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99 Forester L, 5 spd
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1,306 Posts
Discussion Starter · #1 ·
I have a stock 2.2l OBS with Supers most recent header. I know a lot of people here just run the aftermarket 3in WRX pipes. How much power do you feel you have lost due to the piping being too big and loosing backpressure?
 

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99 Forester L, 5 spd
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1,306 Posts
Discussion Starter · #7 ·
I would say stick with 2.5" as the max for an N/A build. After that your just adding unnecessary weight to an already slow car ;)
I totally agree with this, but I found a WRX 5zigen exhaust local that I can afford. The seller says it should be 2.75in piping, still too big but, not as too big as those 3in Ebay $130 exhausts. lol
 

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RIP 08.24.2013
01 legacy L
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8,015 Posts
could build your own with a welder and some 409 stainless. Tis what I plan on doing *shrug*.

The idea with building an exhaust is to keep exhaust velocity high while pressure buildup to a minimum.
The faster the exhaust can leave the car the more of a 'scavenging' effect you can benefit from at lower speeds.
 

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2002 Forester
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464 Posts
Size only matters when you're talking about the right parts of the exhaust. If you're talking about the primary piping coming out of the heads, yes, size is extremely important. Along with the intake manifold, heads, and cams, this helps control flow velocity into and out of the combustion chamber. Sizing will affect power band.

Then the primary pipes collect into the secondary section. This primary and secondary piping is important in terms of size and length to influence the power band.

The collector is typically terminated by a resonator. This creates a low pressure zone that terminates the tune length and defines a tune frequency for pressure wave tuning.

After this, size means very little. Bigger is better. You can even run without anything, and it won't matter. If the header is done right, dumping to free air after the collector will yield the most power.

Any restriction, ANY is bad for power. However you do need to size the heads, cams, and primary piping on both the intake and exhaust small enough in order to create enough flow velocity to scavenge and clear the combustion chamber. This means some backpreassure is automatic based on velocity needs. However, sizing only needs to be done around the combustion chamber.
 

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99 Forester L, 5 spd
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1,306 Posts
Discussion Starter · #14 ·
Agreed, great explanation mvw2. Thank you!

Looks like I am going to save my self some money and buy the full stainless ebay 3in. I should probably get new, longer hangers too.
 

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'MURRICA!
Hybrid Camry
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5,700 Posts
Here's the sad truth: no matter how big or small your piping is, your car is slow and you'll probably make a difference of <5hp (probably <1hp)

Buy cheap, save your money, swap sooner.
 

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99 Forester L, 5 spd
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1,306 Posts
Discussion Starter · #18 ·
Holy i-don't-know-how-to-search batman! You're worried about power loss in a 168hp n/a econobox? Get over yourself...

3", 2.5", 12" do what you want.
I did search, didnt find the answer I was looking for and so I asked. Also, I have an OBS, not an RS. I have a 130hp-ish econobox and would like to preserve as much as that power as I can since I have no plans of swapping this car. Finally, I have no complex about myself and you no reason to be a dick, sir.
 

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RIP 08.24.2013
01 legacy L
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8,015 Posts
No engine on the planet needs or wants or benefits from backpressure - backpressure is a restriction, not an attribute. The idea behind proper scavaging is the reduce backpressure, and increase pumping velocity and volume

The truth that often gets overlooked, when someone gets a huge exhaust and wonders why they have no low end torque it's probably because their exhaust gases are moving the same speed as your local swamp.
 
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