how to calculate compression ratio

[canner78]

how do you figure out your compression ratio. i have .060 over 360 with flat top pistons no valve reliefs. the block is decked to 0 with .042 head gaskets and i have 68 cc J heads. i went to this compression ratio calculator http://mopowerstyle.com/index.php?pid=8

it says my compression ratio is almost 10.9 to 1 which seems to be high

i was also wondering how high of lift cam i can go with. i don`t think there is much clearance with no valve reliefs

 

[perfacar]

hi, the C.R. is 10.85! cam duration is where the clearance problem will come .
duration and lobe center placement are the two critical numbers.
in this motor, duration should be kept small.

 

[fishy68]

10.9 is real high for street gas especially if you can't run a big cam to bleed off some cyl. pressure. Did you cc the heads to make sure they are actually 68cc's? Reason I ask is even though the J head is rated at 68cc's most are 70 or more. My X heads were 70.8 cc's after milling them .010. Must have been closer to 72 cc stock but yet their rated at 68cc.

 

[canner78]

i never built the engine, the guy i bought the car from built it and told me it was around 9.8 to 1. he told me they where 68 cc j heads and that he had the block decked to 0. I told him that i was going to put in a bigger cam and he told me that with the flat tops that have no valve reliefs the cam in it is a big as i can go. it is a 450 lift 268 advertised duration cam.