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Does anyone know the internal or external diameter of the factory standard exhaust downpipe?
I did search but either I somehow missed it it wasn't there. Thanks in advance.
 

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its not just the diameter, but the constricted flow coming off the turbo.

Take a look at the photos in the GMS exhaust thread here:

Post 1 shows the cat/downpipe from right where it comes off the turbocharger. It does a 90 degree sharp bend and is constricted right there, followed immediately by the catalyst.

There is good description there as well. I suspect the dimension is given (likely around 2" to 2-1/4" but I am not sure) somewhere in that thread.. But it's the shape as much as the diameter up there that is an issue.
 

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Does anyone know the internal or external diameter of the factory standard exhaust downpipe?
I did search but either I somehow missed it it wasn't there. Thanks in advance.
Pretty sure it's 60mm the down pipe replacement I put in from Scara is 70mm.
 

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The idea behind the stock design is to get the engine up to operating temperature quickly. That helps reduce emissions and that is important from a compliance standpoint. Ideally you would have the turbo a bit farther back and larger diameter pipes but those fly in the face of keeping emissions down.
 

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Correct. 2.25'' or 60mm diameter, 800 cell, which is spot on for 240hp, but the problem is the bottleneck in the upper part of the stock cat which is designed to be the bottleneck to achieve high temps and consequently superb emission control at the cost of performance.

More detailed discussion on stock exhaust parts available here: GMS - Modular Exhaust System 3''
 

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I guess because I still haven't managed to get the fricking cat off the turbo...
It is 2-1/8" (54mm) at the bottom flange of the cat, though.
 
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