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2012 XT Touring 4EAT
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Discussion Starter · #1 ·
The vehicle is presently running Cobb's off-the-shelf Stage 1 map. The stock intake has been upgraded to a Mishimoto CAI. Fuel used is always 93 Octane. The transmission is the 4EAT (is there any other tranny available for USDM?). Turbo is stock TD04 - it can hold 15psi boost until I run out of road. I replaced the stock BOV with a GFB recirculating BOV (the stock valve was behaving erratically under boost).

The vehicle is running great - much better as Cobb Stage 1 and CAI than with stock map.

I would like to get to Stage 2 and a shop tune, but have some questions:

Is there any performance advantage to upgrading just part of the exhaust system? Changing the 'sound' of the exhaust/engine is not a goal, although if it happens that will probably be okay. I am concerned about low-end torque - the more the better.

For example, any performance advantage going to a 3" cat-back exhaust but keeping the stock down-pipe?

Or, adding a 3" down-pipe and/or mid-pipe but keeping the stock axle-back components (mufflers and pipes)?

Or switching to a catless down-pipe - either larger diameter or stock diameter?

Any other suggestions for a 'next step' along the path to Stage 2? Any 'brand' preferences?

Thanks.
 

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2009 Forester XT
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From what I've come to understand, downpipe is more improvement than cat back, and 3" is what most aftermarket companies offer. I'm no physicist, but supposedly exhaust gasses need room to expand just beyond the turbo, then they cool off and compress before the cat back, thus not as much need for cat back.

This was the advice given to me by my tuner, and the stock cat back is quiet while still meeting hp goals (stock injectors were the tap out point for me at 330 whp).

In hindsight, wish I went with catless for least restriction, and the fact that I purchased my catted Perrin used (who knows how clogged it may be). At least that wouldn't be a concern with catless.

Cobb suggests full turbo back for stage 2, or just downpipe? I don't recall, but I'd imagine cat back system shouldn't be necessary for any certain reason.... but the sounds would be oh so awesome with full catless TBE.

I have to add that my mod path went down a way different route, so HP output doesn't have much to do with original question. Just my positive experience with replacing only the downpipe on the exhaust.

Sent from my XT1585 using Tapatalk
 

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2010 Forester XT Auto
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The downpipe alone will give you the torque you want. The 3" TBE is needed if you upgrade to a larger turbo. Get a catted pipe, no need to pollute for an additional 5hp. Bellmouth or Divorced is personal preference, neither has a huge gain on the other.

To add a bit of rumble, just pick up a used WRX midpipe and mufflers for cheap.
 

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1999 Forester S
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A couple thoughts here:

1. Remove the CAI and reinstall the OEM intake. These cars don't like changes to the diameter of the MAF housing. Besides that, the OEM airbox and filter don't restrict the intake until around 400 whp.

2. Turbocharged cars LOVE exhaust modifications. The more free-flowing your exhaust, the more power you'll make. Start with the downpipe and CBE (after you remove the CAI, of course).

3. Get a protune. OTS tunes work well enough, but they're compromised and not individually tweaked for your vehicle and mods.

That's it. TBE and a protune gets you to "stage 2." You'll be seeing 40-80 hp more than stock, especially if your tuner ups the boost pressure at the same time.
 

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Discussion Starter · #5 ·
From what I've come to understand, downpipe is more improvement than cat back, and 3" is what most aftermarket companies offer. I'm no physicist, but supposedly exhaust gasses need room to expand just beyond the turbo, then they cool off and compress before the cat back, thus not as much need for cat back.
Thanks. Exactly what I was looking for.


In hindsight, wish I went with catless for least restriction, and the fact that I purchased my catted Perrin used (who knows how clogged it may be). At least that wouldn't be a concern with catless.

Cobb suggests full turbo back for stage 2, or just downpipe? I don't recall, but I'd imagine cat back system shouldn't be necessary for any certain reason.... but the sounds would be oh so awesome with full catless TBE.
As I read Cobb's site, it's full TBE. My guess is that by the time you exit the down pipe, it doesn't much matter as long as the exhaust system is not restricted. I will probably keep everything stock from the axle back. Not sure yet on the mid-pipe.
 

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Discussion Starter · #6 ·
Remove the CAI and reinstall the OEM intake. These cars don't like changes to the diameter of the MAF housing. Besides that, the OEM airbox and filter don't restrict the intake until around 400 whp.
The Mishi CAI improved the airflow to the engine, to the point that it was running lean without a tune - I was off the OEM map. It might have been even more finicky yet if I were using mid- or low-octane gas. Conclusion: the OEM airbox was not as free flowing. Whether it was 'restricting' is another question.
 

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2015 Forester 2.5i CVT
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Thanks. Exactly what I was looking for.

As I read Cobb's site, it's full TBE. My guess is that by the time you exit the down pipe, it doesn't much matter as long as the exhaust system is not restricted. I will probably keep everything stock from the axle back. Not sure yet on the mid-pipe.
Agreed, and remember exhaust gases that expand too quickly (ie large diameter catback setups) can hinder power especially torque and low to mid rpm power. Exhaust gas needs some room, but too much and it loses velocity and therefore scavenging ability. Not to mention many of the exhausts are unnecessarily loud, more power is good but I don't like to attract police or annoy the neighbors. :)
 

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The Mishi CAI improved the airflow to the engine, to the point that it was running lean without a tune - I was off the OEM map. It might have been even more finicky yet if I were using mid- or low-octane gas. Conclusion: the OEM airbox was not as free flowing. Whether it was 'restricting' is another question.
This isn't how a closed loop fuel system works.

Intake restriction has no bearing on hitting target AFR. The MAF, MAP and AF sensor work together to adjust the injector duty cycles to attempt to match the target AFR based on the volume and density measurements it's seeing along with the feedback it's receiving from the AF sensor.

If the intake is restricting the amount of air it's pulling in, that should have no bearing on the AFR provided all those sensors' accuracy is unchanged.

If there's an adjustment in fuel trims indicating a lean or rich condition after changing the intake, the only explanation is the MAF is reading differently...and if it's reading differently you really should tune for it. There's a margin of safety with the stock tune which is what most of these manufacturers take advantage of when they design their intakes by intentionally throwing off the MAF to lean out a bit without significant risk, but that risk goes up when you introduce canned tunes that weren't developed for those intakes.
 

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2009 Forester XT
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Simple solution (and most beneficial) is to pro tune. Then all "off the shelf" concerns would become null and void. Plus OP would be safe as well as take advantage of all parts installed.

Sent from my XT1585 using Tapatalk
 

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07 FXT
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A couple thoughts here:

1. Remove the CAI and reinstall the OEM intake. These cars don't like changes to the diameter of the MAF housing. Besides that, the OEM airbox and filter don't restrict the intake until around 400 whp.

2. Turbocharged cars LOVE exhaust modifications. The more free-flowing your exhaust, the more power you'll make. Start with the downpipe and CBE (after you remove the CAI, of course).

3. Get a protune. OTS tunes work well enough, but they're compromised and not individually tweaked for your vehicle and mods.

That's it. TBE and a protune gets you to "stage 2." You'll be seeing 40-80 hp more than stock, especially if your tuner ups the boost pressure at the same time.
First off, my experience is with the 2007 era cars, but much will be applicable. Reducing pressure drop in the intake CAN make a significant difference, especially on a car with a small turbo that runs out of steam at high RPM. In fact, the gains are multiplied by the turbo's compression ratio (approx 2.2 at 18 psi boost) such that 1/2 psi less drop in the intake can net more than 1 PSI at the turbo outlet. In my experience a good CAI will also help throttle response, more noticeable on manual tranny cars. So with a free flowing intake, if you can hold target boost later in the rev range you will net more power.
 

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1999 Forester S
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The Mishi CAI improved the airflow to the engine, to the point that it was running lean without a tune - I was off the OEM map. It might have been even more finicky yet if I were using mid- or low-octane gas. Conclusion: the OEM airbox was not as free flowing. Whether it was 'restricting' is another question.
You came here asking for tips, but want to argue about it with the people giving you free advice? :thumbsup: NOT installing an aftermarket intake without a tune is rule #1 when it comes to turbocharged, EJ-powered Subarus.

Agreed, and remember exhaust gases that expand too quickly (ie large diameter catback setups) can hinder power especially torque and low to mid rpm power. Exhaust gas needs some room, but too much and it loses velocity and therefore scavenging ability. Not to mention many of the exhausts are unnecessarily loud, more power is good but I don't like to attract police or annoy the neighbors. :)
This is a turbocharged car, not a normally asperated V8. How are exhaust gasses supposed to scavenge through a SINGLE, small turbine wheel spinning over 100k rpm? A TBE, or a CBE as you mentioned, has no effect on cylinder-to-cylinder exhaust gas scavenging, that's the job of exhaust manifold.

First off, my experience is with the 2007 era cars, but much will be applicable. Reducing pressure drop in the intake CAN make a significant difference, especially on a car with a small turbo that runs out of steam at high RPM. In fact, the gains are multiplied by the turbo's compression ratio (approx 2.2 at 18 psi boost) such that 1/2 psi less drop in the intake can net more than 1 PSI at the turbo outlet. In my experience a good CAI will also help throttle response, more noticeable on manual tranny cars. So with a free flowing intake, if you can hold target boost later in the rev range you will net more power.
The SJ uses the same EJ255 as the '06-07 WRX, so your experience is completely applicable. I'm not saying intake can't make power, but common knowledge with turbocharged EJ engines is that intakes are, for the most part, a waste of money. They don't make a ton of power on our cars and the tricks to designing an intake for a Honda just don't pan out on a turbo Subaru.

Any reduction in "restriction" on the intake side when installing an aftermarket intake is due to a less efficient filter element. I for one am not keen on letting more dirt into my engine. Like I said, it's been shown on the dyno that the OEM intake box is not a significant flow restriction until your engine is producing around 400 hp. Again, the dimensions where the MAF sensor bolts to the intake are CRITICAL in our cars. You can tweak your tune to account for changes in these dimensions with a pro-tune and all will be well, but many intake manufacturers don't take this into account and people end up dusting their engines because of it.
 

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Any reduction in "restriction" on the intake side when installing an aftermarket intake is due to a less efficient filter element. I for one am not keen on letting more dirt into my engine. Like I said, it's been shown on the dyno that the OEM intake box is not a significant flow restriction until your engine is producing around 400 hp. Again, the dimensions where the MAF sensor bolts to the intake are CRITICAL in our cars. You can tweak your tune to account for changes in these dimensions with a pro-tune and all will be well, but many intake manufacturers don't take this into account and people end up dusting their engines because of it.
I disagree. First off, most aftermarket intakes DO NOT change the diameter of the MAF, unless you specifically seek out a "big MAF" intake. My tuner advised me not to do this, as you lose resolution, so we do agree on this point. And I agree all intakes should be tuned for. But intakes can add power, it depends on the car setup. On a small turbo that cannot hold boost to redline (includes all the stock and VF turbos) decreasing losses in the intake will help with boost up top. The "stock intake is good to 400 whp" myth is just that, a myth.

This was my experience.

https://www.subaruforester.org/vbul...take-good-enough-up-400-whp-88927/#post977853
 

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Discussion Starter · #13 ·
Intake restriction has no bearing on hitting target AFR. The MAF, MAP and AF sensor work together to adjust the injector duty cycles to attempt to match the target AFR based on the volume and density measurements it's seeing along with the feedback it's receiving from the AF sensor.

If the intake is restricting the amount of air it's pulling in, that should have no bearing on the AFR provided all those sensors' accuracy is unchanged.
Okay, but there may be some limits to the adjustments possible. I am not suggesting the OEM intake was restricting the air flow, or at least not by much. But, it is reasonable to expect the OEM map was designed around the amount of air the OEM intake would deliver. Advertising hype aside, the various CAI's probably do put more air in the engine.

If there's an adjustment in fuel trims indicating a lean or rich condition after changing the intake, the only explanation is the MAF is reading differently...and if it's reading differently you really should tune for it. There's a margin of safety with the stock tune which is what most of these manufacturers take advantage of when they design their intakes by intentionally throwing off the MAF to lean out a bit without significant risk, but that risk goes up when you introduce canned tunes that weren't developed for those intakes.
As you suggest the problem might be the MAF itself. It was probably designed to sit in the middle of the intake duct. If placed in a larger duct, it might not measure as accurately - unmeasured air could pass by. Or, there could be some other quirk of the duct shape to cause the MAF to measure air in a manner not anticipated by the OEM map. Presumably, Cobb's maps for an SF intake attempt to take this into account.

Or, there could be a temperature issue. The ECU knows the air temperature. There must be some factor programmed in to translate measured air temperature into usable air density. If the "C" part of CAI has any functional meaning (other than the advertising function), that might upset the mixture, if only slightly.

Yes, a real tune is in order. I'm trying to get the pieces in place before doing this.
 

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Oh, I never said an intake wouldn't make more power, please don't misunderstand me. Based off your mod list you're a lot further down the path than OP is, you've raised the boost pressure, opened the exhaust a little, bigger turbo, etc., so of course an intake will show benefits.

IMHO, an intake is the last thing I would install, for several reasons, poor air filtration being chief among them. OP was asking "what to do next" after basically installing only an intake.

I disagree. First off, most aftermarket intakes DO NOT change the diameter of the MAF, unless you specifically seek out a "big MAF" intake. My tuner advised me not to do this, as you lose resolution, so we do agree on this point. And I agree all intakes should be tuned for. But intakes can add power, it depends on the car setup. On a small turbo that cannot hold boost to redline (includes all the stock and VF turbos) decreasing losses in the intake will help with boost up top. The "stock intake is good to 400 whp" myth is just that, a myth.

This was my experience.

https://www.subaruforester.org/vbul...take-good-enough-up-400-whp-88927/#post977853
 

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2009 Forester XT
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I've not seen proof that stock intake is not good up to 400whp.

I would like to see is a stock intake not give 400, no matter what other mods are present, in other words 395 or so max with stock box, then when swapping to aftermarket intake see like 405. If you're not making 400, no way to say the stock box won't do it.

BTW, my stock box is making 330whp, in excess of the example given supposedly "mythbusting".......???????

Again, also not saying aftermarket won't see gains, just haven't seen proof that the stocker won't do 400.

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IMHO, an intake is the last thing I would install, for several reasons, poor air filtration being chief among them.
The filter on the AEM intake looks to be high quality, I suspect it has more surface area than stock, I doubt it passes more contaminants.
 

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I've not seen proof that stock intake is not good up to 400whp.

I would like to see is a stock intake not give 400, no matter what other mods are present, in other words 395 or so max with stock box, then when swapping to aftermarket intake see like 405. If you're not making 400, no way to say the stock box won't do it.

BTW, my stock box is making 330whp, in excess of the example given supposedly "mythbusting".......???????

Again, also not saying aftermarket won't see gains, just haven't seen proof that the stocker won't do 400.

Sent from my XT1585 using Tapatalk
It is hard to make absolute claims from one or two examples, because there are so many variables. For instance, the scenario I presented with the VF car showed great gains, because the smallish turbo runs out of steam up top. Gains in the inlet help out up top where the turbo is struggling. Now I am running a larger turbo (Dom 1.5), it has the grunt to hold my peak boost of 19 PSI all the way to redline. If I swap back to stock intake, it will still probably be able to hit 19 PSI at redline, so there is a good possibility that the dyno results between the two cases would be less dramatic. But that doesnt mean the better intake is not helping, because the turbo would have to work harder to hit that same peak boost with the stock intake. That will mean the boost builds a little slower, and the air is getting heated a bit more, perhaps to the point where detonation starts limiting the peak boost to a slightly lesser value. Every setup will be different.
 

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The filter on the AEM intake looks to be high quality, I suspect it has more surface area than stock, I doubt it passes more contaminants.
I'm not saying it's not a quality product, or that it is larger or smaller than OEM. I am making a broad generalization about aftermarket PERFORMANCE air filters. My position is that to make measurable gains in power you give up some particulate filtration in the name of increased flow, meaning they pass more contaminants.
 

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exhaust

back to the original question on exhaust:

I'm running up and downpipes, blouch 380xtr, injectors, custom tune, all with STOCK catback exhaust. (nice and quiet). Dyno was 275hp and 290 ft/lbs. So you can get decent power using stock exhaust.

just my 2c

Jeff
 

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Discussion Starter · #20 ·
I'm running up and downpipes, blouch 380xtr, injectors, custom tune, all with STOCK catback exhaust. (nice and quiet). Dyno was 275hp and 290 ft/lbs. So you can get decent power using stock exhaust.
Jeff: How well did this up/down pipe and other mods work with your 4EAT ('slush box' ;-)?

In some of the earlaier posts, the responders are talking about gains at high RPM. My engine rarely gets above 3K, unless I'm in the mood to blow-off some bimmer. Does your setup deliver much extra during the daily drive?
 
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