Why PPP downpipe is not bellmouth design?
 

Looking at different downpipes available the PPP looks like it hasn't got one opening for the exhaust and wastegate gases as this: Zero/Sports Super Metal Down Pipe (Subaru Impreza STI/WRX)
but 2 different openings (divided wastegate?), one for the exhaust and one for the wastegate gases.
Could some post a pic of it and what is the reason behind this?
Cheers :)

MRT in Aus reckon that the twin port deign is better for exraction of the gases. Because having the wastegate's gases exit separately, and joining the turbo's gases later on about 1 to 2 feet furher down the pipe, is supposed to be better for gas flow i.e. alleviates turbulence.

Seems, too, that Prodrive know this (or have performed appropriate tests).

I have a 'divorced' twin exit 3" downpipe on my WRX.

Thanks.
My worry is about the 'flange' on the downpipe: will a downpipe from a 2004 Sti fit my 2007 WRX since the turbos are different design?
What downpipe do you have and what options are there for downpipes with sports cats apart from prodrive?
Reading this: CobbTuning.com - Exhaust Design ("Also, we have had a few situations where the splitter caused problems allowing the wastegate to function properly by not allowing it to open to its full extent, or even open at all. That caused either boost spiking, or no control over boost what so ever") it may be why prodrive didn't use a downpipe as part of the PPP in 2006+ cars (the reason being not able to control boost?)
Cheers :)

- The flanges will be of the same pattern, so will fit any OEM-fit turbo in the Impreza range (but not twin scroll items).

- I think mine is a Magnex :wonder: item (got it 2nd hand for c.£60 :D).

- That's a fair point by Cobb I suppose. If the port is large enough i.e. so the gate doesn't catch against the sides, etc., then I can't see it being a problem. I've had NO overboost issues and d/p has been on for about 8 months now. If you're worried about that, then by all means fit a genuine bellmouth design.

TBH, if you were to do a comparison of the two designs in back-to-back road tests, I bet you wouldn't feel any tangible difference whatsoever... which is what matters, I suppose.

- Seems I've done MRT a disservice. They manufacture this type, which is a good design..

Split Bell Mouth This design separates the gasses in the beginning of the turbo outlet and joins them at the rear of the bell mouth section. It works well and has some of the advantages of the bell mouth and some of the advantages of the divorced wastegate designs. The main deterrent for this is the cost and complexity of adding the splitter. I am a fan of keeping things as simple as possible while still making the product work well.


- Looks like the divorced item is a bit of a mixed bag then :wonder: (green = good; red = bad)...

Divorced Wastegate Keeping the gasses from the turbo outlet and wastegate separate until farther back in the system is an attempt to combine the advantages of not collecting the gasses and the real world. Combining them far back is closer to optimal than collecting them closer to the outlets. It is also critical to power production and spool-up to join the pipes smoothly and avoid turbulence. The disadvantages are that you add a lot of cost and complexity. You have big temperature differences on each pipe and that makes for a system that can crack. Putting in flex or expansion joints helps, but adds even further complexity and yet another part to fail. With all of the exhaust systems we have tuned with on the dyno we have seen that it is generally harder to bring boost on as quickly with these types of systems as compared to the bell mouth type systems. Perhaps it helps the wastegate function too well. Also, we have had a few situations where the splitter caused problems allowing the wastegate to function properly by not allowing it to open to its full extent, or even open at all. That caused either boost spiking, or no control over boost what so ever. Since the wastegate could not function the turbo ran as if it did even not have one, and the poor turbo just ran whatever boost it could make uncontrolled. The fix was not hard, but the least amount of stuff to go wrong the better. I know that I would not be happy having to pay for someone to install the exhaust only to have another place diagnose the problem, remove the exhaust, repair the part, and re-install the exhaust.

Bugger. :lol1:

Just get a proper bellmouth mate. :)