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Originally Posted by SeanWRT
Heard a while back that you guys are making a catted downpipe as a stock replacement. Is that still in the work? Is there an option of going 3.5" outlet to fit your catback system? Do you use HJS core, most importantly for a catted system.
I like the idea of increasing diameter a little bit for more power. Few months back we actually tested how N55 catback pipes diameter effect power at 450whp+. Bigger diameter definitely helps.
At the rear end where one pipe split into two, rather than two 3.5", actually two smaller pipes (still bigger combined than single pipe upstream) are preferred, as exhaust flow gets cooler and shrinks as it travel thru pipe so less cross section is needed to maintain velocity, though smaller pipe makes less decibels. What's more is this system bends radically, twice, right before muffler while most competitions bend only once. Two bends hurt flow, which may or may not make a difference.
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We are working on a catted downpipe and it's currently in longevity testing. I can't say exactly which cat it is right now but it's not an HJS core. We do plan on having an option for it to mate in to our 3.5" system.
As you know, volume is directly related to velocity and velocity is what's important which is why the two smaller pipe design you're referring to works. That said, two smaller pipes with a combined volume larger than a single 3.5" will create more friction because of surface area. You'd be better off stepping to a smaller single pipe if you were really concerned with exhaust velocity at the rear of the exhaust, although I can see that hurting top end power eventually. We do have two bends at the rear, one with a tighter radius than the other as pictured below. At the rear of the system the exhaust is much cooler and therefore a bit slower as well, so we haven't noticed this being an issue at all. On a turbocharged engine, power is made at the turbo exit (downpipe) and beginning of the exhaust system when exhaust velocity is high.