Rb25 Block And Crank +rb26 Pistons = Cr?

[Stronzo]

Not according to CR calc. This is assuming the head is 64cc and the piston is 15cc dome.

thank you for your help.

afaik RB25 stroke is 71.7, not 72.

And it seems to me that rb25 ans rb26 have slightly different combustion chambers (rb26`s a bit larger):

some data pieces found via google:

wiseco`s catalogue says rb25 head is 66.5 cc, and rb26 head is 70cc

http://www.wiseco.com/PDFs/Catalogs/SC06_Nissan.pdf

here is 62cc@rb25:

http://forums.nicoclub.com/zerothread/186816

here is 63cc@rb25 and 64cc@rb26^

http://www.skylinesaustralia.com/forums/lo...hp/t119495.html

where is the truth?

i just want to know what CR will my engine have...

[Adriano]

You will have to cc your head, they vary due to valve height, casting differences and whether it has been milled before, remember to cc each chamber.

[proengines]

Adriano is right, you do need the exact volume of what you are using, the only way to do that is to cc your head/piston dome etc..

let me correct what I wrote before with the correct stroke for an RB25, this is using a 71.7mm stroke, 86mm bore, 1mm head gasket, 15cc piston dome, 85mm piston top OD with the top ring 6.4mm from the top with a range of different chamber sizes:

64cc- 8.46:1

63cc- 8.6:1

62cc- 8.74:1

61cc- 8.89:1

60cc- 9.04:1

Now the same using an RB26 crank and pistons, 73.7mm stroke, still zero deck and 15cc dome, 1mm head gasket, 85mm piston top OD with the top ring 6.4mm from the top:

64cc- 8.67:1

63cc- 8.81:1

62cc- 8.96:1

61cc- 9.11:1

60cc- 9.27:1

These numbers are assuming that the volumes are what's written above, a smaller dome will drop the compression, a thicker head gasket will do the same and vise versa. Don't rely on this or what's in a catalogue or you find on a google search, use the volumes that are specific to your own engine. The domes on the forged pistons available for these engines range from around 5cc to 16cc from what I have seen and this makes a huge difference to the compression you will achieve. Also, the compression height of these pistons vary by .2-.3mm depending on the brand which will change your deck clearance. The best way to check this is to dummy the engine up and measure it.

Also, many people ignore the volume around the piston top down to the first ring but this is a part of the chamber volume and on one of these engines comes out around .9 to 1cc depending on the ring depth and the piston top diameter.

If you want to give me the actual numbers you have I dont mind calculating it for you.