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hey guys, ive got a rb25det which is in the build and im getting a tomei head gasket but cant find what compression relates to what size, 1.0mm, 1.2mm, 1.5mm and 1.8mm. if any1 could help that would be great. note i am also using a rb26 crank and rb26 forged pistons though this shouldnt affect the outcome. cheers.

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yeah all im getting at is the bottom end is basically a rb26, there could be a chance the deck height is slightly different... just assuming the deck height stays the same. then there is the head which is 2cc smaller compared to the rb26 head, so a slight increase of compression would occur but nothing the right size head gasket cant fix. so all in all i should be able to get the comp ratio pretty much back to standard right? the calculators ive tried cant be right because when i put specs in for a standard rb26 it gives a comp ratio of 6.7:1 which is waay off. ive seen a neo motor with a 26 crank running a 1mm head gasket, didnt give me comp ratio but apparently was good :D if anyone has done this before any info would be handy as the motor is at the machine shop atm, cheers guys.

Things that will and do affect your compression ratio.

stroke (using 26 crank)

piston compression height (1mm difference between RB25 and RB26 pistons)

piston dish/dome (I don't have factory specs but good chance they are different)

deck height (deck height will be affected by other factors such as crank throw, conrod length, piston compression height, whether the block has been decked).

combustion chamber volume (this varies from engine to engine and will be affected if the head has been skimmed)

To calculate your comp ratio properly, you need to CC your head and pistons (I don't know how you would CC a dome piston, but a dish piston is easy enough) and enter all the values into the compression ratio calculator.

yeah cheers guys, i know what effects cr and how to cc, done it couple times at work on the race car. just was looking for the lazy way out haha but na it is good to speak to some1 thats done this mod before for a few tips, looks like ill be doin it the hard way... and to cc a dome piston is pretty easy, u must cc the piston with it positioned exactly 1 inch down in the cylinder bore then fill the area above the piston with fluid, then subtract this volume from the calculated cylinder volume at 1 inch and u get the piston dome displacement in ccs.

on a std engine does any1 know the cr difference in the diff thickness headgaskets though?

thanks bob! well i worked it out, if its 0 deck height it will come to 8.8:1 comp with a 1.5mm gasket. sooo i will have to see what the deck height comes to once its set up and then take it from there, thanks for the input guys, much appreciated!

another question about this iam running a standerd rb26 what is the best metal gasket set to use on that 1mm 1.2 or more?

1mm.

thanks bob! well i worked it out, if its 0 deck height it will come to 8.8:1 comp with a 1.5mm gasket. sooo i will have to see what the deck height comes to once its set up and then take it from there, thanks for the input guys, much appreciated!

No probs.

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