Jump to content
SAU Community

RB26 Crank and pistons into RB25 NEO. What about VCT?

Finn_GT-T

By Finn_GT-T,
in Engines & Forced Induction

 Share

Recommended Posts

Finn_GT-T Apprentice

Finn_GT-T

Posted
Posted

Hi guys,

i am doing a rebuild of an RB25det NEO engine. I have the chance to get an RB26 long nose crank for a good price. My idea now is to put the crank into my rb25 block an buy forget rb26 rods and pistons to get a cheap stroker and solve the short nose problem of the rb25. The crank should fit into the block but what about the combination of the pistons and the neo head? Will there be an issue with colliding pistons and valves when using the vct of the neo block? Will the compression rate be ok? Any experiences with that combination?

GTSBoy Grand Master

GTSBoy

Posted
Posted

Neo rods are 26 rods anyway. So any aftermarket 26 rod is same same also.

The trouble with the Neo is that because they used the 26 rods, they had to have a piston with a 2mm (or maybe it's only 1.5mm) pin height difference so they don't poke out of the block. Those Neo pistons have a different dome (to vanilla 25s and 26s) to work with the ~10cc smaller combustion chamber, to yield approx the same compression ratio as all the previous engines. But you can't use the Neo pistons with the 26 crank.....because then the 2mm stroke difference brings back the deck height problem. So you end up needing to use 26 pistons. But....the 26 pistons are suited to a 10cc larger combustion chamber, resulting in sky high compression. This would commit you to E85, large cams and careful tuning. I haven't done the calculation to work out what the compression would be (well, not in the last 15 years anyway) so can't really tell you how high it would be. I just remember it was "quite high", something north of 11:1 I reckon. The last time I thought about it was pre-E85 in the world, so maybe it's more realistic now. It's not very realistic on 98 though.

But, this seems like a complete waste of time. 100cc is an undetectable increase in capacity (4% !!). You would do better to spend money on turbo or headwork. The oil pump drive can be dealt with the on the 25 crank as easily as on any other RB.

Finn_GT-T Apprentice

Finn_GT-T

Posted
  • Author
Posted

I was hoping it was a common mod because it is a cheap (but small) stroker for the 25.  I've already found a few threads on this subject on UK Skyline forums. They say it works, but don't mention what pistons they used. In the end, as you said, the main problem would be the pin height of the pistons (if using 25 pistons) or the compression ratio (if using 26 pistons).

The compression ratio could be lowered if I use a thicker head gasket. The thickest one I found is 1.8 mm thick. Wiseco offers pistons that lower the compression ratio to 8.25 compression for the rb26.
If I have the head and block machined, I may lose 0.1 to 0.2 mm that would also slightly increase the cr.


So maybe the cr problem could be solved. I just wonder if the pistons and valves do not collide. Especially if I use the VTC of the neo head. I don't get the problem solved if I enlarge the combustion chambers in the head a little. Does anyone have experiences with this?

You are right, the increase in capacity would be low but with the right pistons it would be an easy modification because i am rebuilding my engine anyway with new rods and pistons.

My current setup is a gtx3076 gen2 on the stock neo with with 1.2 mm metal head gasket and i want to keep the turbo because I installed that setup just 1,5 years ago. I am just aiming for a bit more low end torque.

r32-25t Grand Master

r32-25t

Posted
Posted

I’ve seen it done and it works, the rb26 has a lower compression ratio then the rb25 and may not be as high as you think.
 

After measuring everything up, if the compression ends up to high you can machine the dome on the piston down to bring the compression down to what you require 

Finn_GT-T Apprentice

Finn_GT-T

Posted
  • Author
Posted

I think I would feel a little bit bad about machining down a shiny new set of forged pistons :(  I would also worry about the stability without the dome... maybe that's a too high price for the little increase.

Do you know which pistons they used in the engines you saw running?

r32-25t Grand Master

r32-25t

Posted
Posted

People have been machining pistons since the beginning of engines, that’s how they lower compression or make room for the valves when using cams with huge lift, it’s even how they’re made in the first place 

  • 2 weeks later...
Finn_GT-T Apprentice

Finn_GT-T

Posted
  • Author
Posted
On 1/30/2022 at 3:53 AM, GTSBoy said:

The trouble with the Neo is that because they used the 26 rods, they had to have a piston with a 2mm (or maybe it's only 1.5mm) pin height difference so they don't poke out of the block.

I've just had another silly idea :) 

Maybe I could use conrods from the tomei rb28 stroker kit. These are 3 mm shorter than the std rods. Does anyone know the exact pin heigth difference between the rb25 and rb26 pistons?

Finn_GT-T Apprentice

Finn_GT-T

Posted
  • Author
Posted

Ok, i've found the right search term for this info.

If my information is correct, the compression high between the two differs by 1.5 mm.

RB26: 30.0 mm

RB25: 31.5 mm

That would be too much difference using the stroker rods.

CefiroA31 Apprentice

CefiroA31

Posted
Posted
On 2/14/2022 at 6:57 PM, Finn_GT-T said:

Ok, i've found the right search term for this info.

If my information is correct, the compression high between the two differs by 1.5 mm.

RB26: 30.0 mm

RB25: 31.5 mm

That would be too much difference using the stroker rods.

RB26 crank, rods and pistons with machined down domes modeled after RB25DET NEO lower smaller piston domes to lower compression about 9.0:1 = RB26DET NEO.

  • Like 1
BK Grand Master

BK

Posted
Posted
On 14/02/2022 at 9:27 PM, Finn_GT-T said:

Ok, i've found the right search term for this info.

If my information is correct, the compression high between the two differs by 1.5 mm.

RB26: 30.0 mm

RB25: 31.5 mm

That would be too much difference using the stroker rods.

Actually, RB26 standard rod length is 121.5mm. HKS or Tomei RB26 2.8 stroker rods are 119mm as their strokers use stock pin height RB26 pistons.

Conversely, the RB26 Nitto 2.8 setup uses 121.5mm rods with lower pin height pistons.

Finn_GT-T Apprentice

Finn_GT-T

Posted
  • Author
Posted (edited)

Thank you for the infos. I would need 120 mm rods if I wanted to use the rb25 pistons with the rb26 crank.

Machining rb26 pistons to suit the rb25 neo head is not that easy. the rb25 neo pistons do not have a flat surface but a little dome and the pockets for in and ex valves. To find someone with the right cnc skills would be too much effort for a 61 ccm higher displacement.

These are the cp pistons for the neo, that I are already have here:

image.thumb.png.d24762743c51cfbad56790d0c0c9bb29.png

Edited by Finn_GT-T

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing


  • Similar Content

  • Latest Posts

    • Kinkstaah
      Boost cut after swap(RB25DET NEO) R34

      By Kinkstaah · Posted

      Stock ECU (or more accurately stock tune) absolutely refuses to go over 10psi and behaves like you have seen. The Nistune is the same if it is the stock tune. If the Nistune chip has been tuned, the resulting tune could be literally anything for any combination of parts. The Nistune just makes the stock ECU Tunable.
    • Kapr
      Boost cut after swap(RB25DET NEO) R34

      By Kapr · Posted

      So stock ECU does not like anything above 10 psi?  That Nistune one is just for "try" if it will be any different, I know it need to be tune for that. I know but YOU may know about these problem but i/we dont. They few little Skylines here let alone people who know anything about tham so that is why iam asking here  
    • Duncan
      V37 Radiator removal

      By Duncan · Posted

      So now we have a radiator with no attachments whatsoever. It lifts up with a particularly tight spot between the drivers side air box mount and the lower radiator outlet, but if you've got this far you will sort that too. This is the lower mounts with the rad out so you can see where the rubber bushes go, it is a straight shot upwards Done! Assembly is the reverse of disassembly, with blood less likely to be shed.
    • Duncan
      V37 Radiator removal

      By Duncan · Posted

      Right, onto the second last trick. The Air Con condenser is mounted to the front of the radiator and stays in the car when the radiator is removed. There are 2x 10mm headed self tappers holding the top of the condenser to the radiator, remove those The bottom of the condenser is attached to the radiator with clips. You need to lift the condenser out of those clips and clear (up, then forward). f**ked if  could work out how to do that last bit with the front bumper on. I hope you can, and you share the trick.  Bumper removal probably deserves its own thread one day once I've recovered the will to live, but basically you need to remove the wheels, front inner guard liners (clips and 10mm headed bolts), the self tapper between the guard and the bumper at the rearmost point of the bumper (same as an R32 that bit), any remaining clips at the top/front of the grill, an absolute bastard design with a plate that holds the top of the bumper above the headlight each side (only 1 bolt which is tricky to get to, but the plate catches 2 places on the bumper and must be removed....carefully!) and push clips between the bumper and guard under the headlight. If you've done all that you will be faced with wiring for the fog lights on both sides and in ADM Q50 RS at least, 4 nasty tight plugs on the driver's side for the ADAS stuff. So, the clips at the bottom look like this on drivers side (looking from the front) And on the passenger side (also from the front), you can see this one is already out Clearance on both of these are super tight; the condenser needs to move up but the upper rad support mount prevents that, and the radiator can't move down far because it is (rubber) mounted. Once you achieve the impossible and drop the condenser off those mounts so it does not stop the rad moving, you are good to go
    • Duncan
      V37 Radiator removal

      By Duncan · Posted

      OK, next the shroud needs to come off and there are a couple of tricks. Firstly, there is a loom from near the passenger side headlight to the fans, coolant temp sensor etc and there is no plug to undo.  In my case I was OK to leave the shroud on top of the engine so I just undid the passenger side fan plug and about 10 of the clips which gave enough free wire to put it aside. The fan plugs were super tight, the trick I used was a small falt screwdriver to push down on the release tab, then a larger flat screwdriver to lever the plug out of the fan unit....be careful with how much force you apply! If you need to remove the shroud altogether for some reason you will have to deal with all the plugs (tight) and clips (brittle)....good luck. I removed all of the clips and replaced them with cable ties that I will just cut next time. Also, in the Red Sport / 400R at least, the intake heat exchanger reservoir hose is bolted to the shroud in 2 places with 10mm headed bolts; so remove them (the hose stays in the car; no need to undo it at the t fittings down at the radiator lower mount. Once you've dealt with the HX hose and the wiring loom, there are 3x 10mm headed self tappers holding the top of the shroud to the radiator; remove those.   The shroud then lifts out of the bottom mounts where it sits on the radiator, up and onto the engine out of the way. Simples
×
×
  • Create New...