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04 STi and Fxt
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Discussion Starter #1
If I swapped this into my 04 FXT, wouldn't it effectively raise my boost to the 14.7psi that the STi runs. Vinny told me it raises maf voltage as well. So, would i need to swap mafs too? While I would be at it, should i swap the 04 sti boost control solenoid?

Thanks!
 

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Administrator
2004 Forester XT Premium 4EAT
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29,449 Posts
I'm assuming you're already running a VF39?
 

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2004 Subaru FXT MT
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1,841 Posts
If I swapped this into my 04 FXT, wouldn't it effectively raise my boost to the 14.7psi that the STi runs. Vinny told me it raises maf voltage as well. So, would i need to swap mafs too? While I would be at it, should i swap the 04 sti boost control solenoid?

Thanks!
MAF's are the same, BCS is the same. Different pill and WGA obviously.

MAF cal is the same, unless my memory is failing me. I don't see why it would be different, but I don't have the rom open in front of me.

Unless if you are doing a spot on STi replication, I'm not sure why people would want to do this. You would need to get tuned anyhow, and the stock FXT ECU is perfectly capable.
 

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04 STi and Fxt
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169 Posts
Discussion Starter #4
I'm assuming you're already running a VF39?
This has nothing to do with running the STi ecu. You do not need the vf39. I would like to effectively increase my boost from 11.6 to 14.7 for no cost. I have all of these parts in my garage, off my 04 STi. I wont be needing them anymore, so figured I would swap them in for a few extra ponies until I can afford an Accessport.

This mod would be totally free, so it is not a big deal either way. Do it or dont.
 

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04 STi and Fxt
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169 Posts
Discussion Starter #5 (Edited)
MAF's are the same, BCS is the same. Different pill and WGA obviously.

MAF cal is the same, unless my memory is failing me. I don't see why it would be different, but I don't have the rom open in front of me.

Unless if you are doing a spot on STi replication, I'm not sure why people would want to do this. You would need to get tuned anyhow, and the stock FXT ECU is perfectly capable.
Why would I need to get a tune? It is stock. The only real difference would be the turbo, which flows less than the vf-39, but several stock sensors will make up for air/fuel inconsistencies(?) oh, and maybe the cams. I know they are both avcs, but don't know if the sti cams are more aggressive(?)

HMMM
 

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none none
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Why would I need to get a tune? It is stock. The only real difference would be the turbo, which flows less than the vf-39, but several stock sensors will make up for air/fuel inconsistencies(?) oh, and maybe the cams. I know they are both avcs, but don't know if the sti cams are more aggressive(?)

HMMM
It won't work, the wastegate duty will be completely wrong for the TD04.
 

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2004 Subaru FXT MT
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1,841 Posts
mafs are not the same according to my tuner.
Doing a side-by-side comparison of the MAF scaling, the curve matches perfectly. The only difference is that the FXT scaling is mapped out to 4.69, where as the STi scaling is out to 4.92.

This tells me that the MAF sensor is the same, so is the diameter of the tube in the airbox. I really doubt there is anything different in the rest of the airbox.

So, unless you are consuming more than 300g/sec, moving to the STi ECU doesn't make any sense. Considering it would take me 5 seconds to copy the STi scaling over to the FXT ECU, it really doesn't make any sense.

To the OP: You will not find any gains on a stock FXT by moving over to a STi ECU. If you want more boost, get tuned. If you insist on using the STi ECU, go for it. It won't hurt anything. But you will actually reduce the amount of boost you are running. While the target boost is much higher on the STi ECU, the accompanying initial and max WGDC's are lower. You won't hit the targets....
 

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04 STi and Fxt
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169 Posts
Discussion Starter #9
Thanks a lot! Thats the kind of info I have been looking for. I absolutely do not insist on using this ecu, I just figured since it's just sitting in my bedroom collecting dust, maybe I could use it. I will not do that now:Banane14:

What engine management would everyone recommend. I am new to the cheaper end of the market and would like some advice. I am leaning towards an Accessport.

Anyone?



To the OP: You will not find any gains on a stock FXT by moving over to a STi ECU. If you want more boost, get tuned. If you insist on using the STi ECU, go for it. It won't hurt anything. But you will actually reduce the amount of boost you are running. While the target boost is much higher on the STi ECU, the accompanying initial and max WGDC's are lower. You won't hit the targets....
 

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2004 Subaru FXT MT
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Thanks a lot! Thats the kind of info I have been looking for. I absolutely do not insist on using this ecu, I just figured since it's just sitting in my bedroom collecting dust, maybe I could use it. I will not do that now:Banane14:

What engine management would everyone recommend. I am new to the cheaper end of the market and would like some advice. I am leaning towards an Accessport.

Anyone?
You have a number of good options. The most inexpensive and seemless option is opensource (romraider).

The more mainstream but expensive options include Cobb's AP and TurboXS's UTEC.

I personally only use opensource because I don't like paying more for something than I have to. Going rate for a road tune with a reputable opensource tuner runs $100-200.

The AP or UTEC is going to run you $600+ to get the piggyback, and other $300-500 to get tuned. :shrug:

In the end, find a good tuner and do whatever they are most comfortable with.
 

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02 Foz 4eat
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Agree with thefoos, I only did opensource. Cheap option and same results. If you haven't already, may as well get a catless uppipe to put on there before the tune. Cheap upgrade.
 

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BLUE WASABI, (#2 Info Provider)
2008 SG Model D
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9,051 Posts
I ran the STi ECU on my '04 (as did some others) and it worked great. I installed it with the STi boost pill, STi TMIC, VF39, STi catless uppipe and STi catback. I of course wanted more power than the stock STi so I ended up getting an '04 STI COBB AP and got it tuned with a larger exhaust and other bits.

This time around I just went the easy route and got a COBB AP for my stock '08 ECU. Opensource is a good cheap route IF you have someone that knows what they are doing and can tune it properly.

Good luck.
 

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04 STi and Fxt
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Discussion Starter #13
Thanks guys! I picked up a catless up-pipe a couple days ago. So, I want to get the engine management settled before i throw everything together. I am still thinking AP. One of my friends used the Utec. Neither of us liked it much. I dont know anyone who I would be comfortable with using Opensource. There are lots of really good tuners in Colorado who have really good results with the AP.

I will let you all know what I decide here and in my member journal. Cant wait to pull the trigger on the almighty, yet small, 68HTA!
 

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2004 fxt A/T
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2,567 Posts
mafs are not the same according to my tuner.
The MAF's are the same, at least in this case (04 fxt-04 sti). I had to swap out MAF sensors into a friend's 04 sti while tuning to trouble shoot a problem. The MAF's had identical number/letters on top. A bit of a mute point now but I thought I would document on the interwebs for any one that cares.
 

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2005 AW FXTi
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446 Posts
I ran the STi ECU on my '04 (as did some others) and it worked great. I installed it with the STi boost pill, STi TMIC, VF39, STi catless uppipe and STi catback. I of course wanted more power than the stock STi so I ended up getting an '04 STI COBB AP and got it tuned with a larger exhaust and other bits.

Good luck.

Ditto, with few exceptions:

-Invida catless UP
-Cobb catted DP
-Stock FXT catback

Netted 245/255 awhp/tq on a Mustang dyno yesterday with a Fineline Imports stage 2.5 tune. I'm going to install my catback and a Tomei UEL header/UP combo in a couple weeks and see what happens. The tuner says I have plenty of headroom left in my setup if I can get the exhaust to flow better.

To be clear though, theres no benefit to running the 04 STi ecu over the stock FXT unit if you're getting tuned. I'm probably behind the times in stating this.
 

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Ol' Timer
2006 WRX Spec-C 6MT
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5,962 Posts
There are lots of really good tuners in Colorado who have really good results with the AP.
Let us know how that goes, since Utah lost Cobb's dyno we've been on the lookout for solid tuners. Since CO is extremely close in altitude and weather, it would be worth the drive for a good tune.
 

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05 Forester XT
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541 Posts
There are lots of really good tuners in Colorado who have really good results with the AP.
I know of Flatiron tuning (got my AP from them), and SCR performance (in Loveland)
SCR will do open source too. I also see somebody advertising on the Denver Craigslist every now and then.

Who else?
 
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