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Old 09-18-2013, 11:14 AM   #1
AMCjavelin74
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Default How good is the factory header (2.5 N/A)

I love my 2004 Impreza Outback (yup a wagon) and while it has an exhaust leak (up by the exhaust manifold) I figure if it's bad then upgrade it. Well, I thought I'd ask the Subie community "how good is the factory exhaust manifold?" as it looks to me more of a header, which is a HUGE improvement over some GM engines (I had a GrandAm with a 3400 and it had a log style exhaust manifold. Think farm tractor exhaust. Those things are JUNK for flow.) I know on newer vehicles the exhaust manifolds are actually pretty good (LSwhatever, and such).

It seems to me on a car that at most will see a CAI/WAI/something like that and probably a 2.25" exhaust the factory header looks to be pretty decent actually. I know the turbo cars see an improvement, but they're also moving much more air and fuel at WOT than the NA engines. (14.7psig/1 bar of boost effectively doubles the displacement. Actually it's 2bar or 29.4psia considering atmospheric pressure is already at approx. 14.7psia a=actual g=gauge for the ungeeky)

Thanks,
matt

Cliffs Note Version
My factory exhaust manifold has a leak in it (2004 2.5 NA). Debating on replacing with a stock replacement or maybe an aftermarket unit. How good is the factory manifold? I know on newer cars they're a lot better than the old style "log" manifolds found on farm tractors and such.
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Old 09-18-2013, 01:16 PM   #2
RaceFaceXC
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They are not terrible, and they are well tuned for the engine intended use. They do not do you any favors at high rpm, but neither does the intake manifold, heads, cams, etc. The 06+ factory exhaust manifold is an improvement.
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Old 09-18-2013, 02:38 PM   #3
AMCjavelin74
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Quote:
Originally Posted by RaceFaceXC View Post
The 06+ factory exhaust manifold is an improvement.
I take it it's a direct bolt-on I can make brackets and stuff like that but exhaust fab is a b(t$h!

(hint for home fabricators: stock plenty of cardboard, sharpies, and good boxcutter razors to make "prototypes". Then getting to know a local metal supply or fab shop (often they almost give scraps away) is good to source raw materials for DIY fab and of course skill for more complicated things)
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Old 09-18-2013, 05:11 PM   #4
RalliSport Racing
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Quote:
Originally Posted by AMCjavelin74 View Post
I take it it's a direct bolt-on I can make brackets and stuff like that but exhaust fab is a b(t$h!)
Not even close to a direct bolt on:
Wrong oil pan
Wrong cat pipe
Wrong o2 sensor locations
etc
All that plus the cost of a used 06+ factory header (usually around $250).

Borla replicas are around $130
OBX EL are around $300

Both are direct bolt on for you
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Old 09-18-2013, 05:58 PM   #5
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Quote:
Originally Posted by RalliSport Racing View Post

Not even close to a direct bolt on:
Wrong oil pan
Wrong cat pipe
Wrong o2 sensor locations
etc
All that plus the cost of a used 06+ factory header (usually around $250).

Borla replicas are around $130
OBX EL are around $300

Both are direct bolt on for you
Damn, I sold my used oem manifold for $80. Only had 20k miles on it
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Old 09-22-2013, 07:31 AM   #6
ericm555
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Here is a link to an EL header dyno.
http://forums.nasioc.com/forums/show....php?t=1031584
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Old 09-24-2013, 04:06 PM   #7
AMCjavelin74
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Old 09-27-2013, 03:03 PM   #8
RecoveringSaabaholic
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Quote:
Originally Posted by AMCjavelin74 View Post

Fairly certain that does not have a cat, but rather a resonator where the cat should be. Will cause CE lights to pop, and will not pass inspection in most states. Just FYI.
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Old 09-28-2013, 12:41 AM   #9
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An aftermarket header will hardly gain anything over the factory header... Except noise.

Now IF you're digging into the block, going with bigger cams, valve train work ect, then you may find some benifiets from a higher flowing header. But only in the High RPM ranges. For a low-mid range engine like the NA EJ25s you're not doing anything by changing the header except blowing your cash and making more noise.

If my stock header was shot, bent, rusted out, stolen, lost or whatever then I'd probably "upgrade" but only because after market headers are probably easier to buy now than stock ones unless you find a junkyard gem. I've spent thousands on exhaust systems over the years and all I've noticed is they make more noise not more power or better milage as you would think. Most cars have an exhaust that's pretty well engineered for the type of vehicle and engine under the hood.

Just my 2 cents
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Old 09-28-2013, 11:09 AM   #10
RaceFaceXC
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Quote:
Originally Posted by flyboynextdoor View Post
An aftermarket header will hardly gain anything over the factory header... Except noise.

Now IF you're digging into the block, going with bigger cams, valve train work ect, then you may find some benifiets from a higher flowing header. But only in the High RPM ranges. For a low-mid range engine like the NA EJ25s you're not doing anything by changing the header except blowing your cash and making more noise.

If my stock header was shot, bent, rusted out, stolen, lost or whatever then I'd probably "upgrade" but only because after market headers are probably easier to buy now than stock ones unless you find a junkyard gem. I've spent thousands on exhaust systems over the years and all I've noticed is they make more noise not more power or better milage as you would think. Most cars have an exhaust that's pretty well engineered for the type of vehicle and engine under the hood.

Just my 2 cents
Not exactly. Most cars come with a cheap to manufacture exhaust manifold that insulates or has a high capacity for heat so the cat works better. They also tune the manifold for the performance desired, usually great midrange efficiency, which is a compromise to higher rpm performance and efficiency. If a properly designed header that is tuned for higher rpm performance, like TWE or rallisport racing EL merge collector design, along with a good tune on the Ignition timing, decent gains can be found. Like 20 hp/tq across a large part of the rev range.
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Old 09-30-2013, 09:55 PM   #11
AMCjavelin74
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/\ My factory manifold is cracked, so I figure it's a good time to upgrade.

Quote:
Originally Posted by RaceFaceXC View Post
Not exactly. Most cars come with a cheap to manufacture exhaust manifold that insulates or has a high capacity for heat so the cat works better. They also tune the manifold for the performance desired, usually great midrange efficiency, which is a compromise to higher rpm performance and efficiency. If a properly designed header that is tuned for higher rpm performance, like TWE or rallisport racing EL merge collector design, along with a good tune on the Ignition timing, decent gains can be found. Like 20 hp/tq across a large part of the rev range.
I think you're pretty right for aftermarket exhaust. Some newer vehicles are a pretty good compromise of the whole cost/fitment vs performance and efficiency battles (LSx exhaust manifolds, most modern cars too). A lot of cars though have garbage for exhaust (ever look at the stock exhaust manifolds on a GM 3400v6? Looks like it belongs on a farm tractor.)

So, where the OEM cuts corners, the more reputable aftermarket companies put in the effort to make a quality product that offers benefits over stock. While I don't see 20hp in my n/a basically stock 2.5 as possible gains, I think there are some mid/top end to be found, and going with smaller primary tubes (the smallest that will not restrict the exhaust flow) there will be enough velocity for exhaust scavenging= keeping low end torque.
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