Working out the CC's to get the ratio

[Kingsley78]

Ive gotta pull my head off soon to fit 17cc domes. Id like to cc it to work out the corrected ratio.

Ive never checked the CC's on an engine with domed (blaster) pistons before. Whats the way to go about it?

Prevoisuly ive filled the cylinder at TDC with trans, with perspecs to fill it to the deck and done the same to the combustion chamber in the head, then messured the amount of oil.

My memory 1ml of oil equalled 1cc id area, it this correct?

[Kingsley78]

yes the is correct.

but just because you changed the shape of the piston head or the domes in the the head, the size of the engine will not change.

 

Thats right but it does change the ratio....

 

How do i work out the cc of the piston with the dome above the deck?

Do i messure the cc from the shoulder of the piston to the deck, then add the squish thickness and minus the piston dome cc from the head dome?

[No Limit Powersports]

Its really not that easy to fig compression ratio on a two stroke. On a two stroke the motor does not start building compression untill the exhaust port closes so to know the compression ratio you will have to know the exhaust height, deck height and head volume.

[Kingsley78]

ok this will give you a ratio from TDC and BDC, although this isn't the correct CR for a 2-stroke. Have one fo the pistons at TDC. Put the head on. Fill up the the jug through the spark plug hole, use something like premixed gas or oil or something like that. Measure how much it takes to fill it up. And if you are running a stock bore and stroke engine use this formula, (347/2 + x) / x, where x = cc of liquid it took to fill the jug.

 

 

Im running a 66.5mm bore and 58mm stroke (4mm stroker) .

 

Where have you taken the figures from?

Im guessing i delete the cc space that the head of the spark plug would take up or isnt this a concern?