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Old 06-13-2013, 11:52 AM   #1
Spiffy07
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Default Stage 3 WRX w/cobb cat-back. is the Cobb CB creating a bottleneck?

I tried searching this but couldnt find any clear answers.

Okay so I recently upgraded to a stage 3 setup with the following mods:

-Process West Top mount intercooler
-Process West Splittler & Shroud
-Blouch TD06H-20GXT Billet 8cm^2
-Blouch Oil Feed Line
-AVO Silicone Turbo Inlet Pipe
-TGV Deletes (ported and polished)
-DeatschWerks 300LPH Fuel pump
-DeatschWerks Bosch 1000 CC injectors
-GrimmSpeed 3mm Phenolic Thermal Spacers
-GrimmSpeed 3 port EBCS
-TurboSmart Boost Tee MBCS
-Invidia Divorced WG catted DP (with bung welded in pre-cat for true AFRs
-Cobb Cat-back
-Cobb Heat sheild
-Cusco catch can
-K&N drop in filter
-Custom grounding kit
-Snorkus delete
-Crushed bpv mod

…The car pulls hard to redline and has no issues what so ever with knocking/pulling timing. On a Mustang Dyno up here in Utah we got it to a very stable 330whp/330wtq. I’m happy with her new found power, especially considering the crap 91 octane that passes as “Premium” up here in the mountain states.

Now, to mate the Cobb Cat-Back with the Invidia down pipe I had to get the GrimmSpeed adaptor that tapers down from a 3inch flange down to a 2.5in diameter where the stock doughnut gasket connects the catback to the downpipe. This has worked no problem fitment wise and I have had no exhaust leaks so far.

My question is; With the taper in the Cobb cat back the exhaust system is not a “true” 3inch exhaust. Would I gain any power if I removed the 2.5in restriction and had a true 3in TBE? Say, have an exhaust shop cut out the taper and weld in a 3inch flex pipe with a flat flange.. Would there be any benefit to be gained? Or should I just stick with what I have? Please note that I have a catted downpipe.

This inquiry is for info only at this point. I just want to know my options.
Thanks!
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Old 06-13-2013, 12:18 PM   #2
sc00ter
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I'll let the real experts chime in here with factual data but I'd be extremely surprised if making it a "true" 3" exhaust, as you've mentioned, would make significant (if any) HP gains. With those mods I'd say it's more about the tune at this point.

However, if anything is the bottle neck, I'd actually say it's the TMIC. I know some will call blasphemy! But I've read quite a few posts on various Subaru forums with data showing that the Process West TMIC is actually worse in terms of flow and cooling than even the stock STI TMIC.

Quick example: http://forums.nasioc.com/forums/show....php?t=2348764
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Old 06-13-2013, 01:10 PM   #3
Spiffy07
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Thank you for your comment,

Every time we tried pushing it a little further power wise it wouldn't like it. It would create more power on the dyno but it would also start pulling a little timing here and there on the top end. A new tune would be required after removing the 2.5in restriction to take advantage of any power gains to be had… if there are power gains to be had by doing this is the question.

Interesting thread about the PW TMIC vs the STI TMIC. As much as I don’t want this to turn into a discussion about intercoolers, I want to mention that the PW as a heat ex-changer does works extremely well in conjunction with the splitter PW provides (which was not used in the experiment posted).

The difference between tube & fin design vs. bar & plate design is also significant. Pressure drop in a tube & fin intercooler is always going to be more than the bar and plate of the same size. Say the map calls for 20psi, the turbo has to work harder to get that 20psi to the intake manifold. If you were to stick a pressure gauge on the turbo you would see it boosting 23-24psi (maybe more). The more pressurized air the turbo creates, the hotter the air gets. Also, while the exterior airflow of a tube & fin is superior, the interior airflow is inferior to a bar and plate design since the intake charge flows into the open core as opposed to a flush tube. That is why it is so important to use the splitter provided with the bar & plate PW TMIC. The PW splitter seals tightly around the intercooler surface forcing air through it. Without the splitter, air will take the path of least resistance. On top of all that the crimps on the STI TMICs have tendencies to pop on high boost applications.

My own personal experience with both TMICs, after driving on a hot summer day (any day really) the STI TMIC on my car was always very hot to the touch. The PW unit is always cool to luke warm to the touch. Even in stop and go traffic the PW unit stays much cooler than the STI tmic ever did on the best of days. Granted, the STI tmic I had was from an 04 STI.

lol in other words the TMIC is not an issue in my scenario
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Old 06-13-2013, 01:20 PM   #4
sc00ter
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Well I just learned my fun facts for the day. Thanks for this!

Now hopefully someone will pull your thread back on topic...
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Old 06-13-2013, 01:33 PM   #5
tuzzio
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Personally I don't think you'll gain anything out of it, besides a shallow wallet and disappointment. I'd be more concerned with why you're making such low numbers in a 2.5 with a 20G
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Old 06-13-2013, 01:39 PM   #6
Trea17
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Quote:
Originally Posted by tuzzio View Post
Personally I don't think you'll gain anything out of it, besides a shallow wallet and disappointment. I'd be more concerned with why you're making such low numbers in a 2.5 with a 20G
I'd dare to say fuel is a large issue here, but regardless it should be 350+.
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Old 06-13-2013, 01:43 PM   #7
Spiffy07
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Quote:
Originally Posted by tuzzio View Post
Personally I don't think you'll gain anything out of it, besides a shallow wallet and disappointment. I'd be more concerned with why you're making such low numbers in a 2.5 with a 20G
well,, besides the 91 octane gas..? Or the 2.5in restriction in the exhaust? ..I don't know. I was told that this particular mustang dyno reads 8-10% low. Which would bring the car up around 350whp/360whp. Number wise makes more sense. This is at a peak of 23psi tapering down to 20psi.

Last edited by Spiffy07; 06-13-2013 at 09:18 PM.
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Old 06-13-2013, 09:13 PM   #8
Spiffy07
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According to this article: (http://www.ft86club.com/forums/showthread.php?t=20572) Mustang dynos read 12% lower than Dynojets. Dynapack dynos read 8 to 10% higher than Dynojets.

my dyno graph:


Theoretical dyno numbers:

Mustang dyno (baseline): 324whp/323wtq
Dynojet dyno (12% higher): 363whp/362wtq
Dynapack dyno (20 - 22% higher): 389whp/388wtq - 395whp/394wtq

Which one is closest to reality? Who knows. What i do know is that based on these numbers, comparing power from one dyno to another is pointless. What i'm curious about is if anymore power can be made if the 2.5in restriction in the exhaust is removed.
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Old 06-13-2013, 10:26 PM   #9
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Mustangs and dynapacks are kind of a crap shoot. They're both very easy to change to read whatever you want without using a correction that would show up on the dynosheet. It gets even worse when you dyno a turbo car at high elevation since the standard SAE barometric pressure factor for any of those dynos (dynojet included) doest properly account for turbo cars at elevation.
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Old 06-14-2013, 12:34 AM   #10
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Quote:
Originally Posted by Leafy View Post
Mustangs and dynapacks are kind of a crap shoot. They're both very easy to change to read whatever you want without using a correction that would show up on the dynosheet. It gets even worse when you dyno a turbo car at high elevation since the standard SAE barometric pressure factor for any of those dynos (dynojet included) doest properly account for turbo cars at elevation.
right, which basically makes it a tool to identify loss or gain on the same machine. Which brings me back to the question, will removing the 2.5in restriction in the exhaust allow for more power potential?
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Old 06-14-2013, 12:41 AM   #11
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It's the fuel, not the 2.5" restriction. You would get at most 5 hp from going with a full 3" catless turboback.
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Old 06-14-2013, 04:49 AM   #12
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You need to look at Xluben's exhaust threads. That guy has a very built car (around 500 hp/tq), and has run pretty much every exhaust known to mankind on it.

If I recall correctly, the only time he saw any power changes with exhaust was when he went to a single pipe system ... think it was the Greddy TiC. Otherwise, every other catback exhaust he tried had zero effect on power ... even the stock system.

So if your chasing power, you need to go single exit. If you stay dual exit, it simply does not matter what you have on there.
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Old 06-14-2013, 08:34 AM   #13
Spiffy07
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Quote:
Originally Posted by GrumpyPitbull View Post
You need to look at Xluben's exhaust threads. That guy has a very built car (around 500 hp/tq), and has run pretty much every exhaust known to mankind on it.

If I recall correctly, the only time he saw any power changes with exhaust was when he went to a single pipe system ... think it was the Greddy TiC. Otherwise, every other catback exhaust he tried had zero effect on power ... even the stock system.

So if your chasing power, you need to go single exit. If you stay dual exit, it simply does not matter what you have on there.
Thanks for the Info. I will check out his threads. My car is a pre 08 so it already has a single exit.
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Old 06-14-2013, 03:23 PM   #14
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Quote:
Originally Posted by Spiffy07 View Post

Thanks for the Info. I will check out his threads. My car is a pre 08 so it already has a single exit.
Ok sorry bout that. I'm on my phone so can't see your car info.
But his exhaust adventures show that the catback taper will have no effect on your power level.
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Old 06-14-2013, 11:05 PM   #15
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Your also around 4200'-altitude has big effect...
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Old 06-15-2013, 03:15 PM   #16
Spiffy07
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Quote:
Originally Posted by point78 View Post
Your also around 4200'-altitude has big effect...
That it does.
Atmospheric pressure at sea lvl = 14.7psi
Atmospheric pressure in Salt Lake City (4350ft) = 12.6 PSI
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Old 06-15-2013, 05:36 PM   #17
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The Cobb system is fine at your power levels. Now, a catless downpipe might give you a nice bump in power.
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Old 06-16-2013, 02:14 PM   #18
Spiffy07
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Good to know. I will probably stick with what i have for the time due to how hard it is already to get the damned car to pass emissions. Stupid Utah and their "no tampering" laws.
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