CTS HFC - did I oops?

[Dfraney]

Ordered a CTS high flow cat thinking it was the cat + downpipe. Turns out it was just the cat (site called it downpipe). With that being said, will I need a 3” downpipe for this cat? Any recommendations on downpipes? Apparently it’s a direct stock replacement which leads me to believe a downpipe isn’t required.

I do plan on routing a custom 2.5” exhaust, is it “safe” (I.e. no restrictions) to run the HFC to the stock 2.25” downpipe to a 2.5” exhaust? Tuner states upgraded downpipe isn’t necessary for stock turbo.

[grayjay]

Not "unsafe" to run exhaust through a 2.25" bottleneck but it somewhat negates the reduced backpressure benefit of the HFC and 2.5" exhaust. 3" downpipe from cat leading to 2.5" exhaust can be a benefit since it helps promote turbulent (non-laminar) flow and since there is more flow restriction immediately post cat due to the increased pressure resulting from the heat generated by chemical breakdown through the cat and before exhaust has any time to cool. Smooth megaphone shaped transition between the 3" to 2.5" is also better than an abrupt transition.

[1.8Turbo4me!]

go check out Techtonics Tuning 2.5" downpipes. I run that with my 034 HFC.

https://techtonicstuning.com/product...18t-downpipes/

[Dfraney]

Not "unsafe" to run exhaust through a 2.25" bottleneck but it somewhat negates the reduced backpressure benefit of the HFC and 2.5" exhaust. 3" downpipe from cat leading to 2.5" exhaust can be a benefit since it helps promote turbulent (non-laminar) flow and since there is more flow restriction immediately post cat due to the increased pressure resulting from the heat generated by chemical breakdown through the cat and before exhaust has any time to cool. Smooth megaphone shaped transition between the 3" to 2.5" is also better than an abrupt transition.

Gotcha. This was the answer I was looking for. Thank you.