Originally Posted by
EvolutionArmory
No doubt. Big enough to the point of diminishing returns. And I agree with you totally that a turbo needs to push more air out before it can get more air in.
The question is, does 2.5 cut it once you go above K03 power and 3 inch is the diminishing return on a turbo that small? I think 2.5 will flow K03 airflow just fine but not be able to handle upper RPM airflow as efficiently as it should on a K04 or bigger.
I think my car would benefit from a 3 inch system. Does my spike in N75 duty at 6500 support my theory in your opinion Jake?
Yes for optimal turbo performance you want an exhaust big enough to where it has the same restriction as no exhaust. For most people this unrealistic as for as noise and how big of pipe you can fit where. People oftentimes get confused because they hear people say that your engine needs back pressure from the exhaust or you lose performance which is somewhat true, but not at all for turbo cars because the engine gets all the back pressure it needs AT the turbo, which is why anything after the turbo needs to get out as fast as possible, which is where larger exhaust comes into play.
Your car would definitely benefit from a 3" over 2.5" however most of torque gains to be had will be down low, as the turbo can ramp up much faster and harder without the restriction. If I remember correctly, Derek picked up 25wtq when using his electric cutout at the DP.
The spike in the n75 duty is because of the high RPM. The higher the RPM of the engine, the more CFM the engine is flowing, thus the turbo is needing the wastegate fully shut in order to maintain the requested boost. So by going to a larger exhaust, you may actually see the n75 spike, even more, do to the efficiency of the larger exhaust. Not that its a bad thing, as long as the software and turbo are working together to meet requested vs actual.
The flowing of the exhaust is the reason we did the Downpipe-Y. A lot of aftermarket downpipes had used the stock sized flange, with a large pipe on either side effectively making it a cork. We made flanges with a true 3" ID and included gaskets with a larger 3" ID so that there was no cork. The downpipe then splits into dual 2.5" when used with an S4 system for most of the length of the exhaust providing as much exhaust flow as you reasonably can. We knew we could use the S4 2.5" systems (like Fast Intentions) because we had already made much more power using that same system on the S4.
The dual 2.5" from the S4 is equivalent to a single pipe that is just over 3.5"
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a blanket also. Let me 
your results when you do 
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