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Old 03-10-2010, 09:06 PM   #1 (permalink)
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Default OEM catless vs. aftermarket up pipe DEBATE

There seems to be an on going debate whether or not it makes sense to add an aftermarket up pipe to a car that already has a catless oem up pipe(ie, All sti and 06+ wrx). we’ve cut apart the oem catless and found some pretty nasty ridges around the oem flex joint which definitely aren’t good for flow. We’ve seen dyno comparisons on nasioc but I don’t recall seeing actual flow comparisons. Over the next few days we will be running some comparison tests on the flow bench to get some hard data for you guys.


Justin




CONSOLIDATED RESULTS FROM THREAD:

UP PIPE

Subaru OEM Cat'd Up Pipe at 28" - 164.7 CFM <--- BOOOO!
Subaru Gutted OEM Up Pipe at 28" - 216.1 CFM
Subaru OEM Catless Up Pipe at 28" - 244.6 CFM
Grimmspeed Up Pipe at 28" - 292.6 CFM

CFM increase of 20% or 48 CFM at 28" of water

I am honestly quite shocked by the difference between the catless oem and an aftermarket up pipe. My initial thought was that the aftermarket version would flow better but not by this much. I can only imagine what the difference is between an aftermarket and a cat'd oem.

Remember, this isnt max flow, this is just the flow at the industry standard of 28" of water...it is used for comparison, not determining max flow.

Subaru OEM Catless Up Pipe


GrimmSpeed Up Pipe




Ok here are the Cat'd OEM Up Pipe results
Subaru OEM Cat'd Up Pipe at 28" - 164.7 CFM <--- BOOOO!
CFM increase of 78% or 127.9 CFM at 28" of water vs. an aftermarket up pipe.

OEM Catted Up Pipe


Here is a graph of all three tests:




Subaru Gutted OEM Up Pipe at 28" - 216.1 CFM

I was rather shocked by this number compared to the oem catless up pipe. The oem catless is still about 30cfm more than a gutted oem.


Subaru Gutted OEM Up Pipe





CROSSPIPE

Subaru OEM Crosspipe at 28" - 230.4 CFM
Grimmspeed Crosspipe at 28" - 315.5 CFM


Subaru OEM Crosspipe




GrimmSpeed Crosspipe
CFM increase of 37% or 85.1 CFM at 28" of water vs. oem crosspipe.
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Old 03-11-2010, 03:52 AM   #2 (permalink)
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^ Souunds good. I slapped an OE catless on when I did mine ages ago, and now wish I had gone aftermarket.

I noticed the same ridges, ported it out to some degree, but thought the bands were part of the flex assembly so didn't want to get crazy.

Plus, I thought the up-pipe diameter is pretty pathetic for any turbo of decent size. Not sure how much of a fat up-pipe w/ a turbo housing made to accept it would improve things (though you're basically rotated at that point).
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Old 03-11-2010, 05:49 AM   #3 (permalink)
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I would add that to make sense of the up-pipe flow data you should add a turbine housing really.....

On a stock TD04 the flow benefit of an aftermarket up-pipe will almost certainly see no gain in power and possibly a slight degradtion in spool performance due to the reduced gas velocities, there will be a swapover point (depending on turbo) - or band - depending on the turbine housing/wheel sizing, but I feel pretty confident in saying that will be with a much bigger turbo than a TD04L

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Old 03-11-2010, 03:41 PM   #4 (permalink)
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Doesnt the STi up have a tapered pipe in the interior increasing the straw effect, assisting in spool?
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Old 03-15-2010, 08:12 AM   #5 (permalink)
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RESULTS ARE IN NOW!


Subaru OEM Catless Up Pipe at 28" - 244.6 CFM
Grimmspeed Up Pipe at 28" - 292.6 CFM

CFM increase of 20% or 48 CFM at 28" of water

I am honestly quite shocked by the difference between the catless oem and an aftermarket up pipe. My initial thought was that the aftermarket version would flow better but not by this much. I can only imagine what the difference is between an aftermarket and a cat'd oem.

Remember, this isnt max flow, this is just the flow at the industry standard of 28" of water...it is used for comparison, not determining max flow.

Subaru OEM Catless Up Pipe


GrimmSpeed Up Pipe



Justin
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Old 03-15-2010, 10:53 AM   #6 (permalink)
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Very interesting results. Thanks for posting this up. Looks like you also have your catted results in on the web site.
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Old 03-15-2010, 12:58 PM   #7 (permalink)
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Forgive the uneducated question, but since we are talking about driving a turbo wheel, wouldn't gas velocity be more important than volume?
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Old 03-15-2010, 02:18 PM   #8 (permalink)
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Both are important. Realize you still need to get that exhaust out of the motor and if the turbo is big, it does need enough volume of air w/ velocity to get the wheel moving.

Think in terms of water. A water hose at high pressure isn't going to move a huge water wheel on a mill either.
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Old 03-15-2010, 05:52 PM   #9 (permalink)
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However, the difference between water and air is air's compressibility. Velocity.....velocity....velocity every one always harps on this due to the conception that it speeds spool. Velocity comes from the compression of air and at some point air goes sonic and it doesn't matter how much pressure is behind it the volume will become static. If you increase velocity i.e. decrease orifice size, you will hit the static limit sooner.
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Old 03-15-2010, 07:12 PM   #10 (permalink)
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^^^Opening a BIG can of worms... lol

Thanks for posting the flow-bench results - interesting to see the big jump in CFM.
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Old 03-15-2010, 07:41 PM   #11 (permalink)
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Quote:
^^^Opening a BIG can of worms... lol
I understand the test and I greatly appreciate Grimmspeed actually giving us real data. So much of the information on forums (NASIOC in particular) starts as opinion. Then as soon as any one that is viewed as an expert states this opinion it all of the sudden becomes fact. No one ever wants to put up their own time and money to do actual testing for data points. I commend you for doing this test, now go break the hearts of thousands over on NASIOC.
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Old 03-16-2010, 04:51 AM   #12 (permalink)
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Interesting results, but I would be even more interested to see it done with a stock range of turbine housings (TD04, VF39 etc), after all (purely hypothetical) if the max flow of the the TD04 turbine housing is 140cfm at 28", then changing the up-pipe will see no significant increase in total flow and the 'possible' reduction in velocity would be harmful to spool.

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Old 03-16-2010, 06:05 AM   #13 (permalink)
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Quote:
Originally Posted by Hardrvin View Post
at high pressure isn't going to move a huge water wheel on a mill either.
Good visual. Thanks!
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Old 03-16-2010, 06:44 AM   #14 (permalink)
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Scubadoober, thanks for diving into it more. I didn't see a good way to explain it, but you did a good job. Yes, I did not mention the compressable nature of air versus the water.

Inheriently I believe that is the wall the heads hit, and the reason that more boost does not result in more power on the 2.5L.
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Old 03-16-2010, 02:01 PM   #15 (permalink)
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Sheesh I just read this same thread over on NASIOC (don't ask I was bored last night)and the differences are striking. I did notice that you kept us in the dark a lot longer over here.
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