So I know that for 04 5spd fxts, you can slap on a vf39/sti intercooler/catless uppipe/sti exhaust manifold/sti ecu and its the cheap way to basically become an sti (stock power). You could probably get away without swapping the exhaust manifold as well..

What Im not understanding is why you couldnt just flash the fxts ecu with an sti tune? Isn't it the same thing as swapping ecus or is there something that stops you from being able to flash an sti tune onto an fxt ecu?

As far as I know the 04 stis motor is identical to 04 fxts except Intercooler/UP/ExM/catback (I dont count the painted manifold or bpv as it makes no functional difference at stock/stage 2 boost levels) and transmission/gearing isnt factored into maps. Inquiring minds want to know.

 

I am just throwing this out there, but isn't the FXT motor the EJ255 and the STI is the EJ257. From what I remember they are slightly different and the ECUs are different.

 

AFAIK no, you can not.

The FXT is not just a detuned STI motor, there are some subtle differences in the setup. There is a reason why there is a FXT tune and a STI tune on cobb's OTC downloads, looking at the site cobb has up, the downloads are 2 different files (different names) so, I suspect it means they are 2 different ones.

 

That is correct, the FXT is very similar to, but not identical to a STI. Not the least of which is the different turbo, so I definitely would not expect the STI maps to be a good idea.

 

Read all the responses and there seems to be a lot of (mis?)information out there so Im just trying to get to the bottom of it.

Check out these two threads regarding the matter. I see Stan posted there as well so maybe he can chime in as well. I also understand some of the information is old and might have been posted before Cobb made an FXT vf39 tune.

*This is for information purpose. I already have a decent tune on my FXT.




('03-'05) advantages and disadvantages of 04 STi ECU - Subaru Forester Owners Forum

How to go faster than an STI for cheap! - Subaru Forester Owners Forum



Also what specifically allows/disallows the ability to use the STI ecu in the 04 5spd FXTs and nothing else?

 

the 05+ FXT have immobilizers so the 04 STi ecu wouldn't work. I had a built 04 FXT running on a 04 STi ecu with an AP and protune so if you want real world answers on this stuff, feel free to hit me up on PM. some troubling misinfo in this thread.

 

Sounds good. I'll hit ya up.

 

The 04-05 fxt are ej257 motors then they went to the ej255 in 06-07. Even though the motor in 04-05 is a sti motor its different in many ways, like intake manifold and exhaust manifold. And a lot of other things so it will be completely different.

 

Uh..no

04 has a EJ255, even marked on the top of the motor, ask me how I know

 

ahh your right forgot my information lol thinks its just the 05 that has the ej257?

 

All 04-08 FXTs came with the EJ255. I believe the 5 vs. the 7 at the end isn't a reference to the block itself, it's more of a designation to differentiate between model lines. Internally the 04-05 XT engines are almost identical to the 04-05 STi with forged internals, sodium filled valves, AVCS, etc. There are only minor differences in pistons, heads and cams I think.

 

Whatttt? I dont think Subarus come with forged internals.

 

Pistons here in the states are most definitely not forged.

 

They aren't in Japan either AFAIK, forged was only on some years and possibly only the Spec C motors. They were replaced with hypereutectic ones after 2002 based on what I am reading - so the question is does the FXT have those same pistons or not? I'd guess not.

 

I guess Car & Driver had it wrong then in their famous XT road test back in 2003.

Subaru Forester 2.5XT - Road Test - Car Reviews - Car and Driver