Another compression ratio question

[Rusty 356]

I am in the process of checking c/r on a 3.0 liter engine. Using jack oil I filled the combustion chamber including the spark plug hole and had a total fill of 76 cc of fluid. The engine is a twin plug with an unknown Ross pistions. Any direction would be appreciated. Std 70.4 stroke, 95 bore.

[afterburn 549]

I will give you my cave man
Adjust position of engine with piston at TDC so as to fill the the combustion chamber
1- put enough liquid in a accurate beaker and fill the chamber
that's your "1 ' number
now wind engine to BDC and with a "measured" amount fill it all again
if you got 7 parts of "1' in then you would be @ 8 to 1 as you had left 1 part in

[cgarr]

7.565:1

[AlfonsoR]

Compression ratio is just a comparison of volumes with the piston at BDC (admission) to TDC (ignition). You take the volume you measure at BDC, then divide that value by the number you got at TDC, in your case 76cc. You want to make sure you fill to the same spot each time, a good place would be to the bottom thread of the spark plug hole. You only want to measure where the gas/air mixture will come into contact upon ignition. On a twin plug, leave one spark plug installed and try to get the other spark plug hole as close to horizontal as you can.

If your engine has a compression ratio of 9:1, then you can expect that when you check it at BDC that you will use 9 x 76 = 684 cc's or pretty close to that.