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2011 Forester XT Auto
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92 Posts
Discussion Starter · #1 ·
2011 XT with a stage 2 protune on it now.
I have a Grimspeed larger intercooler, and a VF52 turbo to install when I do the timing belt job in a few months along with a new tune.
Is a CAI recommended at this point, or just keep the stock box?
 

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2002 Outback H6 4EAT
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1,050 Posts
Stock box is good until past Stage 2 levels.
 

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Premium Member
02 WRX
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148 Posts
Stock box is good until past Stage 2 levels.
So negligible gains with a CAI along with my vf52 turbo and a larger intercooler, and retune?
Unfortunately this is something you will read over and over on forums, and the original message of this is lost on a misunderstanding. The stock box is good until past stage 2 levels, but good at what? What people are saying is that the stock airbox isnt a restriction until some horsepower figure, they usually pick 400hp. However, the stock airbox, even with a drop in filter is more restrictive than a design such as our own even at stock power levels. Where the misunderstanding comes in is that with the stock sized MAF tube you usually run out of sensor at around 350-400hp, which is when it would make sense to go to a larger sized intake if not running speed density or some creative tuning strats.

Its funny because NO ONE ever says "your downpipe is good until past Stage 2 levels" even though a downpipes only goal is the same as the intakes, and that is to decrease restriction. Or "your catback is good until past Stage 2 levels" even though its main job is to create a sound that you like, and second to decrease restriction. The exact same duty as an intake.



Here is a dyno of intake testing on a vf52 wrx with pretty much the same mods as you have. Now you've already been protuned, so the gains are not as dramatic, but what we're looking at here is the difference between the stock intake run with the cobb tune, and the grimmspeed intake run with the same tune but with MAF scaling (green vs blue lines in boost). The MAF scaling was simply done to make the AFRs equal and so the timing values were equal. The grimmspeed intake run always makes more boost, and that is a direct result of a decrease in restriction vs the stock intake. Its making a little more than a PSI of boost, and that is because it is less restrictive by at least that amount. The wastegate duty cycle has not been changed to make this additional boost, but rather it is occurring simply because it is easier for the turbo to ingest air.

Now why do we care? We could just turn the boost up one PSI on the stock intake instead, right? Yes you certainly could, but hopefully it is clear that that is not the same. If you were to simply just turn the boost up a little more the turbo has to do more compression to hit that target boost level, which means more heat into the charge air. More heat isnt good here and is only going to work your intercooler more. Plus, these turbos are already relatively small, and have a difficult time making boost in the higher rpms. It is already turning into a hair dryer at redline, and working it harder is making heat in the charge exponentially worse. Any decrease in restriction that can help make more boost in the higher rpms helps make more power everywhere under the curve without the additional heat penalty of just turning the WGDC up. If you look around 5800rpm there is an increase of boost of about 1.5PSI, and its making a little over 20 horsepower. Id say that an over 10% gain is worth it, but thats just me.

The only time I wouldn't recommend an intake is if you hate the intake noise. I personally love it, so just like an axleback muffler, even if it did nothing I would still probably get one. And the only other time I would not recommend getting an intake is if you didn't want to get protuned. I think in your case where you already have an intercooler and are willing to get another protune, you can take further advantage of an intake and that intercooler to push the engine a little further and still keep trouble like knock away.

Here is the link for more reading: https://www.grimmspeed.com/grimmspeed-cold-air-intake-subaru-08-14-wrx-sti-09-13-forester-xt/ There is also a ton of info in the development thread for that intake on another forum, and again on another forum for the LGT version. Mainly I just want people to understand exactly what the stock airbox is "good" at until stage 2 and beyond, because what it isn't good at is decreasing restriction.

Chase
Engineering
 

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2012 XT Premium Manual
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48 Posts
I have a Process West (Perth Based Company) CAI K&N Filter behind the front right light... LOVE IT... LOVE IT.... LOVE IT!!

Im going to remove the notion of getting big gains and is it worth the $$$ Vs Kilowatts blah blah, lets face it, thinking that you are going to see massive gains with a CAI is like thinking that having red brake calipers will make you stop quicker Hahahhah. It does assist in achieving the kilowatts in its own little way but its not a show stopper and you certainly don't need it.

I installed mine purely for the WOOSH factor. I wanted to drive down the street and hear my turbo windup, suck in the air and woosh as i change gears. Its a turbo, that is what turbos do!!! I will celebrate this, while you enjoy the pleasant tones of a VF52! hahahaha

I hear the argument all the time, I say just do it... but make sure you either chat with your local tuner as to whether you need to have your car tuned.
 
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