Rb26 internals in rb25 combustion chamber cc

[fikemoo]

Right so before any flaming as I know there is a million and one threads on this and believe me I have read them all. I've been planning my rb26 swap for over a year and have pretty much all the information I need ie parts etc. I know I need to use;

Rb26 crank
Rb26 rods
Rb26 pistons
Rb26 crank pulley bolt

But what I am trying to research at the moment is I know the bores are the same the extra displacement comes from the stroke from the crank and thus the 26 pistons have the different pin hight so they don't pretrude the block. What my question is; the difference in compression does it come from the rb26 having a different combustion chamber cc or the different pin hight or both?

I'm going full forged build but say for example I just stuck all the rb26 internals in my rb25 what cr would I have?

I'm aiming for 8.5.1 which I know I can do with pistons but what I'm trying to work out is what pistons I'd need to achieve this. Or would I retinal the normal rb25 cr with 26 internals. I've been searching every evening for the past few days trying to find out whether the 26 and 25 have the same combustion chamber volume and I keep getting conflicting views it's so annoying.

I'm getting yea they are they are 63cc
I'm getting yes they are 64cc
I'm getting no they are different 25 is 66.5 and rb26 is 70cc
I've been told the chambers are identical Inc valves. Even the castings of the head are identical apart from bolting patterns and galleries etc. Which I doubt is true

Any rb gurus that can help me would be amazing. I want to do this properly
Hence my amount of research. Yes I could just slap it together and it would work but it's not right in my eyes

Thanks mike

[alanjuggler]

they're both 66.5CC afaik, I was using a mixture of online opinion along with Wiseco's catalogue.

http://www.wiseco.com/Catalogs/SportCompact/Nissan.pdf

the difference is the deck height that creates the compression change. using RB26 pistons drops the compression slightly by about 0.2-0.25 points

don't worry about it. you've got enough other stuff to worry about.

also, NB, wiseco used to list them as different sizes - 66.5 for the RB25 and 70 for the RB26 - I suspect there was a measurement error that's now been corrected in the latest catalogue.

[fikemoo]

Thanks for the reply alan. And I had notice this revised combustion chamber myself when I downloaded the new version of the wiseco catalogue. And that's awsome news and will help me with my decision in regards to piston choice.

In looking at wossner piston they only do rb26 pistons with the piston pockets/reliefs. And this makes sense due to the solid lifts they can run much higher lift than a rb25. I am looking at getting cams but non will warrant the reliefs I don't think anyway still something I need to work out. Say they don't am I at a disadvantage getting a piston like this. As I am a fan of wossner pistons

Mike

[alanjuggler]

Thanks for the reply alan. And I had notice this revised combustion chamber myself when I downloaded the new version of the wiseco catalogue. And that's awsome news and will help me with my decision in regards to piston choice.

In looking at wossner piston they only do rb26 pistons with the piston pockets/reliefs. And this makes sense due to the solid lifts they can run much higher lift than a rb25. I am looking at getting cams but non will warrant the reliefs I don't think anyway still something I need to work out. Say they don't am I at a disadvantage getting a piston like this. As I am a fan of wossner pistons

Mike

never used Wossner so can't commment, if there's more combustion chamber volume due to the cut outs, it'll drop the compression ratio more. I wouldn't drop the CR on a car that'll see the road. did that with my CA18, bag o' shit below 4k.

you'll have to do the maths, I like the amount of info wiseco publish and I've (ab)used their pistons before, so happy with them.

[fikemoo]

Yeah I've been told wiseco pistons are good and I was originally looking at them. As I knew you used them and went for 8.5.1 and you pretty much got that didn't you?? Only think I noticed with the catalogue and it might just be default pictures but it looks like wiseco pistons don't have the dome so won't that mess up the quench/squish area as I've been told not to mess around with that. As you want as cleaner burn as possible.

As you said lots of math for me to do. The good news is that the combustion chambers are the same son can atleast work with knowing that. I ended up finding some really good second hand rb26 rods so I've got them sitting in doors already. But I might be looking at going fully forged as new turbo is a bit of a beast lol. Think I'm going to go with crower rods. I've heard good things and I can't warrant paying for carillos lol

[fikemoo]

Just did a quick calculation and providing the rb25 combustion chamber is 66.5 aswell as the rb26 using wiseco's 86.5 oversized pistons you get bang on 8.5.1 bazingaaaa

[alanjuggler]

Just did a quick calculation and providing the rb25 combustion chamber is 66.5 aswell as the rb26 using wiseco's 86.5 oversized pistons you get bang on 8.5.1 bazingaaaa

iirc when I did my calculations using wiseco's figures, I ended up at 8.4:1 static.

when I get some cash, I'll replace the headgasket with a metal one (rb25 gasket is thinner than the RB26 oem - 1mm to 1.1mm for the RB26, bear that in mind for your CR calcs) along with some ARP headstuds. I reckon I can push these steel rebuilt OEM turbos to about 1.2-1.4bar but doubt the OEM HG would survive long.

[fikemoo]

I am going metal headgasket myself and I've been told to not use the headgasket to lower compression etc. Would you suggest sticking with the 1mm headgasket or use the rb26 thickness? I'm basically overhauling everything. Idea being it shouldn't have to be opened up again

[alanjuggler]

You can't use the actual RB26 gasket due to the VVT oil way passages, it was more just to remind you about the sums for the calculations.

I'd stick with 1mm so the squish is effective and you get good off boost response. it depends on what you want - if you're going to start chasing numbers, you need to decide now. in the 4-500hp range, I'd definitely keep it at a normal level. it's just way more enjoyable to drive.

if you're using RB26 rods, I'd make sure you replace the rod bolts - look at the Z32 rod bolt part number for ARP (they don't actually have an RB26 listing but the Z32 ones fit), that's what I've got in mine.

[fikemoo]

Yes I remember you telling me that before about the z32 arp bolts and this was planned to be done anyway luckily my crower rods come with the arp 2000 bolts or the uprated arp 625+ volts. I think they are called that will need to check. And power wise it has been decided I am going to chase higher numbers. The turbo is good for around 800 I'm not going to push that but I'd like to build the engine capable of holding 700. And I know not to use the rb26 gasket I'm not that silly haha. It was the thickness I was thinking of. As you rightly said I don't want to mess around with the squish

[fikemoo]

Edit: stupid phone don't know why it posted that twice sorry

[alanjuggler]

I'll tell you stuff multiple times, my memory is terrible for that kind of stuff hadn't realised you meant aftermarket RB26 rods.

ehh, if you're chasing that kind of power, you'll need deep pockets to do it properly. do some serious reading about what other people have done - there's no point reinventing the wheel and if you do need to drop the CR a little to get there, it's best to get it out of the way now than regret it later.

I still wouldn't touch the squish, if you need to drop it, I'd get custom pistons done instead.

also, bear in mind the RB25DET gearbox won't last an amazingly long time at that power, unless you're babying it. they don't break with straight power but if you're shocking the box with launches etc, I wouldn't expect it to last ages.

[fikemoo]

Yeah this is what my next door neighbour has been telling me. Only use the piston to change the compression ratio so I might have to look at custom ones. He seems happy to run that sort of power with 8.5.1 but I know about all the oiling modifications that need to be done and that will be all done at the time the engine is out.

Gearbox/clutch is another thing I have though about aswell and I know it's all money when aren't cars haha. The idea is to collect the parts over time as I don't need to strip the engine any time soon. The ew turbo and manifold can go on and it be remapped so similar power I'm running at the moment and it'll run that until I'm ready for the complete strip and rebuild.

I know everyone goes over to steel/metal headgasket and I will be doing this. People talk about going 1.2mm but I don't like this idea because of the squish. I'm looking at all the aussies big builds at the moment because they seem to know their shizzle about the rb's and have picked up some stuff and some ideas. Ie wiseco pistons the aussies swear by them so I'm sort of dead set on them. I've read a lot about if the car isn't pure track use then going for these high pressure pumps is useless as they are rubbish at the lower rev ranges so I'm going to get a brand new standard pump and I'm going for the later r33 crank with the long nose drive. Still loads more to think of but I think I'm going in the right direction lol