Hi guys, sorry for bringing back a dead thread, but I found some information on the "semi-bellmouth" design downpipes that I think might be interesting for some of you.
It's a post from the fabricator at Lachute Performance, its in french but the pictures definitely speak for themselves.
Montreal Subaru Club :: Voir le sujet - Comparo "semi-belmouth" Lachute VS OEM **Photos**
First picture is to show the difference between OEM vs Semi
Second shows the wastegate door fully opened and again OEM vs Semi with fully opened wastegate door.
Basically what he is saying is that Lachute Performance always went with the "semi-bellmouth" design because after all the dyno tests that they did, it NEVER made LESS power than a traditional "bellmouth". And that if they would have seen a power loss from that design they would have changed it a long time ago.
***Also mentioned that if they didn't see any difference it doesn't mean there is none. They just think that the difference between the two is nonexistent and can be neglected.
Now the point that they always defended is has stated above, that the wastegate door opens up and deviates the gases in the exhaust flow. Making the semi vs bellmouth equal, unless used with a TWIN SCROLL turbo, because the wastegate door opens up the other way.(then a full bellmouth would be more effective.)
If you look at the third reply there's a picture of a VF37 twinscroll.
I hope maybe some of you learned something from this. I know i did!