Post
Wed Jan 16, 2008 9:19 am
here is what i suggest (these are rough numbers not to be taken as gospel)
perhaps start by calculating the overall chamber volume of a standard m20b25 and since you know the head CC and gasket size you should be able to find the dome something like
overall cc's = head cc + gasket cc - dome cc
so some basic numbers
53.3cc = 40cc + 9.7cc - dome cc
therefore dome = -3.6cc meaning its actually a slight dish overall
then perhaps press a piston in some plasticine to make a mold to fill with carefully measured liquid.
Don't forget that your 'effective' dome has changed since your piston sits higher by some amount so you may need to adjust for this aswell
if for example it sat 0.5mm higher then the dish would be
-3.6 cc + pi/4*(8.4^2)*(0.05) = -0.8cc
therefore there is really no dish or dome and the chamber cc is just head cc + gasket cc or around ~50cc
giving a CR = 10.3:1.
did you find out why the piston sits higher when in theory it should have been 0.5mm below? I know its only theory but i would assume an 84mm crank is pretty darn close to 84mm and a 130mm rod is 130mm or are these just approximate numbers? it doesn't make a whole lot of sense otherwise.
