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Discussion Starter · #1 · (Edited)



This graph shows that exhaust area is not linear as you go up in diameter.

For example, if you wanted to match the flow rate of a single 4 inch pipe, you would need dual 3 inch pipes! So if your thinking that you want dual pipes, and are considering doing 1.5 inch, you should realize that you will actually flow more air by going with a single 3 inch pipe.

just food for thought.
 

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I think a great option would be to go from a 3" down and mid-pipe, to a dual exhaust 2.5" back section.
 

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Ive always wanted duals, but the only thin keeping me from doing it is tbat i do t know a good body shop around here to do that cut in the bumper
 

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Cool guys...
 

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I think this was brought up sometime ago some seasoned boosted guys swore all you needed was 2" or 2.25" to completely outflow 3", guess not huh
2x 2" pipes won't, but 2x 2.25" pipes will outflow a single 3" pipe.
 

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Yes 2.25 barely outflows a single 3", in fact I will say adding to the extra weight of the materials, more surface area for friction (Laplace law), I will call it a wash with an edge going to the single 3".
I can't argue that and I completely agree. About the only reason I'd opt for a dual exit is for purpose of packaging/clearance around the rear subframe and potentially lower noise.
 

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Yes, the volume of air flow of two 2.25" pipes is greater than one 2.5" pipe. The math is rather simple here. By useing two pipes and smaller diameter they A.) create a greater scavanging effect to speed up the exhuast gasses flow away from the engine and B.) keep down noise. There is a fine balance to making use of scavanging and noise control in a dual exhaust setup. What it really boils down to is there is not enough room under the IS's real axle. Yes two 2.5" pipes will fit under it but you must use mandrel bends, no Resonators and you can forget about lowering the car. I have had a true dual exhaust, it is loud, raspy and because of the number of bends in it a 3" single will move air faster because it is straighter with less friction than a true dual exhaust.

Here is a parts list if you choose to go down this path.
2.5 inch or 2.25"
1 X-pipe
8 45* mandrel bends
4 90* mandrel bends
I hope this help somebody save some time and money. God knows I have invested in my exhaust to learn what works best on the IS.
 

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speaking of getting underneath the rear axle...
This is my 4 inch Turbo-back exhaust, no cats, no muffler and only a resonator bhind the tip.
\
Car rides really low and have not scrapped it yet....

 
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