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HKS piping (60mm = 2.36") is actually slightly larger than the Borla's (2.25"). But, when you look at the cross-sectional area of the piping, it starts to make a difference. The HKS Sport is only 10% larger in x-sectional area, while the HKS Hyper is actually about 29% larger. That's a pretty big difference. In my experience, I noticed more low-end loss from the 2.5" Thermal than I did from the Borla. I think, though, that the mufflers play a big role here. The HKS mufflers are baffled and packed, and thus not only suppress sound, but add back a bit of backpressure. The theory, as I've read and heard it, is that the NA engine needs a bit more backpressure at low RPM's to keep the fresh a/f mixture from being "over-scavenged" into the exhaust manifold during low-RPM valve overlap. The reduction in backpressure at the exhaust port results in over-scavenging, and thus a weaker mixture in the chamber. Someone also gave me the analogy of blowing out a candle through a straw. Try blowing out a candle through a really large straw. It just doesn't work. But, if you get the right size straw, you can be really effective at blowing out the candle. Also, I believe the stock NA exhaust is 2" OD.
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