Monson
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Quote:
Originally Posted by 99subi
why would you go catless on n/a you need back pressure
Depending on the spec of your engine, 2.25" or even 2.5" exhaust piping produces the appropriate backpressure. There's a lot more going on there to produce a tuned exhaust with proper pulsing, backpressure and flow than just having a cat in-line to produce backpressure...
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Old 05-18-2005, 12:46 PM   #15
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Cool project. Are there pics in the conversion forum?
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Old 05-18-2005, 12:59 PM   #16
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Quote:
Originally Posted by Matt Monson
Depending on the spec of your engine, 2.25" or even 2.5" exhaust piping produces the appropriate backpressure. There's a lot more going on there to produce a tuned exhaust with proper pulsing, backpressure and flow than just having a cat in-line to produce backpressure...
yes i see what your saying i have the apexi 2.25 inch catback but i dont think i would ever remove the cats
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Old 05-18-2005, 01:27 PM   #17
tonyakavw
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No pictures yet, but I'll start a thread over there soon.

-Tony
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Old 05-18-2005, 06:28 PM   #18
FuJi K
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It's about getting the gasses out, wrapp the WHOLE exhaust system and have no cats and you'll see good gains.
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Old 05-18-2005, 07:59 PM   #19
Honkey3k
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the stromung High Flow Cat and Catback gave an additional 12whp on the dyno...
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Old 05-19-2005, 11:14 AM   #20
Matt Monson
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Quote:
Originally Posted by FuJi K
It's about getting the gasses out, wrapp the WHOLE exhaust system and have no cats and you'll see good gains.
I think you are getting towards the point of exhaust tuning. The point is exhaust gas velocity. People always say, you need backpressure on an NA car. This is a misstatement. You need port velocity and some back pressure is a neccessary evil of getting that.

All of this is really mute for us 914 guys because all of the early cars were catless from the facotory, so we don't have an obligation to cat. The 1975 and '76 guys kind of got hosed because they got cats.

Going catless on an RS not only has very little gains, but if you ever get popped for it, it is a federal offense with up to a $10k fine!
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Old 05-20-2005, 08:18 PM   #21
FuJi K
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Yes, you gotta go big/extreme if you want the most out of the exhaust system in an NA car. Then again you need a little back pressure, just a little so it can scavenge out.

"It's only a TEST pipe, officer!"
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Old 05-21-2005, 12:15 AM   #22
BrysImpreza
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Go with either a Random Tech or Stromung High Flow cat, with my 2004 TS I felt a decent gain in power almost immediately after installing it.
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