Tomei Metal Head Gasket

[adamsrotor]

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.

[Abo Bob]

Using RB26 pistons will definitely affect the CR. What size bores will you be using?

[adamsrotor]

87mm bore. how will it affect it if the crank compensates for the different piston pin height? the squish area should be exactly the same as using rb25 crank and pistons, correct? cheers.

[adamsrotor]

tired to work it out with the cr calculator but wasnt right at all, if you can give me a hand to work this one out bob it would be greatly appreciated, thanks again.

[cuppas]

isnt it cr = (cv+sv)/cv?

[adamsrotor]

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 if anyone has done this before any info would be handy as the motor is at the machine shop atm, cheers guys.

[Abo Bob]

RB25 and RB26 cranks are pretty much the same. The rods are the same length too. I think it's the pin height in the RB26 pistons that gives the extra displacement and lower CR.

[Beer Baron]

well RB26 has 2mm longer stroke. where that 2mm comes from i'm not sure off the top of my head (don't know much about RB25) but something is different...

[Adriano]

The rb26 crank has the extra 2mm stroke. I would get your engine builder to cc the head and piston tops. It is the ONLY way to accurately figure out the final comp ratio.

[adamsrotor]

yeah might have too, ive got all the gear at work to cc everything so might have to get to it hmmm. yeah it does have 2mm more stroke from the crank having more throw and the piston pin height raised...

[daisu]

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.

[Kranker]

^

Presume you workout the displacement of a dome piston (roughly anyway) by pressing it into some kind of clay then measuring the cc of the impression.

Might not be spot on but should be close is done carefully

[adamsrotor]

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?

[Abo Bob]

Check here.

[phenline]

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?

[adamsrotor]

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!

[Abo Bob]

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.