Hi all.. first time post so I hope it’s in the right place 😀

I’m looking to design and build my own down pipe for a Jdm hatch sti. Ive done so much reading up on open vs divided vs divorced and people saying there is a difference but it depends on a few different things.
Now, I can understand that depending on what turbo you’re using the bell mouth seems the best option vs the divider type down pipe due to what the back of the turbo looks like ie the vf35 has an open back where the divider sits so I can’t see that it would make much difference with just that thin divider over the open mouth down pipe.. verses the back of the vf49 having two separate compartments separated by a physical wall and gasket for turbine and wastegate enabling you to actually separate turbine and wastegate properly.. to create pipework that mimics an ewg if you like.
What I’m considering building is a sports cat downpipe where the turbine will be fed into the sports cat via a 2.25/2.5 tube and the wastegate into a separate similarly sized pipe that will enter back into the exhaust past the cat in the bottom of the downpipe theoretically dividing the flows of gases and mixing them further down the exhaust where turbulent air shouldn’t be as much of a problem?
the idea is to lessen the restriction of the cat by only using it for the turbine side of the exhaust which should still enable it to pass an mot as the waste gate shouldn’t open (letting none catted gases from the wastegate into the exhaust) while sitting there not under any real load from just being rev’d in the mot bay..
this in turn would relieve any back pressure from the back of the waste gate and possibly give better control of the wastegate??

thoughts?

theoretically this should be the optimum use of the cat and wastegate shouldn’t it?

be gentle people 😄 cost isn’t an issue as I’ll do the work myself

 

This is what I mean by the backs being different, with the 49 you can actually properly separate each side of the exhaust as apposed to the 35

 

My rcm500 twinentry has a wall ,my milltek downpipe is open neck one,
493 horses on vpower no issues/ restrictions at all being 3" later on.

 

Thanks for the reply.. I’m not saying an open mouth wouldn’t work, more that while I’m doing it I’m going to go way over the top and over engineer it if you know what I mean 🙈 hoping someone who does this sort of thing might chime in 😬👍🏼

 

I think one of the jap manufacturers builds a downpipe with two distinct tubes that blend.

 

Hi, yes I think I’ve seen that.. they merge quite close to the turbine and the cat, I was thinking of bypassing the cat with the wastegate pipe

 

Yes hks makes it. Not sure it is worth a hassle.
You need to check if You have enough space for a pipe bypassing a cat.

 

How mine would differ from that is I’d reintroduce the waste gate pipe passed the cat at the bottom of the downpipe, one to reduce back pressure from turbine to the cat and waste gate, and to eliminate turbulence on the turbine wheel

 

All depends from turbo You will use.
Doubt there is any turbulence in open neck one. My graph looks so good tbh

 

Anyone else have any insight into downpipes?

 

Yes Sherlock