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Acl Forged Piston Comments?

OGR88 R32

By OGR88 R32,
in Engines & Forced Induction

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r33_racer Grand Master

r33_racer

Posted
Posted (edited)

im fairly certain std rb25 headgasket is 1.2mm thick, and a 86.5mm bore first oversize (20 thou), 85mm stroke, 87mm headgasket bore size, 62cc combustion chamber head, 11.5cc dome and the rb25 piston will sit 1.2mm below deck height...not sure if i worked that out correctly though. I based it off CPs which i believe have a 0 deck height. Then i worked out 8.7:1.

Yours is a little less cause your oversize again at 40 thou over. But then your CC size is 5cc smaller. So it bumps it back up again.

I use to work it our manually until i was pointed to this site which helps alot....saves all the maths on the paper and in my head.

http://www.csgnetwork.com/compcalc.html

I just worked ours out with the cps and i get 8.2:1 CR which is exactly how they advertise them. So i know this calculator works good. That and it worked good when i rebuilt my rb25 with acl forgies. I just worked out an rb30det with acl 25 pistons with your 34 neo head and at 40 thou over with a 1mm hg i got bloudy 9.6:1CR.... thats gibson motorsport gtr territory there.

Ok one more time...Spool did the rb25 pistons have any dome on them or were they flat as a tack? Cause im pulling the specs off the acl website for their pistons. If its wrong then it could explain the weird CRs im getting in comparison to what your getting.

Edited by r33_racer
Spoolup Mentor

Spoolup

Posted
Posted

sorry I was working on acl rb30et piston

which has -4.5cc dome, 0 deck height , a compressed gasket usually works out at 1mm compressed, 88.8 mm gasket bore size ( the last few Ive had measured this )

and the more oversize you go the higher comp ratio you end up with as your swept volume increases.

So with the ACL RB30ET piston with a R33 RB25 head you end up with close enough to 8.0:1 comp - plus or minus as every engine differes a little, depending on actual comb chamber volumes, compressed head gaskets ( depending on brand etc )

Its a very precise calculation.

r33_racer Grand Master

r33_racer

Posted
Posted

interesting on the headgasket...i was under the impression that a 1.2mm gasket is 1.2 when compressed. And my 1.6mm headgasket is 1.6mm when compressed. Well that .2 difference makes a difference to the CR.

My apologies, your right about the higher comp to volume increase. I had it back to front.

I was working everything off rb25 pistons not rb30et ones.

Spoolup Mentor

Spoolup

Posted
Posted

with the head gasket - I am referring to graphite gaskets. So when u meausre it on the bench, it will definately be less when compressed. A laminated steel one should theoretically be what it is advertised at. So like u said a 1.2mm should be 1.2mm when compressed. But never take it for granted, it is easy to measure with a small piece of malleable lead or similar.

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