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Rb25det Rebuild Issue - Help

Muskett

By Muskett,
in Engines & Forced Induction

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Muskett Apprentice

Muskett

Posted
Posted

G'day just in need of some advice, currently having my rb25det rebuilt,

Mods relevant to the issue are

Cp Pistons Sc7305 86.5mm 9.0:1

Bridgeway forged rods

Block was decked

Been told Comp ratio is only 8.4:1 and pistons are .029" out of the block deck.

Any help appreciated. Cheers guys.

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https://www.sau.com.au/forums/topic/454457-rb25det-rebuild-issue-help/
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GTSBoy Grand Master

GTSBoy

Posted
Posted

Doesn't seem possible. Are you sure that they are normal RB25DET pistons, or is it possible that they are for a Neo?

If they are the correct pistons, what is the value of the volume of the "dome" portion of the piston? We can neglect the slight poke of the piston out of the block and use the normal 25 chamber volume and the dome volume to calculate what the CR theoretically is.

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GTSBoy Grand Master

GTSBoy

Posted
Posted

Just doing a real quick and dirty calc, ignoring deck height issues and the gasket thickness, the 63cc chamber, 416.3 cc swept volume and 8cc domes gives 8.56:1 ratio. So close enough to the 8.4 reported before.

FWIW, doing the above with Neo 51.5cc chamber gives 10.5:1.

Ergo, it is hard to believe that these are 9:1 pistons under any circumstances.

Muskett Apprentice

Muskett

Posted
  • Author
Posted

Yes

  On 05/03/2015 at 7:18 AM, GTSBoy said:

Just doing a real quick and dirty calc, ignoring deck height issues and the gasket thickness, the 63cc chamber, 416.3 cc swept volume and 8cc domes gives 8.56:1 ratio. So close enough to the 8.4 reported before.

FWIW, doing the above with Neo 51.5cc chamber gives 10.5:1.

Ergo, it is hard to believe that these are 9:1 pistons under any circumstances.

It's wierd

zebra Grand Master

zebra

Posted
Posted
  On 05/03/2015 at 7:18 AM, GTSBoy said:

Just doing a real quick and dirty calc, ignoring deck height issues and the gasket thickness, the 63cc chamber, 416.3 cc swept volume and 8cc domes gives 8.56:1 ratio. So close enough to the 8.4 reported before.

FWIW, doing the above with Neo 51.5cc chamber gives 10.5:1.

Ergo, it is hard to believe that these are 9:1 pistons under any circumstances.

Yep, just got 8.48:1 using a 1.0mm thick gasket

Muskett Apprentice

Muskett

Posted
  • Author
Posted
  On 05/03/2015 at 8:05 AM, zebra said:

You would need a 12cc dome, to get 9.06:1

The sport compact section in the CP catalog is farking rubbish, and gives next to no info on their pistons

Yeah it's basic, been looking through it just now

Ryno Experienced

Ryno

Posted
Posted
  On 05/03/2015 at 9:15 PM, Ben C34 said:

Good on you or your builder for actually checking.

I'm betting heaps get slapped together with no idea of the compression ratio.

Very true, I had CP pistons put in my RB25DET and I don't believe the CR was checked.

Muskett Apprentice

Muskett

Posted
  • Author
Posted (edited)
  On 05/03/2015 at 8:53 PM, zebra said:

as unfortunate as it is for you. its good info for anyone wanting to buy CP slugs in the future.

Zebra could you calc using 17cc instead of the original 8cc stated?

The stroke measurements were incorrect in a book he was using or something along those lines? It has now come up with 10.5 compression ratio with taking into account the .029" out of the block.( which is probably from being decked too much)

Head cc is 62.8cc

Edited by Muskett
Muskett Apprentice

Muskett

Posted
  • Author
Posted
  On 06/03/2015 at 8:59 AM, zebra said:

9.9ish:1 using a 1.0mm gasket and 0.8mm out the bore

edit, given that the piston is 0.74mm out of the bore, it works out at 9.84:1 with a 1.0mm gasket

1.3mm gasket is 9.53:1

He came up with 10.5:1 with the standard nissan gasket and the 0.74mm out of the block ( obviously can't use this combination but for calcs)

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