Jump to content
SAU Community

Recommended Posts

hello,

i want to put rb26 pistons and rods in my rb25 block leaving rb25 crank. As i know rb26 piston have ~ 2mm lower pin location and CR of such conversion will be to low -- lower than 8.5. What can be done to leave CR in between 9-8.5?

Edited by Stronzo

to deck 2mm fron the block -- yes that`s the only way i was thinking about. I want to use rb26 pistons as they are stronger than rb25.

p.s. Well maybe if i can find neo 25 pistons -- i can use them. But they are still weaker the 26`s.

The RB26 runs a bigger stroke and same sized rods as the rb25.

For all that to fit within the rb25's deck height (which is the same as the rb26) they run a piston with a lower pin/compression height.

There's a bloke here in Adelaide that stroked his rb26 out to an rb29 using a destroked (machined) rb30 crank, rb26 rods and custom pistons with a very very small pin/compression height.

NEO pistons will lower your compression, once again why? Aftermarket rb25 forgies are getting quite cheap, and depending on your power level, probably unnecessary, stock pistons are good for 300 rwkw at least safely "IF TUNED CORRECTLY"(NISMOID did your car havee stock internals?With 350 rwkw)

The RB26 runs a bigger stroke and same sized rods as the rb25.
But the stroke is controlled by the crank. The stroke is twice the offset of the big-end journal from the centre of the crank.

If he uses a RB25 crank, he will have RB25 stroke. If he has RB25 stroke, then he has RB25 C/R.

But the stroke is controlled by the crank. The stroke is twice the offset of the big-end journal from the centre of the crank.

If he uses a RB25 crank, he will have RB25 stroke. If he has RB25 stroke, then he has RB25 C/R.

I think you have missed what I was getting at. ;)

The Rb26's pin height has been setup as such to suit the stroke, rod length and the blocks height.

If he were to use an rb25 bottom end with rb26 pistons (with their lower pin height) he WILL have end up with a lower static comp and very much less than ideal deck clearance.

i`m back again, thank you for your replies, now i`ll answer your questions:

why i want to use _stock_ pistons (not forged) -- because here, where i live we sometimes have +35 C in summer and -35 C in winter. Forged pistons are pain for a street car with low to moderate power output (imho) and are nearly impossible to use at winter :). that`s why.

what`s my power goals -- 350+. no more than 400 hp at the flywheel. i think it will be enough for me :). In Russia in Far East region where 95% of all cars are used cars from Japan -- experienced skyline tuners say that can be acived at 26 pistons only.

why i don`t want to use 26 crank -- it`s quite expensive if i buy it new and hard to find it in good condition if i buy it used.

The reason i`m doing this -- i mesured compression in my rb25det neo, and it is: 12-12-11.5-10.7-11-12 -- i`m not happy with it so i`m thinking what to do next. i have variants:

1. try to fix my current engine (not interesing, and hard to order parts before disassembling motor)

2. buy 26 motor -- i have a cheap proposal but it has no documents and hard to understand what is inside of this motor. and there is much job to install it.

3. build a mega-motor from my old rb25de -- install oil injectors, good pistons, turbo -- etc. this is the most interesting but i think the most expensive variant.

so now i`m desperate -- i can`t make a decision :mad:

Edited by Stronzo
  • 4 weeks later...

ok, got rb26 crank, pistons and rods at yahoo auctions. no way back :D

Can anyone tell me what CR i will have in a combination of:

rb25 block and head

rb26 pistons, rods and crank

stock head gasket

As far as i know CR will be slightly higher than 8.5 as rb25 combustion chamber is smaller than rb26. rb25 cc volume is 62-64cc, what about rb26?

RB25 std everything is around 8.5:1 (31.3mm comp height, 72mm stroke, 15cc dome)

RB25 w/- RB26 pistons is around 7.8:1 (30.3mm comp height, 72mm stroke, 15cc dome)

RB25 w/- RB26 crank & pistons is around 8.7:1 (30.3mm comp height, 73.7mm stroke, 15cc dome)

this is working on both heads being at 64cc and 1mm head gasket, I measured a 26 head on Thursday and it came out at 64cc. It will vary slightly depending on the dome volume and the head gasket thickness.

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


  • Similar Content

  • Latest Posts

    • Does that German restaurant still exist in the old place out the NW end of Goulburn? When I say "out the NW end of"...I am really being vague. It was 1997 when I was last there, and the only point of reference I can recall is that it was on the opposite side of the main drag from the big merino. And when I say "opposite side of the main drag", I don't mean "on the main drag". It was either a couple of streets back from there, or might have even been out in the sticks a bit further. Was an old farm building or mill or somesuch. And when I say "the big merino" I might actually be thinking of a completely different part of town, because I just looked on maps and the big bugger is not where I remembered him to be! The food was good, consisting largely of various German mystery-meat sausage/loaf things and kartofflen.
    • So while the second sentence is completely correct and the whole point of the conversation, the first sentence bears consideration. If this bloke is just hoping to throw big turbos on and drive it around, because there are no helpful facilities at all in his tropical paradise** then he likely has zero chance of even knowing what the TP is on the last column in the stock maps, let alone know whether the ECU is operating anywhere near it or past it. So the point is very very moot. And, per what I said before, at stock boost on those turbos, you may well be off the end of the map. **I'm just back from Vanuatu, so I know exactly what small Pacific nations can be like wrt paradise without requisite facilities. But it's not even that simple. I put a high flow on my car and had to drive it around with a proper tune because of the lack of opportunity*** to put the bigger AFM and injectors into it to allow it to be tuned. I had to turn the boost down to less than I had before, and back off the boost controller's ramp, because it was exploring parts of the map that it didn't drive in before, and really couldn't access for tuning on the dyno either, and so was pinging. It was still well within the last column, because when I first**** set up the Nistune on the Neo I rescaled all axes of the maps to give some more space to explore. ***Family dyno was broken ****This was 13 years ago, and the TIM thing wasn't a thing then and so TP would definitely grow when pushing past the stock tune's limits.
    • Yep, this bit another local owner. I caught it before putting the transmission back into the car, what I noticed was the pressure plate fingers weren't flat and even. It's more obvious with the pull style clutch because the throwout bearing ring was visibly not flat once everything is put together. Nismo should really update their instructions to call out this specific detail. I'm not even sure the clutch as-shipped orients everything properly.
    • It ended up being that orientation of the float hub in relation to the clutch disk, when I installed it, I heard a loud click and being stupid, I decided to not take it a part and check it. The hub didn't properly align with the clutch disk and was causing the issue. Definitely an odd one! Dahtone Racing was able to fix me right up, stand up blokes!      
    • Right, but I'm saying on the stock ECU measured airmass from the MAF is no higher than stock. So it's accounting for the higher flow rate iso-manifold pressure. You just have to keep turning down the boost until you're within the stock tune's load scale. If you run off the end there's no telling what will happen. This does mean there's zero benefit to the turbos you're running vs stock, if anything it's just a straight downgrade because the transient response is worse, you don't even get the ECU's boost solenoid helping to pull the wastegate closed during initial spool, and peak power is only whatever the factory map can give you before you hit the R&R corner. On a -9 I would bet that you would have to change out the wastegate spring once you have a real ECU and you're tuning it for real. I'm not saying this is a remotely ideal state of affairs, it's just a way to keep it driveable until you can get a proper tune done.
×
×
  • Create New...