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04 Xt 5 speed
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Discussion Starter · #1 ·
So my question is this. What all do I NEED to swap the VF39 on my forester? I have seen that a 04 sti intercooler and ecu is recommended but is it required and why? The reason I ask is because I have a vf39 sitting on my shelf just waiting to be installed.

I don't see the point in the ecu as I could just tune mine. Unless there is something else that I do not know about. I understand why someone would recommend the intercooler upgrade and yes it is the next part on my list of things to buy.


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07 FXT
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828 Posts
only reason to swap in the STI ECU would be if you wanted to run a stock STI calibration with no further need of tuning. That implies that the turbo, intercooler, injectors, intake and downpipe are all stock STI (or their equivalent XT version) pieces. This is only an option for the 04 XT with manual tranny.

Almost everyone will swap in an aftermarket downpipe, at which point you will need to be tuned anyway. So unless that first paragraph applies to you, skip the STI ECU.
 

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04 Forester XT
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145 Posts
You wont need to upgrade the injectors, the OEM 04 ones are just fine for a VF39 swap unless your going to go nuts.

I went a bigger fuel pump just for peace of mind, I have a stock STI intercooler, VF39, cobb downpipe, grimspeed uppipe, and am getting ready to put in a grimmspeed crosspipe. I run a OTS VF39 COBB tune right now, and im getting tons and tons of boost and plenty of power.
 

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04 Xt 5 speed
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Discussion Starter · #4 ·
Would I be ok to run with the stock 04 intercooler until I pick up a sti IC? I have a 3" dp and 3" mid pipe with a high flow cat and a 04 sti cat back.

Thank you guys for clarifying that up for me.


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2004 Forester XT Manual 5sp.
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1,662 Posts
Overbear knows what's up.

You'll need, at minimum:

VF39 turbo
STI TMIC
Cobb Access Port with VF39 map loaded
Downpipe
STI Axle back

But you'll want:

3" downpipe
3" Catback
Catless up pipe (OEM or aftermarket)
A custom tune.

A custom tune completely changes the car from the Cobb OTS tune.
 

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2007 FXT vf37 swapped 4EAT w/ VDC
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782 Posts
Would I be ok to run with the stock 04 intercooler until I pick up a sti IC? I have a 3" dp and 3" mid pipe with a high flow cat and a 04 sti cat back.

Thank you guys for clarifying that up for me.


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Yes, you can run the stock intercooler. It's not ideal. You may have to retune when you get the STi intercooler.
 

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2004 Forester XT Manual 5sp.
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1,662 Posts
Yes, you can run the stock intercooler. It's not ideal. You may have to retune when you get the STi intercooler.
I'd say this isn't a good idea.

The Cobb OTS VF39/STI TMIC tune is specifically designed to run on a car with a VF39 and STi TMIC as well as other Stage 2 mods (downpipe, cat back).

If you're going to baby it, sure. But if you really want to drive it, I'd say it's not wise. The STi TMIC is substantially larger and offers better cooling capacity than the stock FXT TMIC.
 

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2005 Forester XT 4eat
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4,581 Posts
My tuner tuned an XT with an STI turbo and the stock XT TMIC. The torque curve was not right, and he had to do some messing around with the throttle mapping at high rpms. I do not recommend.

If the STI TMIC is on your list, then wait until you have it to install the turbo.
 

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04 Xt 5 speed
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19 Posts
Discussion Starter · #9 ·
My tuner tuned an XT with an STI turbo and the stock XT TMIC. The torque curve was not right, and he had to do some messing around with the throttle mapping at high rpms. I do not recommend.

If the STI TMIC is on your list, then wait until you have it to install the turbo.
My tuner was telling me that bc it is now almost winter not to worry about the top mount and just to make sure I get one by spring. But what he did recommend is a larger fuel pump. He will be doing a OS tune and he comes highly recommended.


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2005 Forester XT 4eat
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It might be fine for winter, but what's your rush? The airflow through the XT intercooler isn't really so great for a bigger turbo. Another thing to consider, your tuner may just want to tune your car now, because this time of year is usually slow for them. You'll get better results tuning your car for what you're running than you will by getting a tune now and changing something.
 

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2004 Forester XT 5MT
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511 Posts
It might be fine for winter, but what's your rush? The airflow through the XT intercooler isn't really so great for a bigger turbo. Another thing to consider, your tuner may just want to tune your car now, because this time of year is usually slow for them. You'll get better results tuning your car for what you're running than you will by getting a tune now and changing something.
This.

I trust my tuner and what he tells me to do. ANYTHING I change, he wants to know so he can do an update and ensure my car is 100%. I was in your shoes and about to build my car backwards and he got me straight. He wouldn't give me a tune until I did the STi intercooler. After that was installed he gave me the go ahead to do the up pipe, down pipe and exhaust. Some tuners are looking out for YOU and your car, some tuners are looking out for THEIR wallet.
 

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04 Xt 5 speed
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Discussion Starter · #12 ·
This.

I trust my tuner and what he tells me to do. ANYTHING I change, he wants to know so he can do an update and ensure my car is 100%. I was in your shoes and about to build my car backwards and he got me straight. He wouldn't give me a tune until I did the STi intercooler. After that was installed he gave me the go ahead to do the up pipe, down pipe and exhaust. Some tuners are looking out for YOU and your car, some tuners are looking out for THEIR wallet.
Thanks for the looking out guys. I'm holding off for now. Might as well get it all knocked out at the same time anyway.


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2017 VW Golf SportWagen 5MT
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Finding a new tuner is what you may need to do before anything else.

All you really need is a VF turbo and STi TMIC with correct BPV and hose, if you don't have a catless uppipe you need that too. Stock downpipe should be fine if you want to save some money. Don't aim for high boost target, a good tuner will get you great driveability with good fuel and timing maps. It's a street car, you are not racing. COBB's OTS maps run too much boost.

Stan
 

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04 Forester XT
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145 Posts
Finding a new tuner is what you may need to do before anything else.

All you really need is a VF turbo and STi TMIC with correct BPV and hose, if you don't have a catless uppipe you need that too. Stock downpipe should be fine if you want to save some money. Don't aim for high boost target, a good tuner will get you great driveability with good fuel and timing maps. It's a street car, you are not racing. COBB's OTS maps run too much boost.

Stan
Not having that problem at all in mine and I run a OTS map. Don't know where you got this info (yea I have seen the posts too but most of them are idiots that push too hard or don't have the proper supporting mods)
 
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