The Bellmouth downpipes VS Divorced Wastegate discussions have animated many forums over the years. Well, we have a winner here. Drum rolllll. Bellmouth Divorced Wastegate. Sounds weird ? In fact, each type seems to have dynosheets proof that they are the best choice. COBB Tuning makes Bellmouth, PERRIN Performance makes Divorced Wastegate, Invidia are Bellmouth with Divorced Wastegate plate and they've all prove they can be a good choice when it comes to performance. Designedly speaking, the Divorced Wastegate seems to make a point in creating a smoother flow in exhaust gas. Internal wastegate being at a 45 degrees angle when fully opened, it makes sense that creating a division in the downpipe will promote less turbulence in the flow of the exhaust gas which should lead to extra torque and HP. On the other side, the Bellmouth design also make a point when it states that putting a divider in the middle of the flow will only create restriction in exhaust flow. After all, why are we so empress to delete the tumblers on our Subaru ? To make more flow... Hey, but there is no cross path flow there, if there was maybe we should keep the tumblers....doh :)
In fact, both design have proven they can achieve good results and the complete setup is more important that the design of the downpipe itself.
DTM world are on the side of DWG, many Subaru builders are more prone to the Bellmouth design. So which one is really better ? We've run the COBB downpipe on a WRX265 and was very pleased the result. We run the Invidia DWG on the STi 12 and are really pleased too.
So, as for most of performance parts, the choice is not obvious, but there're some things that you should always consider when making your decision.
- Talk with your builder/tuner about the options. He will probably be more comfortable and experienced with some setups than others.
- Get a good builder/tuner, otherwise it makes the previous statement pointless :)
- Try to avoid very cheap downpipes, while they can work, they tend to have more issues with fitment and longevity. Reputable brands tend to have thicker flanges and better material while providing better fitment.
- Consider your goal and complete setup. Will you bolt on to the stock catback ? Do you want to go external wastegate later?
- Make some research of your own, we'll never consider that we have THE truth :)
Also, be comfortable with your choice. YOU drive the car. Different setups can lead to totally different cars. Our stock turbo WRX265 was a monster when coming out of the box, but cruelly lack top end. Our actual 2.35l STi 2012 (yes destrocked 2.35l) is smoother on the neck from a stop (although it can be pretty nasty too) but would flyby the WRX anytime on the highway, not even close, still holding HP at 8500 RPM.
And most of all, have fun tuning your car, cause in the end, that's what it should be all about!
See ya soon!