How do I calculate compression or guess

Some data liberated from krazykuda's post!

How do I calculate for compression with speed pro Pistons & J heads?

Am looking at the H116CP40 Speed Pro Pistons,
stock stroke 3.58?" Crankshaft,
(LA 360)

& a

nominal? Deck height? 9.6" or
9.58?"

Head gasket volume??? 12.4cc?
(I know this varies, but have no other data-what gaskets are recommended?)

H116CP40
Comp Dist 1.66"
Piston Dome= +5cc

Not sure of swept volume for the cylinder-> 752cc?

4.04" bore
Area of cylinder? 12.819 cc?

-for stock J heads & another set of J heads milled .040"

""The #3418915 'J-Heads'

'J-Head' Combustion Chambers are typically at 68.0 - 72.0 CC's.

Also, the Intake Port with the 2.02" Valves has measured as much as 3 CC's less than the
'X-Heads' {157 to 160 CC's}.

But, the Exhaust Ports on the 'J-Head" have measured as much as 3 CC's larger than the
'X-Heads" {73 to 70 CC's}.

Port wise, they are nearly identical, with a very minor variation in Port Volume.

Small Block Cylinder Heads

CC's / Port Volume / Size / Flow

1968 - 1971 {340} Casting #2531984 'X-Heads'

Open-Shaped Combustion Chamber

Factory CC Level..........................65.0 to 69.0 CC's
Recommended Minimum CC's......63.5 CC's

Intake Gasket Port Size..............1.16" x 2.27"
Exhaust Gasket Port Size............1.25" x 1.75"

Intake Valve..............................2.02"
Exhaust Valve............................1.60"

Intake Bowl Size........................1.75" {87% of Valve}

Intake Port Size........................160 CC
Exhaust Port Size.......................70 CC

Intake Flow.............................215 CFM @ .450" Lift........221 CFM @ .500" Lift
Exhaust Flow...........................140 CFM @ .450" Lift........142 CFM @ .500" Lift

Intake-to-Exhaust Flow....................65.5%..............................64.1%..............""""... "The notable "X" and "J" heads (named for an X or J cast into the head near the spark plug hole) fall into the large port category, as do cylinder heads with "915" as the last three digits of the casting number."


Recommended head gaskets?

Street driven cars.
One motor gets the
crane 693902 H-260-2
1500-4500 RPM lobe sep 112
Intake Duration @.050"Lift 204
Exhaust [email protected]"Lift 216
ADV Intake Duration 260
ADV Exhaust Duration 272

The other gets the Elgin P/N CL3003+103+VS9010-16
Same as OE Purple #2899206 340 275hp cam

Not sure if the cam specs really matter but some of you guys have made comments about running too small of a cam with higher compression ratios.