I have a 2006 2.5 STi that was getting its engine rebuilt but upon checking the heads, they required a lot of work to rebuild and could then potentially be out of tolerance when they were done. Decision made to look for some new ones instead.

I'm told I need B25 Single AVCS heads. Trying to find a set has been pretty much impossible. Now I have 2 other sets lined up but I want to know if they will be plug and play. I'm not worried about if they need different thickness gaskets etc - I just don't want to have to **** about with wiring or the likes. The choices are:

D25 heads as currently fitted to a Spec D ( I can only assume they have been changed at some point as I thought they were WRX heads??)

Or

V25B heads which I believe are from a 2007 Hawk STi somsame model as my one but one year newer.

Will either of these sets of heads work and what should I be looking to pay for them?

Thanks

 

hi , your correct, d25 are 2.5 wrx heads and some peeps reckon flow better that the b25 heads . if you google both heads youll find alot of info from the usa. one chap split open 3 different subaru heads 2 being the d25 and b25 and on looking into this believed the d25 heads look likely to flow better. i ran d25 on my last 2.5 build.

maz

 

D25 defo Wrx heads

B25 are Sti's and would have been on spec d originally, you'll probably find the complete engine was swapped for a Wrx one, both labelled ej25 on blocks.

So B25 flow more than D25 in std form. If your port them the D25's have more material to remove so depending on how you look at it

They will flow more than sti heads or if ported equally will flow the same but stay stronger as have more material everywhere round ports.

The oil actuators and sensors are the same the oil line is slightly different not sure how as I have only been given part numbers. Not physically held them

The cams springs and valves need swapping over (sti is uprated)

I have been waiting for a while to find an amazing set. But they cost a pretty penny!!

 

well put jampot , i couldnt quite remember the whole read and your right " chap said had more potential for work on d25 heads !

 

Sounds like there's a still a bit of work to do with a set of D25 heads then. I was hoping to keep this as simple as possible to be honest.

Does anyone know much about the V25B heads? I've found some information on them but I'm struggling to get a definitive answer if they will work with my current set up.

 

The dome of the combustion chamber looks to be much better on the D25 heads. However each has their own up's and down's, which isn't a lot to make a big difference power output depending how big your setup is.

The D25 heads do have more room to port than the B25 heads. found this also .

 

You can see how on the D25 heads the coolant passages have been redesigned to allow more flow threw the head because after comparing the cross sections you can see that the inner cooling passages are now smaller to allow for bigger exhaust ports (I'll take a picture of this). Also much more importantly you can see the different style combustion chambers! IMO the D25 has the best design because of its hemispherical based design and the lack of valve shrouding. The STi's b25 however has a very squared off combustion chamber.

 

the above taken off NASIOC

 

The main difference in the castings is the combustion chamber capacity, 25D WRX heads are 51cc as std, 25B STI heads as std are 54cc, hence the reason the WRX runs a higher comp ratio.

 

So am I right in thinking it would just be a case of swapping over my valves and springs from the B25 to the D25 heads - I'm sure I read they were uprated?

I have also been offered a very low mileage set of V25B heads. The heads will get a check over and some porting done but nothing mad. I'm not going for huge power - somewhere around the 400 mark would be fine.

 

The 25B STI have the better spec valves, just swop into the 25D castings.


Just check the exhaust valve & seats, 2.5's are quite hard on exhaust seats compared to the 2.0.

 

For 400 just get std b25's

Not sure on differences with v25b's didn't actually know there was another set of heads for 2.5's

 

A bad porting can be far worse than std porting.

There are velocity's and all sorts of things that need to be worked out. Not a simple case of the biggest holes equals best!!!


Porting is an art and a science. ��

 

If I could find a set, I would! Didn't think it would be this difficult to find a set to be honest.

Another question - if a Spec D has D25 heads, I'm assuming that's not from the factory and it will have had some work done? I know someone breaking a Spec D and when I asked for them codes from the heads and pics, they are D25??

 

B25 are easier to come by than d25.

I'd imagine the spec D with those heads would have had a complete Wrx engine dropped in. I may be wrong!!!!

This is just a guess!! But a lot of trouble for zero gain! If it's original spun a bearing or dropped a rod cost wise it's cheaper to drop in a ej255 than build or drop a ej257

If fitted for performance gain after upgrade to Sti or equivalent plus a port job your talking over 1000 1500£ extra
Your friend breaking would know as who ever he would have got it from would have made him aware. If you see where I'm going with this

 

https://forums.nasioc.com/forums/sho....php?t=2821899

In the states if your on nasioc. Send him a message see if he is happy to send uk. These will work out cheap too considering.

You have cams from your old heads. Worth a shot though?!?!

 

My cams were not in good condition either - there was a lot of material had gone through the engine!

I spoke to RCM earlier and managed to get some definitive answers so
I've gone for a set of low mileage V25B heads which will get checked over and refreshed before being used.

Thanks to everyone for their input. :-)