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i know some people that can make me a 3 inch exhaust to a muffler no cats at all. This meaning a full setup from the turbo all the way back and an upipe . I was even thinking of making one myself but they can probably do it for under 200. My question is is 3 inch to big and also how much of a benefit would i get from having no cats at all? I know some people that will pass my smog for 50 bucks but do they check at the track? maybe i can make a fake cat?
 

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I was considering a smiliat setup. the mechanic said I'd be better off putting flanges on either side of the pipe where the cat SHOULD be and run a pass-through cat to keep the sensor from burning up...also when you have to get the car inspected you just take off 4 bolts, put the cat back in, pass, then take it out again.
 

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um...I wasn't sure about that either...isn't there an issue with no cats and a CEL? maybe the pass-through cat keeps the CEL from tripping?

PS - do you lose torque with a 3" exhaust? before the turbo kicks in? I'm still trying to decide between 2.5" or 2.75" or 3".
 

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blarg said:
um...I wasn't sure about that either...isn't there an issue with no cats and a CEL? maybe the pass-through cat keeps the CEL from tripping?

PS - do you lose torque with a 3" exhaust? before the turbo kicks in? I'm still trying to decide between 2.5" or 2.75" or 3".
The "loss" of torque is exaggerated, IMO. Depending on the size of the turbocharger, the [spool] lag that is inherent to the turbo is often mistaken with such perceived losses. Where legally permitted, going 3" or bigger and catless maximises the turbo's flow characteristics. So much so I have noticed the propensity to overboost easily. I have conducted tests between two WRX .. one with VF24 + factory exhaust (note: heavily backpressured and catted) and another car with VF24 + 3" turbo back exhaust no cats. The one running stock exhaust never experienced any overboost while mine, always overboosts in higher gears. This is another story altogether.

Coming back to my original note, a 2.5" or even 2.75" exhaust system (assuming completely catless) in GENERAL will cause top end to suffer. The restriction pre- and post- turbo are key considerations to have good top end pull. No good if you're running a restrictive air filter with a 3" exhaust as this will compromise flow effectiveness of the turbocharger. You have to be realistic also in your goals. The stock TD04 isnt likely to give you kick ass torque at 6000 rpm compared to a higher flow rated turbo such as the IHI VF series for example.

Regarding the exhaust diameter, I have noticed that the Japanese like to advertise their 2.7" or even 2.8" something pipes as 3" .. take a caliper to your Jap exhaust next time and you'll see this is true. The BPM Twin Dump downpipe I had was a real 3"

Hope this helps!
 

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I'll second the 3.0" vote. Check out the book "Modern Engine Tuning", in the last paragraph of the exhaust chapter, the author goes on to mention the improvements that a 3.0" exhaust makes to the wrx (albeit an older version).

Also, anyone that doesn't recognize the name above, remember it, his word is the gospel.

jb
 
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