What's the purpose of a splitter in a downpipe?

[Matt Monson]

I've got someone on another board trying to convince me that the splitter in an aftermarket DP is just a marketing ploy that does nothing for improving the performance of the turbo. What are your thoughts on the subject?

[vrg3]

It helps reduce turbulence, since the flow from the wastegate is separate from the flow through the turbine. The ultimate realization of this is the implementation of a separate exhaust system for the wastegate.

Cobb Tuning's web site has a good writeup about downpipe design, I think. It was linked to in another thread. I can't seem to find it at the moment though.

It is true, though, that most aftermarket WRX downpipes tend to give performance numbers within a few horsepower of each other irrespective of design, and that divorced wastegate downpipes cost a lot more.

[Matt Monson]

I am quite familair with the Cobb article and several others. I actually refrained from including any of my own tech in this thread to avoid making any suggestion that others agree with me as to why it is a valid design feature.

[legacy92ej22t]

I think it makes a big difference. I'd put my CES twin dump with BEP (Boost Enhancement Plate) up against any other DP on the market.

The OEM design is crap for flow, the Bell mouth is better but very turbulent, the bellmouth with divider is a bit better but the twin dump w/ a seperated wastegate dump pipe rules all. With twin dumps, the farther back the wastegate gasses reenter the main dump the better for flow too. Because of this, I think a CES twin dump will out perform others like the Perrin twin dump which has a wastegate dump that reenters the main dump within a few inches of the flange. I would put that type (the Perrin TD) more in the class of a divided bell mouth personally.

[BAC5.2]

It would seem that if the wastegate exhaust could re-enter the exhaust flow after a bend in the exhaust, then that would negate any turbulence of exiting before the bend.

Get what I'm saying? Maybe it's the lack of sleep talking

[stillkckn]

The way I understand the wastegate/main flow separation issues is, as follows:

1. Wastegate exhaust flows with higher pressure (and lower volume) than the main exhaust from a turbo.
2. The stock exhaust system (with the flat "deflector" plate opposite the wastegate opening) was designed WITH THE INTENT TO REDUCE BOOST as quickly as possible, since the role of the wastegate is to do just that.
3. If a baffle is inserted into the turbo housing (whether welded to the DP or as a separate piece, like the BEP), that turbulence and backflow will no longer affect the primary, and not limit power (nor kill boost as quickly!).
4. How you manage the wastegate flow, once you've cut it off from the main pipe, is a separate issue, but it makes sense that you want to reduce the possibility for reflected waves of pressure getting back to the turbo, if you want to remove that limitation.

As to the question of all designs resulting in the same HP gains, I don't think that those results are reflective of a fair comparison of the POTENTIAL of each of these approaches. For example, it may be possible that simply swapping exhaust systems and running dyno numbers, with a given car, given day, given ECU setup, etc., all the numbers might well come out nearly the same (within the reproducibility limits of the dyno), but that might well be the result of ECU-induced limits, and have little or nothing to do with the differences among the exhausts.

We (I'm with MSR) spec exhausts with ceramic coatings and TIG-welded joints, all 304 stainless, etc. These all add cost, and they don't necessarily add one bit of performance advantage over a mild steel, MIG welded system, if you're only going to run a five minute test. If you wan't to keep that system for years.....


All comments wellcome.

[rsstiboy]

this is why they have a splitter and why some have a bellmouth:-

in rally cars they need a wide band of torque, to acheive this they split the wastegate gasses from the exhaust gases to help with this. because it is more for on/off throttle applications, and use more of the rev range.

in drag cars where you need max horsepower, cars use a bellmouth design because when fullboost is acheived you dont want backpressure going into your wastegate splitter pipe, hence creating more power.

if i did my car again, and still might when i change my turbo, i would go for the splitter design, its far better for street applications.