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I was planning on installing the JDM V8 RA Spec-C Heads, but my tuner (Kingpin) convinced me to send my USDM STi heads to Cosworth Racing as core, for one of their fully-built heads.

I read on www.fastwrx.com that the Cosworth Racing 2.5L Heads flow 15% more @ Inlet Ports & 31% more @ Exhaust Ports, does anyone know how much more the JDM STi heads flow over the USDM STi heads?

Also, I was planning on using the DPR Stage 4 or 5 cams to take advantage of the higher redline (8500-9000+), and large turbine turbo (.85 or .95 A/R T4 Turbine/GT35R Comp.). The DPR Stg 4/5 cams are advertised as 323 & 330 duration (intake & exhaust) camshafts, whereas the JDM V8 RA Spec-C Cams are something like low 260 to high 260 duration cams w/ "Large Port" Heads. I'm highly doubting the duration is measured at the same point, does anyone know what the equivalent duration would be for the DPR & JDM cams?

Another plus about having my 2.5 Heads built for my 2.65L Sleeved (Large Bore & Stock Stroke) block is that the 2.5 Heads were designed for my bottom end, as compared to the 2.0L JDM Heads. Money saved from not having to get the JDM heads combustion chamber re-worked to match the bottom end. Also, the Cosworth Heads are ready to have the cams of my liking dropped right in.

Are the USDM Head castings completely different than the JDM STi heads? Since i'm going with the Autronic SM4 Standalone EMS, does anyone know if i'd be able to swap in the JDM V8 RA Spec-C Intake Manifold and bolt my 05' USDM STi's Drive-by-Wire Throttle Body on to it? Reasons for the swap: 1) JDM V8 RAC Intake Manifold uses "Top-Feed" Fuel Injectors 2) 1-pc "Long Runner" Manifold without TGV's
 

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Hey man, how did the swap go? I'm thinking about doing this my EJ257... Do you think they flow better than head work (cams, p&P, etc.) done on the USDM EJ257 heads ?
 

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The jdm and usdm heads have different castings. Check this out for some info TOMEI TECHNICAL REPORT | SUBARU EJ25 (I'm pretty sure the ej207 they tested is a standard sti head and not the spec c big ports). Also keep mind that they are comparing the ej207 to a ej255, although I'm pretty sure the castings on the ej255 and ej257 are the same (I could be wrong).

You'll notice that each intake port (8 total, 2 per cylinder) is 4mm larger on the ej207 compared to the ej255.

cliffs: the jdm sti heads are superior and flow a lot more air than the usdm heads.
 

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The jdm and usdm heads have different castings. Check this out for some info TOMEI TECHNICAL REPORT | SUBARU EJ25 (I'm pretty sure the ej207 they tested is a standard sti head and not the spec c big ports). Also keep mind that they are comparing the ej207 to a ej255, although I'm pretty sure the castings on the ej255 and ej257 are the same (I could be wrong).

You'll notice that each intake port (8 total, 2 per cylinder) is 4mm larger on the ej207 compared to the ej255.

cliffs: the jdm sti heads are superior and flow a lot more air than the usdm heads.
So what options do us USDM STi owners have in getting a better flowing head? Will head work on our EJ257 head produce better numbers than the JDMs head? What will cost more?
 

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So what options do us USDM STi owners have in getting a better flowing head? Will head work on our EJ257 head produce better numbers than the JDMs head? What will cost more?
1. You could get head work done (port, polish; better parts like valve springs, valves, cams, etc.).

2. Hard to say, my guess is no considering the jdm head is designed to flow more air from the get go. Plus, I haven't really seen any ej257 longblocks that will make peak hp at or above 7500 rpm - a stage 2 jdm ej207 will make peak hp around 7500 rpm on a version 8/9 longblock.

3. Hard to say. A set of new or used jdm heads would probably cost more than getting head work done on your usdm heads. Plus I don't think it's a bolt in affair - I believe some modification is necessary to make the 2 work together. A lot of the Japanese tuners like team orange are running frankenstein motors, (ej257 block with ej207 heads).
 

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kingpin is not the place to go. they have been long one for a couple of years. either way good luck with your car and i hope your work was on par.
 
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