So i'm fitting 998 +40 thou dished pistons and trying to calculate how much to skim my block to get a CR of 9.2
Using C-AHT88 head (23.5cc comb chamb vol according to minispares) & payen head gasket (2.8cc vol).
Bore rad: 32.78 mm
stroke = 76.35 mm
piston dish vol = 7cc
ring land vol = 0.75cc
head gasket vol = 2.8cc
comb chamber vol = 23.5cc
that gives a USV of 34.56 and SV of 257.74
Which means a CR of 8.46
at TDC the distance from the block face to the top of the piston is 0.15mm.
I made an excel file to figure out how much to skim but that tells me i need to take 1.05mm of the top to get a CR of 9.2.
The problem is, if i skim more than 0.15mm, then the piston will protrude through the block into the combustion chamber, meaning that the unswept volume will also change now. This also means that my excel file is no longer correct in giving me the CR.
if the piston protudes into the combustion chamber at TDC, then the combustion chamber volume will have reduced. but the complex shape means simple geometry cant be used to calculate the new comb chamber vol right?
so how on earth do i calculate how much to actually skim?
Edited by BaronVonchesto, 14 June 2019 - 11:49 AM.