I have a 2006 euro-spec hawkeye STI with a 2.5L engine.

Do I have the same heads as the 2.0 in the JDM hawkeye STI & Spec C?

If not, what are the differences?

Why is it that the JDM cars are permitted to rev to ~8000rpm, where the UK cars are limited to around 7k?

 

No your heads are totally diffrent, there V25B castings, main difference is combustion chamber size 55-56cc, & valve train components, 2.0 litre JDM combustion chamber 46-47cc.

 

I understand Litchfield's first Type 25 mated JDM STI heads with a USDM 2.5L short block.

Is there much to be gained from this?

What would it take for the UK AVCS heads to safely rev to 8k (assuming a fully forged 2.5L bottom end).

 

I would think the improved crank goes a long way to the increased rev limit on jdm cars, think the ports are bigger as well as different cams on jdm heads, I think the my05 onwards jdm are same on sti and spec c. If fitting to 2.5 the pistons, Conrod length and head gasket thickness all contribute to maintaining the correct compression ratio, best to tell who is building engine what heads are being fitted then they can make sure everything ok, you may even have to open the chambers a bit to match the bore size which will also help the compression ratio. I know on my cossie I had the pistons machined to obtain compression ratio I wanted as I had to have the head and block skimmed to fit a wrc gasket. I just asked the tuner and machine shop doing my engine then did exactly what they said then assembled it myself.

 

Does the JDM head use different valve springs, retainers, etc to enable it to safely rev out to 8000rpm?

What would you need to be able to rev like this with a 2.5L and UK AVCS heads?