rb26 head on rb25 block

[Tristan363]

Hello and thanks for reading I've been searching the forums for any info and cant find really anything saying I will have compression issues so here's the question.  I have an rb25 block that is already built im talking ACL bearings CP pistons (8.5:1) Manly rods ARP head/crank studs and all.   Im looking at powering a 9180 borgwarner efr turbo running E85 should be able to make 600-700 hp with room to grow.  Originally was just going to build an rb25 head but I'm toying with the idea of using an rb26 head instead since it already has solid lifters and it has more cam options so i was wondering if I might have low compression because from what i understand the rb26 head has a slightly larger combustion chamber 64.5cc vs 62.5 in the rb25. (https://www.tomeiusa.com/tool/stock_engine_data.html)

Does anyone have any experience  with a swap like this.  And I know i should just buy a rb26 block but im located in the states and rb's are much harder to get a hold of here plus I've already got a built block.   Thanks in advance for any help. I've also got a solid rb26 hhead and all the components sourced and ready to purchase this little inquiry is all thats holding me up.

[RiseofBrutality]

If you already have a built Rb25 bottom end, try and get an RB25det Neo head.
A lot cheaper and it has solid lifters
I currently run an RB26 Tomei Type B cams in my 25 neo head at the moment, 260/260 In/Ex  9.15mm Lift
I had to remove the VCT for this to work and use adjustable cam gears

Anything bigger than 9.5mm lift and you'll need to machine the head.


I've never heard of putting a 26 head on a 25 bottom end

[GTSBoy]

Neo head chamber is ~10cc smaller than normal 25.  Will lead to V.LARGE.COMPRESSION on pistons suited to vanilla head.

26 head will sit on 25 block, because the bore is the same.  26 head bolts are larger.  You will need to drill out the block and retap for larger bolts.  Slightly lower compression can be compensated for with a little less head gasket thickness, a little material skimmed off the head or with more boost.  I am sure, though, that there are likely to be some other traps to deal with if trying this.

Lack of VCT would be a good reason to not do it, even though the 26 head flows better than a 25 head straight up and is therefore going to make better power.  The area under the curve might still lean towards a 25 head because of the VCT.

Edited March 22, 2018 by GTSBoy

[Tristan363]

Ya I knew about the vct and the head bolts which are not really an issue my vct barley ever was hooked up before the swap since the plug was damaged and always slid off so I'm not to worried about that.  I've located two rb26 heads one for 500 and one for 800 both are in great condition and on top of that i have a complete rb25 s1 head that i can sell.  I looks like I'll probably go with the 26 head if i go with a 1mm HG I think that will make it work a bit better.  IDK if any of the rb30 guys could way in on this that have used the rb26 head over the 25 that will probably be the closest swap to what I'm doing.

[mitchum]

I've done this swap, there is no reason to swap to larger headbolts.. same thing the RB30 guys do.. just leave the 11mm studs be dowel pins will keep head located properly.

The front coolant passage on the head will need to be welded and reshaped to the 25 head gasket and the vct oil port in block i tapped and plugged with a grub screw...