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Rb25 Block And Crank +rb26 Pistons = Cr?

Stronzo

By Stronzo,
in Engines & Forced Induction

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Stronzo Collaborator

Stronzo

Posted
Posted (edited)

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
Stronzo Collaborator

Stronzo

Posted
  • Author
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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.

Cubes Grand Master

Cubes

Posted
Posted

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.

Adriano Grand Master

Adriano

Posted
Posted

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)

blind_elk Grand Master

blind_elk

Posted
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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.

Cubes Grand Master

Cubes

Posted
Posted
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.

Stronzo Collaborator

Stronzo

Posted
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Posted (edited)

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...
Stronzo Collaborator

Stronzo

Posted
  • Author
Posted

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?

proengines Rising Star

proengines

Posted
Posted

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.

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