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Old 05-21-2001, 09:50 PM   #1
Scooby Guru
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Join Date: Jan 2001
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02 WRX WFC#040
Clean on 19's

Post What style downpipe is better? (Twin-dump or Single)

As in Twin-Dump like BPM's design:
<IMG SRC="http://home.att.net/~boxer4racing/Bpm/gtexhaustsystem.jpg" border=0>

Or a big Single dump that covers both the wategate and turbo like APS:
<IMG SRC="http://www.airpowersystems.com.au/wrx/housing.jpg" border=0>

Which design works better?
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Old 05-22-2001, 09:04 AM   #2
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2000 Impreza 2.5RS
Blue Ridge Pearl


I can't say I know for sure, tho, I'd think that the single inlet would have more area to flow thru which would be good...
but, on the other hand, the twin inlet might flow smoother...
that clear things up? ;-)
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Old 05-22-2001, 09:14 AM   #3
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I just had this explained to me this weekend. The twin dump provides a separation for the boost bled by the wastegate. The theory is to eliminate any disturbance to the flow of exhaust gases. Hence, a better flowing exhaust will decrease spool up time.
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Old 05-22-2001, 09:17 AM   #4
outback motorsports
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we sell the down pipe flanges for $20.2.5 in.with a 1 in.hole for the waste-gate. see ya steve
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Old 05-22-2001, 10:11 AM   #5
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Location: Austin, TX
2008 STI


For the 1st 8" off the back of the turbo, you want as little disturbance as possible. Any disturbance can give excessive back pressure and rob the engine of power. This will also hurt turbo spool up. By smoothing out your exhaust flow, the turbo will boost harder and faster.

Just for the record, in my DIY turbo setup, I am using the Outback Motorsports twin-dump downpipe flange.

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Old 05-22-2001, 11:56 AM   #6
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Don't forget about a hybrid design like MRT's.
<IMG SRC="http://albums.photopoint.com/j/View?u=246497&a=12990426&p=48971495&f=0" border=0>

I just installed my MRT 3" turbo back. I have some pics of the system and the install to put up as well as my thoughts on the system vs. others. I will be posting all this soon.
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Old 05-22-2001, 12:20 PM   #7
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Theoretically a twin dump with totally seperate lines the entire length ^_^

However in RL, twin with a couple of inches
seperate. On stock turbo cars this
really doesn't have much effect with most
stock turbos due to them running out of
gas at higher RPMs where the disturbance
would be felt most. On cars with larger
turbo wheels and good fill, it can be
felt immeadiately.

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Old 05-22-2001, 04:33 PM   #8
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Answering this question is one of the key understandings of design of downpipes.

Letís consider facts firstly....

A large Turbo exit suffers from the following shortcomings.

Gas turbulence as the gases meet thus affecting natural gas flow.

Generalizing that all turbos have the same cavity depth.

Generalizing that the turbo size does not affect the turbo flange design.

All the above are cost cutting measures to have a generic downpipe.

Moving onto Prodrive...

They are generally viewed as the benchmark most Subaru aftermarket companies strive for....

The utilize a Twin Dump Design with a larger diameter wastegate pipe than the BPM variant for the simple, fact that have better clearances on the WRC car.

There is a further superior method which allows superior exhausts/wastegate flow by not letting the wastegate pipe join the primary thus venting to atmosphere...This bypasses al the R and D required on "when to join the primary to secondary pipe"

Having the separator plate in the flange *exactly* match the turbo is extremely important. It is the accuracy of fit to the turbo that makes it work. The cavity depth of a TD 04 series and a VF series varies quite drastically.
The separator plate that keeps the wastegate charge from interrupting the flow of the main turbine exhaust flow must be made specifically for each turbo so that it keeps ALL the wastegate gases away from the turbine gases.

Minimum turbulence = maximum flow.

This points to exacting standards, and an accurate fit. This is the only true way to keep the wastegate gas turbulence from upsetting the flow from the turbo with an integral wastegate turbo.

The diameter of this wastegate pipe has been chosen through extensive R and D sessions and we will soon release the X Series System for race applications only which will showcase the latest technology in Twin Dump Design.

The BPM TD Downpipes are also HPC COATED for performance gains, elimination of thermal fatigue/oxidation and superior gas flow.

Hope this helps

Greg Nikolettos
ICQ # 85002510
Australia Ph 1300 850 BPM
International + 61 7 3272 8885
Facsimile + 61 7 3272 8045
[email protected] www.bpmsports.com
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Old 05-22-2001, 06:13 PM   #9
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Well I guess that answers it! Thanks!
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Old 05-22-2001, 06:21 PM   #10
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When do you plan to release this X-series downpipe and what are it's features over the one you currently offer?
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