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Discussion Starter · #1 · (Edited)
I was talking to another member last night about downpipe design and I realized that I didn't know enough about the different designs.

Bellmouth, Non-Bellmouth and twin dump designs and the advantages and dis-advantages of the desgins.

Anyone want to help me fill in the blanks? Like how these designs differ.

Why didn't Subaru put a non-bellmouth design on the stock downpipes?

I think this would very informational for not only ME but the rest of the members.

Kurlee Daddee
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Doh! heh

The stock downpipe does not use a bellmouth design...instead, it just has a flat plate that blocks the wastegate, just like how the HKS downpipe is designed. From what I've heard, this hampers performance because the wastegate is blocked, thus leading to boost spikes and inefficiency. However, it's easier to make a downpipe in this fashion, which means that downpipe without a bellmouth tend to cost much less and since since follows the stock design, if you have no plans to upgrade your turbo, it's generally not TOO bad of a design.

A bellmouth on the other hand creates a bell shape (obviously) so that the wastegate does not get blocked, but instead joins the rest of the exhaust and goes down the pipe. That makes it more efficient. The extra work in making the bellmouth makes these downpipes more expensive. If you're going to upgrade your turbo anytime soon, stick with a bellmouth.

As far as a twin dump goes, I'm not sure, but instead of a bellmouth where it all goes into one pipe, the twin dump has a separate pipe for the wastegate that joins the exhaust farter down the pipe instead of at the turbo flange. I suppose this would help in creating a better flow and less of a bottleneck, but I don't know for sure. Obviously, a separate pipe for the wastegate would make this design even more expensive.

I hope this helps for now, Kurlee!
 

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I have the TXS downpipe and it has a huge bellmouth (I can stick my fist in the top end!). The waste gate deflectors and twin dump I think have their merits - having two flows at right angles cannot be a good thing, but there is never going to be LAMINAR flow coming out of a turbine and valve opening!

I would say that anything is better than the stock 'wastegate wall' design, but the debate between bellmouth, wastegate deflector and twin dump is open for interpretation and probably in the noise.

THe main thing is that you have the right tuning to match the unrestrictive hardware. I will try to search out the article, but in Australia a mob did a fairly scientific look at bolt on mods to see the HP gains - the turbo back exhaust by itself (without tuning) did not yeild that much (when compared to what could be done with the exhaust AND tuning).

TXS is getting great HP and torque with theirs. MRT and AVO have twin dump designs and are getting great HP and torque as well.

In the long run - I would have to say go within your budget and see who has good customer service!!

For the record I am very happy with my TXS stage4
I did the staged approach from 1 to 2 and then 4 and the stage 1 to stage 2 jump with the downpipe was the most noticeable for ease of the mod. It felt like the turbo was thanking me!!!

I'm sure the similar mods with other hardware and the appropriate tuning would yield similar results.

I hope this helps in some way.

Andrew
 

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Discussion Starter · #6 ·
Thanks you guys. This info is starting to help me understand the different designs a bit. I am going to install my TurboXS Stealth back tomorrow morning, but was just wondering how the different designs differ.

Talking about the wastegates. Does anyone have any pictures of the back of the turbo and what or how the wastegate opens inside the the turbo. Any way to help me understand the operation of the wastegate and how it would somehow effect exhaust pulses or something of that sort.

Keep the design discussion going.

Kurlee Daddee
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I have tracked down one of paces pictures of a VF30 turbine and wastegate.

The turbine is obvious - the shiny disc to the bottom is the wastegate. it effectively allows exhaust gas to bypass the turbine and hence not spin the turbine (and consequently the compressor) as fast. You can see by the design of the actuator that it will shoot the gasses towards where the gasses are exiting the turbine as well.

Hope this helps.
 

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the stock design is just plain lousy. The reason a stock wrx falls flat on its face at 5500rpms is directly attributable to the stock downpipe design and the backpressure and turbulence and power loss caused by the flat plate on the wastegate side.

Replace the down pipe with any of the other designs and voila! power is there to 6500+rpms. replacing the stock downpipe and the upipe=the single biggest power improvement you can make on the car without having to increasing boost.
 

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Twinz - I would have the disagree with you there - the TD-04 just runs out of breath at the top end. It cannot physically move enough air for the engine to make the power. I had TXS stage 2 which has a great downpipe and the stock turbo - it still ran out of puff at high rpm.

To illustrate - here are some more pics thanks to pace.
Compare the size of the compressor housing.
 

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aussieinstlouis said:
Twinz - I would have the disagree with you there - the TD-04 just runs out of breath at the top end. It cannot physically move enough air for the engine to make the power. I had TXS stage 2 which has a great downpipe and the stock turbo - it still ran out of puff at high rpm.

To illustrate - here are some more pics thanks to pace.
Compare the size of the compressor housing.
Exactly. Even on my dyno with stock turbo, the car keeps making power until the boost starts to fall off. The turbo simply can't push the air (can only hold ~13.5-14psi at redline).
 

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TypeC said:


Exactly. Even on my dyno with stock turbo, the car keeps making power until the boost starts to fall off. The turbo simply can't push the air (can only hold ~13.5-14psi at redline).

Interesting. I have had quite the different experience but that may be attributable to the exhaust mods I have done. I had HKS headers, perrin up-pipe and MRT turboback exhaust installed all at the same time. no other modifications whatsoever, stock boost. The results where very impressive. the car pulled hard all the way to 7k rpms where before the exhaust mods it fell flat at 5500rpms. The same car I had raced before doing these mods would walk away from me when I was flat out in 4th once I hit 5500rpms and the power would just stop. After the exhaust components installed, I pulled him and the power pulled all the way to 7K. It was a total night and day difference in power on the top end. I think the stock turbo can flow better than your giving it credit for given proper exhaust flow mods, but I certainly cant argue with your dyno results.

Maybe the headers are the key here!;)
 

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aussieinstlouis said:
Undoubtably the mods you did there allowed you to extract the best from your TD-04 and compared to stock it feels a lot better - but wait till you try a bigger turbo :D :D :D :D

.....come.....be one of us!!! ;) :cool:

I have a Vf-34, sti injectors, walbro pump sitting at home just itchin' to be put on!!!! ..taking out my TXS unichip this weekend to ship it back for reprogram and then everything goes on the last week of this month..cant wait!!!!:D :D :D
 

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Verdugo said:
...the twin dump has a separate pipe for the wastegate that joins the exhaust farter...
haha ----- verdugo said "farter". teeheehee!


lol anyways, actually i don't know how much of a difference the bellmouth vs. flat plate exhaust is. when i say this i'm referring to when you've already got the cats removed from your car - on your exhaust, the biggest restrictors first and formost are your cats, those gotta go

- so while i agree that the bellmouth/twindump design is better, i dunno if its WORTH the extra money considering how you'll notice significant gains just by running the car catless (ie hks downpipe which has stock design but is 3" and completely catless)
 

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- so while i agree that the bellmouth/twindump design is better, i dunno if its WORTH the extra money considering how you'll notice significant gains just by running the car catless (ie hks downpipe which has stock design but is 3" and completely catless)
This is exactly what I'm trying to figure out. Titan is selling the HKS for $150 right now! This is like $300 cheaper than some of the Bell-Mouth designs on the market. I WILL replace my turbo down the line, but I'm looking at keeping the HP limit to about 300AWHP. WILL the HKS work? It's 3" mandrel bent, eliminates 2 cats, and bolts up to aftermarket Cat-Backs. So what if it's not Stainless Steel.

I understand that the flat-plate design is not AS good, but like Penguinking put it, the CATS are the MAIN restrictions, and getting rid of those is the biggest concern, how much HP actually would be lost through the Flat plate design? Are we talking 10HP, or like 30-40HP? Anyone know?

-Jason
 

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I'll go out on a limb and say no you won't be able to hit 300 whp with the HKS unit and a turbo of VF30 sizing.

Example: With a setup like the TurboXS stage 4 kit they are doing 270 hp at the wheels with the 4" bellmouth. You can add enough other components to get to 280-290 pretty easily but that last 10 hp is pretty hard and EXPENSIVE to do. At this kind of power level I'm guessing a downpipe like the HKS will cost you 7-10 hp (at least) compared to the TXS 4" downpipe. To make that hp up in other ways is going to cost you MUCH more then the $400 or so difference in just getting the 4" downpipe now.

So while it might be possible, it won't be logical to do. Does that make sense?

Carlo
 

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yeah, i understand that as you get into the upper eschelon of power mods, the little things start to matter more and more. for me, though, i'm bound more or less by my budget, so however much power i can get for X amount of money will be greater w/ the HKS dp than with some fancy turboxs or vishnu design. mainly b/c the money saved w/ the hks dp will go into my VF22 turbo fund, which, coincidentaly, is now open to contributions :D

-Robert
 

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I'm right onboard in the same boat as PenguinKing, the $400 dollars would be saved for my VF23 fund. Which may be the difference of being able to afford a new turbo and not.

If I fall shy of my goal by 7-10HP, I wouldn't consider that to be the end of the world...the car would still be faster than hell. And while a Vishnu Twin-Dump is a peice of art, it wouldn't give the the same HP as a new turbo, and at like $600-$700 for some of the Top-of-the-line DP's, I wouldn't be able to get a new turbo for a VERY,VERY long time, if ever.

The HKS mated to the Blitz Nur Spec will give me a 3" mandrel bent, Totally catless Turbo back, for about $700! (And will sound Bad-A$$)

THE CHEAPEST Bell-Mouth DP I've been able to find has been the Modified Stromung DP that is being offered that MisterX has. The price quoted on this DP was $350 after modification. (the regular stromung is a flat plate design as well and sells for about $250). And this pipe only gets rid of 1 cat, wouldn't the removal of the third cat make up for the difference of flat plate and Bell-Mouth? (At least in sub-300HP applications)

-Jason
 

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good point -

although the vishnu dp, for example, is incredibly expensive and has the twin wastegate-dump design, it only removes ONE cat, while the hks gets rid of TWO. so unless you go w/ the full vishnu exhaust (or really any exhaust that'll get rid of those last two cats), you'll probly get as much if not more hp by going w/ the hks dp.

its those cats in the exhaust pipe that are so darn restrictive. its a shame that someone had to be so cruel. maybe u can find a good shelter for them once you take them out of ur exhaust.
 
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