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Old 05-30-2001, 10:43 AM   #1
Minnesota WRX
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Arrow Facts on the OEM WRX exhaust

I wanted to know how the stock WRX exhaust was made so I took the time to unbolt all of the heat shields and this is what I found...

Flange #1 flat at the turbo, 60mm outlet centered on the turbine, spot wielded heat shield - 2nd cat, ~90deg outlet - Flange #2 bell mouth on both sides, flat gasket - Cat #3, bell mouth on both sides with O2 sensor on inlet - Flange #3 held by springs and donut gasket - Resonator about the same size & shape as cat #3 - Flange #4 flat gasket, bell on outlet down to 50mm - Muffler twin 45mm outlets that bell to 50mm - twin 70mm detractive tips.

Some of my thoughts...

- That is a small turbo! But it is for a 2L engine. I'm used to aircraft turbos.
- The bean counters designed the turbo flange. "The Great Wall" causes the wastegate gas to hit the turbine gas at 90deg. Very bad for flow, but it does slow down the turbo which is the reason for the wastegate.
- Until you get to flange #4 the pipe is 60mm, then 50mm. Mandrel bent all the way.
- Because cat #2 is ~ vertical, the outlet bell is at ~90deg to the cat. Not the greatest for flow.
- Cat #3 can be removed or "cored" because the O2 sensor is in the inlet.
- "Sakamoto" (maker?) is stamped on the muffler.

The OEM flange is terrible if you are looking for flow with the wastegate open. But the wastegate doesn't open until a minimum of 9psi (wastegate spring) and with a new MBC/EBC/EPU the wastegate wouldn't open until a much higher psi. The flange isn't as terrible if looked at when the waste gate is closed as the pipe is centered on the turbine. And the whole purpose of the wastegate is to slow the turbine which the OEM flange does very well Without dynocharts, if I were to change it, I like the bell more than the twin outlet flange. The twin outlet still has to use a flat plate to weld the pipes to and with the area within the turbo this flat plate would hurt flow. The bell flange would continue this area and be much better for flow. The bell is made of much thicker metal (iron?) which would be able to take the heat better and not crack.

Replacing the 2nd Cat to move it's location to get rid of the 90deg outlet would probably do as much good as getting a high flow cat for flow.

I think every exhaust needs a slip joint or flex section. Make sure your aftermarket exhaust uses one.

Is the 3rd cat there just so the WRX could get LEV status?

Why does the exhaust go to 50mm at flange #4? To keep up the exhaust energy?

Does the STI muffler bolt to flange 3 or 4? I think #4.

Do we really need 3" exhaust? Flowmaster's '93-96 Toyota Supra Turbo single exhaust system is 2 1/4 or 2 1/2". 60mm = 2.4"

Is the turbo back aftermarket exhaust's better turbo spooling from the improved turbo flange and 1 less cat or getting rid of the muffler and the 50mm pipe?

How much improvement is a full turbo back exhaust over a cat back that gets rid of the 50mm pipe and muffler? .

Just me thinking out loud.

Todd
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Old 05-30-2001, 03:01 PM   #2
yuletak
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Have a Nice Day?

wow, lots of info! i'm not sure what it all means, but hopefully the turbo tuning books i've ordered will help.

i'll come back to the thread after i read the books.
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Old 05-30-2001, 03:55 PM   #3
Zahnster
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Wow great info. Would love to hear answers to these questions as well.

Yuletak: What books did you get? I might be looking for something similar.
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Old 05-30-2001, 07:16 PM   #4
Ginseng
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Excellent work Minnesota. That information is very illuminating.
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Old 05-30-2001, 11:37 PM   #5
Jan Shim
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Both the standard WRX twin-pipe and single "big bore" STi muffler bear the SAKAMOTO name. I cannot say if the STi Genome sports muffler is also the same.

Here in Brunei, exhaust is one of the very first mods to carry out and very quickly there's an abundance of stock exhaust from turbo back. The only way you are going to get power out of the WRX is regardless the size of turbo is to have a turbo back exhaust as big and as straight out as possible. Turbos do not need back pressure, the more free flow they are, the quicker they will spool up .. not forgetting also that the intake bit needs attention too.

One of the most successul manufacturer of exhaust from personal experience is BPM. Their downpipe flange and wastegate separator is just what the 'doc ordered-the stock downpipe with the flat wastegate plate messes up the flow of the exhaust gases causing a very turbulent flow. The better you can separate the exhaust gas from the wastegate gas the better.

We've taken one step further to BPM pipe design when we were preparing for one of the drags. On two STi's we had the BPM wastegate pipe cosmetically altered to vent to atmosphere-woo hoo, instant external wastegate The noise from the new externally venting pipe was sooo nice that the boys decided to leave it that way.
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Old 05-30-2001, 11:49 PM   #6
Zahnster
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In autospeed (?) there was an article for that Rigoli guy with the 10 sec Impreza was saying that exhaust with split wastegate and turbo are not as good as one with the large oval outlet. He had concerns about thinness of the center piece and how well it will hold up.

The BPM Twin Dump looks nice and shouldn't have the issue described above.
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Old 05-31-2001, 12:13 AM   #7
SilverSubie
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i'm definately gettin rid of the whole stock exhaust...i just can't decide on twin dump or single. the only reason iam looking away from twin is that i may want a turbo that dosn't use that setup...i'm not sure. a little confused.

-grant
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Old 05-31-2001, 12:40 AM   #8
Jan Shim
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The Rigoli WRX runs a custom turbonetics and a huge external wastegate, huge injectors, Blitz DSBC, 2.2L Stroker Kit @ 36 psi (limiting factor being the Blitz DBSC). That is not a street car and parts implemented in the project cannot be compared to what we mere mortals are putting in to our road WRX. At 36 psi of boost many things fail including clutch, driveshaft, gears, among other things but a "thin" centre pipe ? Whether the downpipe is a single outlet or twin, the centre pipe of a WRX is a separate pipe that's joined to the downpipe at the flange. It will be for others reason a centre pipe should fail.

Grant, whether a single or twin dump, the flange of the turbo exhaust housing is standard for any WRX IHI/TD bolt on unit. However, if you are getting a Garrett bolt-on kit from BPM for example, the custom exhaust housing already has the gas separator built-in (i have photos of these) so it's important you get a correctly matched twin pipe from BPM for the Garretts.

Unless you're moving away from standard setup and embarking on more exotic mods like having the turbo sitting at an angle so the inlet pipe to air cleaner is straight and run race gates, you're better off with the BPM unit. For anyone considering a single dump with a bellhousing or cobra head style dump, you will still require a gas separator. If it hasnt got on, you'll need to have one done up properly. Experiments by friends have shown this to work too but you'll need to put in a lot of effort to get the "tongue" fabricated properly for it to work.
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Old 05-31-2001, 01:01 AM   #9
Minnesota WRX
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What I like about APS's single bell turbo flange is it is made of ductile iron which can take the heat much better than stainless steel.

I have sean too many exhausts off of aircraft turbos that are cracked. It seams like every time you inspect an aircraft turbo exhaust you can find a crack.

I like the idea of using iron even if it is not the best for flow and is heavy.

Todd
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Old 05-31-2001, 10:03 AM   #10
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Iron will also crack, just look at some 2nd gen DSM tdo4 turbos and you will notice some small cracks in the exhaust housing.

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Old 06-14-2001, 11:30 PM   #11
Minnesota WRX
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The exhaust pipe turbo flange is a flat plate with a 60mm pipe welded to it centered on the turbine outlet.

The turbine housing has a "bowl" in it that has the turbine outlet and wastegate outlet in it.

The reasoning behind replacing the exhaust pipe turbo flange is so the wastegate gases don't disrupt the gases coming out of the turbine. This is eather done using a big mouth bell flange that continues the bowl in the turbine flange. Or using two seperate pipes with a devider to keep the turbine and wastegate gasses seperate until further down the pipe.

Remember the reason for the wastegate. It is to slow down the turbine. This is done by bypassing the turbine and disrputing the flow of gasses out of the turbine.

The aftermarket exhausts will flow better. But I have this need in the back of my head to know why the turbo designer designed the turbine and wastegate outlet in the smae bowl area? I think it is so the wastegate will slow down the turbine quickly when it is opened.

Todd
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Old 06-15-2001, 12:04 AM   #12
Yellow Rex
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Excellent info. I wonder if anybody out there has a drawing or diensioned sketch of the turbo flange?
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